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A Beginners Guide to Visual Prolog

1. o oo oooooo o o 97 ti ia a iaa 217 length A herd betes a 171 Level order traversal 234 LITRO 6 jit ah dd 217 o ION 139 lisnar tented eee ae aera 166 215 list comprehension 179 List domains 00 0 0 ce eee ee 146 list literal cc a 139 list unification 0 0 0 0 ce eee 169 LISTO na Macon the BE Bon ce alas 101 Mt ara a 183 tl iaa 36 Literals dede Wa ie soe ee a Arh 138 logical interpretation 153 loosely cOUplid8 115 lowercaseldentifier 135 lowerCaseldentifiers 139 Main Window 0 oo oooooocoocoomo 19 main lit Bea oe A ee Sa 41 MAIN PIO tro oa re asa 41 MAINE XC sot Ses wens ias 155 WAI ts ais 88 menu iteM ooo ooo 92 menu item 0 0 cece eee 92 mesbox buttoNS o o ooooooo o 33 mesbox default o o o oooo 33 Mesbox_ icon o ooooooommmmomo 33 mesbox suspend o o oooooomo oo 34 Message Window 00005 9 94 messagebox 6 00 0 e cece eee 32 Messages WindoW o oo oooooo o 40 MENOS iia ci 113 Modelhane E eye aks 85 Module as et eee Salts 90 mouse event 0 cee eee eee 23 MouseDown 0 02 cee ee eee 23 MouseUp 000 e eee eee 23 TOU is Fetes Sard dds whee idee a a set 151 Name Field ui sess wea teas ie feces 47 named facts database 147 new menu option 0 eee
2. ooo o 43 Aboutdialog dl8 43 50 AboutDialog L o oooooo 43 AboutDialog prO oo oooooooo 43 ACTIVITIES eee ee 113 AUG se isere ena eee ate cori eae TES 21 addMenultemListener 22 algebraic data types 144 OS Sena weeds Sek ees 54 153 ING ape ancl A anata 60 angle brackets o o 216 anonymousldentifier 135 apostrophes ooooo coco oo oomomo 138 A O 214 appendg micas a rio ria 214 arguments 0 0 E ee 27 53 78 IL A da 34 ER 34 A O 98 AS ds 98 ASSCITZ swede tn as 98 assignment sign 0 eee eee 147 ASIS o Satie ea 167 A A A iene 88 attributes oooooooo o o 47 113 WaACktrack lt gt scars sra cities 69 backtrack point 0 0 0 0 cece eee 65 backtracking o ooooooo 62 64 Backus Naur ForM o o ooooo 215 Base Directory o oooooooooo 10 binary operators oooooooomoo 137 A ER 65 A A ees nace Aea eget aes 143 BNP ses eae 215 DOY A ds 153 DOUNG aa a ee Mada 65 breadth first traversal 234 BrOWSING cia tes 40 bubble Sort od 185 Build MO bes Oe ns 40 Build Build o o oo o o 12 Build Execute ooooooooo ooo 12 Buildings tada saa hts ew bh nas 40 calculating predicates 74 catch_all clause o 152 catchall clause 70 channel
3. class predicates repeat multi clauses repeat repeat repeat clauses run console init InputFile inputStream_file openFile8 Source txt OutputFile outputStream_File create Destination txt copystream InputFile OutputFile InputFile close OutputFile close _ stdIO readChar succeed place your own code here end implement main goal mainExe run main run This is the way to use streams in VIP You create an object with every stream that you need and then communicate with that object to get or to give what you want To help you there are a lot of predicates I shall show you the ones most used But I think you should really take a look in the help 211 file to get an idea of the richness of the class Stream and it s sub classes Here I only give an overview as I think by now you will understand what the predicates are used for Class Stream In class Stream There are a o these predicates close procedure Closes the stream getCRLFconversion gt boolean Enabled procedure Return value specifies whether the stream data will be handled with CRLF lt gt LF conversion if true or passed without modification if false getMode gt mode Mode procedure Returns current mode of the stream binary ANSI or Unicode getPosition gt unsigned64 Position procedure Returns a stream position at which the next read or write operation
4. lt objectName gt lt predicateName gt Here you use a single colon In fact you did this from the beginning Remember how in chapter 2 we created code to show a form It said clauses onFileNew Source MenuTag NewForm form1 new Source NewForm show When you clicked te menu option File New this predicate was called It consists of two sub calls The first is NewForm form1 new Source This says that we call the predicate new in the class form By now you know that this predicate is the constructor of the class It does what it should do it creates a new object from the class form1 Not surprisingly that happens to be a form This form this object is bound to the variable Newform From now on we can use the form by using the variable as a reference That is exactly what we are doing in the second sub call NewForm show We refer to the object by the variable Newform and tell it to show itself and bingo it is there on your screen Please notice the different use of the colon and the double colon To illustrate the difference between class and object predicates let us extend the example above with code that counts the number of person objects that are created This count will increase every time an object is created and never decrease I will extend the declaration of the class with a predicate i e getCreatedCount that can return the current count So please now change the file person cl i
5. 5 shows three buttons lt Abort gt lt Retry gt lt Ignore gt The default button At the fifth place that is the fifth argument you can indicate which button will be the default button When the message window appears the user is supposed to close it by clicking a button Instead she can also hit the lt Enter gt key In that case it is like she clicked the default button e mesbox_defaultfirst 0 means that the first mentioned button in the sequence above is default e mesbox_defaultsecond 1 the second mentioned button in the sequence above is default e mesbox_defaultthird 2 the third mentioned button in the sequence above is default You will understand that it is bad programming practice to indicate mesbox_defaultthird when there are only two buttons 33 Suspending The last position is to indicate whether the user can work in another application while the message is being shown mesbox_suspendapplication 0 means that the user must react to the message before she can continue with your program But she can work in other programs while the message is shown mesbox_suspendsystem 1 means that the user cannot work with any application while the message is shown A last remark concerns the variable Answer When you call the procedure messagebox it not only puts a message on the screen it also accepts the button that the user clicks and returns it In other programming languages this is
6. Height 12 Font MS Sane Sef Maru lt untab raped Tobe lt unseb Oefsul Bulton gnb Tief e Tue Flare O ras Tuwa Figure 4 5 the forms editor with controls and layout toolbars and properties window To add an Edit Field do the following In the Controls toolbar click on the Edit Field Icon It is the rightmost icon on the first row with the word Edit in it Now go to the form template and left click at the place where you want the Edit Field The IDE enters an Edit Field You can resize it according 46 to your wishes To add a button again go to the Controls toolbar and select that is click on the PushButton Icon It is the leftmost icon on the first row with the word OK on it Then go to the form template and click on the place where you want the button to reside When you click on an item on the form template the properties list shows the attributes of the item and their values In the left column you find the name of the property in the right column the value E g click on the button that you added and the properties list shows every attribute of it See figure 4 6 The third attribute from the top of the list is the attribute Text that is also called the Text Field The default value for the Text attribute for a button is Push Button In short you can say the default text for a button is Push Button You may want to change that text Simply click on the text value field in the properties list and type th
7. Some comments here I give the data base a name so I can refer to it in the save command To be able to write to a file instead of the screen I first create this file When the file is created an existing file with the same name is overwritten Then I redirect the standard output stream to that file and with the save command the database contents are written to the output file In writing the program writes to a buffer We do not write directly to the hard disk because then the program will be too slow In stead we use a buffer A buffer is a piece of memory It has fast access and when the buffer if full then a separate program writes the buffer to the file on the disk while the processor can do another fast part of the job So before we close the output stream this is done automatically when the program ends we first must flush the buffer This sends what is left in the buffer to the file If you don t do this the contents of the buffer are lost With the command stdIO setoutputstream DBFile we change the current output stream That means that the write and save commands write to that file in stead of the standard output stream You may still see some lt returns gt on the win32 console That is because the things that you type are echoed to that screen If you want to write to the screen again you have to redirect the output stream with another call to setOutputStream It could be done with NewOutput cons
8. You will find more details of the class console and its predicates in the help file 11 2 The Message Window and the Error Window in VIP In VIP you have encountered the Messages Window It plays the role of a console in this way that it displays all kinds of messages from the system especially various progress messages describing important IDE events like a file or the project is saved a new project is opened a module is compiled a project is built etc In the Messages window you can see a list of all important IDE events it contains a history of the IDE execution You see the Messages Window in figure 11 1 But as the Messages Window is a Windows window you can do much more with it To name a few e The Messages window can be scrolled or re sized at any time to view more events than are visible in the window e It is possible to copy any selected part of text from the Messages window to the clipboard Using the right mouse button anywhere in the Messages window will bring up a local pop up menu e The Font for the Messages window can be changed with the Tools Options command in the Fonts tab or with the Font item in pop up menu e In the Messages Window tab of the Options dialog you can activate it with the Tools Options command it is possible to set the number of lines to remember in the Messages window ISA console target program starts with the clause console init 192 where the Messages window sho
9. erroneous is a very special one It does not fail or succeed but it raises a so called exeption An exception is a deviation of a program from its normal execution path for example when a file cannot be found memory overflow occurred etc If an exception occurs then a specified exception handling predicate should be executed An exception handler somehow analyses what kind of exception occurred and then can do some fixing action for example allow the user to enter a correct file name and recall the predicate or just show a message with information about an exception I shall skip this mode here When you omit the lt predicateMode gt then the compiler assumes that the predicate has mode procedure It is illegal to state a predicate mode for constructors they always have the mode procedure Table 9 1 gives an overview predicate mode succeeds or fails backtrackpoint always fails procedure always succeeds can fail or succeed none or one n o always succeeds one or more nondeterm can fail or succeed none one or more Table 9 1 Predicate modi With the lt predicateMode gt you give the inference engine in fact an indication how you intend to use the predicate In the sequence procedure determ multi and nondeterm we say that a mode is stronger more stringent than an other mode when it permits less options and that it is weaker when it permits more options So procedure is stronger than determ
10. myPredicate integer real string When using variable names the declaration becomes myPredicate integer FirstNumber real SecondNumber string TheString The variable names are neglected by the compiler There are several options that can and sometimes must be added to the declaration When the predicate is a function then the argument that contains the value to be returned must be declared e Optionally you can state the mode of the predicate Optionally you can state the flow pattern of the predicate When you omit mode or flow pattern the compiler assumes values for them When these options are added the declaration looks like lt predicateName gt lt argumentlist gt gt lt returnArgument gt lt predicateMode gt lt predicateFlow gt These options have the following meaning gt lt returnArgument gt The option lt returnArgument gt is preceded by the symbol gt It consists of the type of the return value the domain where the value is from and a Variable Name When you add lt returnArgument gt to the declaration the predicate is called a function You have to take precautions that the clauses return the right value Let s take a closer look at functions 149 When you call a predicate some of the arguments can be used as output arguments Upon return you can retrieve their values and in this way you can make a predicate provide you with the values that you want Another way to re
11. stdIO write The sorted list bubblesort is BubSortedList stdlO nl Run and enjoy the results 186 These three methods are not very efficient During the sorting the lists must be searched more than once A methods that does a better job is called quicksort The method takes these steps 1 take an element Any element will do so as the list is unsorted simply take the first one 2 put the remaining elements into one of two lists one list for the smaller elements and one for the bigger ones 3 then repeat the first two steps with each one of the two lists until finally you hold many lists with each one element in it 4 as you did continuously divide the elements into smaller and bigger categories these lists will be sorted You may find this hard to understand Try to do it by hand with the list 4 2 5 3 1 2 6 4 1 the first element is 4 2 splitting the list in two results in two lists 2 3 1 2 4 and 5 6 with the element 4 between them 1 put the elements equal to four in the lower list take the lower list The first element is 2 split the list in two The result is 1 2 and 3 4 with the element 2 between them take the lower list The first element is 1 Split the list in two The result is and 2 with the element 1 between them As the resulting lists are empty or contain only one element you can stop this branch and continue with the next branch that is the list 3 4 Din Hb U Continui
12. stdIO write Ca3 n _ stdIO readChar succeed place your own code here run end implement main 231 The program prints the tree as it is built You may want to extend the program with a procedure that writes the tree in a better readable way A problem with this program is that we use a lot of variables when we build the tree That is because there is no way to change the value of a Prolog variable once it is bound That s why this program uses so many variable names every time you create a new tree you need a new variable The large number of variable names here is unusual more commonly repetitive procedures obtain new variables by invoking themselves recursively since each invocation has a distinct set of variables 12 9 Binary Search Trees So far we have been using the tree to represent relationships between its elements This is not the best use for trees since Prolog clauses can do the same job But trees have other uses Trees provide a good way to store data items so that they can be found quickly A tree built for this purpose is called a search tree from the user s point of view the tree structure carries no information the tree is merely a faster alternative to a list or array Recall that to traverse an ordinary tree you look at the current node cell and then at both of its subtrees When the tree is adequately structured you only have to search one of the subtrees But that is not always possible
13. student will be ordered to create a new object and then that object receives the data that are entered by the user and stores them somewhere in the database For such a program structure you need several types of files You need e classes a collection of procedures and data interfaces that tell how one class or object can order another object to perform an activity Prolog files that contain the Prolog code of the program packages groups of files that belong together Think of a package as a folder under Ms Windows In VIP the type of file is indicated in the extension Visual Prolog uses the following conventions e cl file contains a class declaration 1 file contains an interface e pro file contains the class Prolog code also called the class implementation pack files contain include directives that indicate the files that should be included in the package e The include directives in pack files point to pro files and to ph files e ph files are package header files that also contains include directives The include directives in ph files point to other ph files and i and cl files e The included files are only shown in the project tree when the project is compiled built These are the extensions that you see in the left pane I shall explain these concepts better in chapter 8 and later chapters because you have to know about object orientation to understand them 4 2 TaskWindow in
14. 4 2 1 3 where the first 3 is the Head and 4 2 1 3 is the Tail More on this in chapter 10 Section A Visual Prolog program consists of several files and each file is divided into different sections by appropriate keywords The keywords inform the compiler what to expect in the section I have given the example of predicates and clauses The keyword predicates tells the compiler that in the section that begins with this keyword it can expect the declaration of predicates On the other hand the definitions of predicates are to be found in sections that start with the keyword clauses Usually each section starts with a keyword There is normally no keyword which signifies the ending of a particular section The presence of a keyword indicates the start of a section and the end of the preceding section The exceptions to this rule are the keywords implement and end implement The code contained between these two keywords indicates the code to be used for a particular class Scope Your program is divided into several parts think of it as the files that make up your project Everything that you declare can be used freely within the file in which it is declared But most of the time it cannot be used outside that part of your program This is what is meant by the scope The scope indicates where e g a predicate can be used If you want to use a predicate outside its scope then you must take precautions E g when you use a predi
15. Edit Go to the Dialog place the cursor where you want the Edit Field and click once The Control gets focus if not select it and in the properties list you see the properties of the control Two properties are important the Name of the control and the Text You find the Name on the first row in the properties list There it says something like Name edit_ctl Now change the Name of this edit control into Name name_ctl This edit field is going to contain the name of the person in the new record Take care that the name of the control name_ctl begins with a lower case letter and keep in the end of the name the ctl It is good practice to change the names of controls into something that has meaning for human beings Now add the other three controls and give them the names weight_ctl length_ctl and iq_ctl You can also change the standard Text Edit of the Edit fields I chose Type a name for the Name Edit Field and for the other ones I chose average values You enter the new text in the Text field in the properties list When you insert the controls you will find out that it is not easy to get them aligned I think this is the moment to play a little with the Layout Tools in the Layout toolbar Select a group of controls and align them Maybe you wonder why there is no field for the ID in the Dialog The reason is that the ID should be a unique code When you enter an ID yourself you may make a mistake and give the
16. Writing this book gives immediately a funny didactic problem you will need to learn about these three subjects programming in VIP programming the GUI and object oriented programming at the same time But it is good practice to write about one thing at a time I shall try to solve this problem by writing incrementally I introduce one subject a little just enough to understand the basics Then I switch to another subject again to give you just enough understanding for the chapters that follow This sounds rather vague and it is Let me elaborate on what I am going to do 1 I start with explaining what you see when you start Visual Prolog What you see is the Integrated Development Environment the IDE This is the place where you will be programming in Visual Prolog In the first few chapters you will get to grips with the basics of the IDE You will do a lot of things that will make you feel like Harry Potter you enter some magic formulae and suddenly it appears that you wrote a working program 2 Then I should like to extend your knowledge to the basics of the Graphical User Interface GUI as that is important in programming under MsWindows But when you program the GUI it is unavoidable that you will need some knowledge of Prolog So first I pay attention to explaining the basics of Programming in Prolog 3 Then we go back to Visual Prolog as an Object Oriented Programming language In the next chapters I tell you the basics of OO program
17. You can use the listBox getAutoNotify 0 gt predicate to get whether the control has this flag IsAutoNotify controlName getAutoNotify IsAutoNotify true and the listBox setAutoNotify 1 predicate IsAutoNotify true controlName setAutoNotify IsAutoNotify to set this flag programmatically AutoVScroll Possible values are True and False They define whether the multi line edit control automatically scrolls text up when the user presses the Enter on the last line The default value is False does not scroll This property has no effect on single line edit controls You can use the editControl getAutoVScroll 0 gt predicate to get whether the control has this flag IsAutoVScrollSet controlName getAutoVScroll IsAutoVScrollSet true and the editControl setAutoVScroll 1 predicate IsAutoVScrollSet true controlName setAutoVScroll IsAutoVScrollSet to set this flag programmatically Border 257 Possible values are True and False They define whether the control is created initially with a border around the control The default value is False You can use the editControl getBorder 0 gt or listBox getBorder 0 gt predicates to get whether the control has this flag BorderlsSet controlName getBorder BorderlsSet true and the editControl setBorder 1 or listBox setBorder 1 predicates BorderlsSet true controlName setBorder BorderlsSet to set this flag programmatically Case Po
18. bill Who to find a friend of Bill friend bill jane girl jane likes bill jane friend bill jim likes jim baseball friend bill sue girl sue girl mary girl jane girl sue likes jim baseball likes bill jane Without cuts in the program Visual Prolog would come up with three solutions Bill is a friend of both Jane Jim and Sue However the cut in the first clause defining friend tells Visual Prolog that if this clause is satisfied it has found a friend of Bill and there s no need to continue searching for more friends A cut of this type says in effect that you are satisfied with the solution found and that there is no reason to continue searching for another friend Backtracking can take place inside the clauses in an attempt to satisfy the call but once a solution is found Visual Prolog passes a cut The friend clauses written as such will return one and only one friend of Bill given that a friend can be found Determinism and the Cut If the friend predicate defined in the previous program were coded without the cuts it would be a so called non deterministic predicate That means that the predicate has multiple clauses that can generate multiple solutions through backtracking In many implementations of Prolog programmers must take special care with non determinist
19. have probably a different value for different objects But sometimes a fact is an attribute of a class as a whole E g when you want to keep track of the number of objects that have been created then this fact is a fact that belongs to the class not to an individual object In that case you have to declare the fact in the section class facts Class predicates In the section class predicates you declare local predicates that are predicates that can only be called from within the scope where they are defined Local predicates are used for actions within a class or object that are not interesting for other objects An example could be the sorting of a list before that list is printed Open The keyword open is used to extend the scope visibility of the class In this section you specify the names of other classes When the compiler compiles a class say a module and it encounters a predicate that is not in this class then it will look in the classes mentioned in the open section The keyword open is to be used right after the implement keyword Implement and end implement Among all the keywords discussed here these are the only ones which exists as a pair Visual Prolog treats the code written between the two keywords implement and end implement as the code that 140 belongs to one class The name of the class MUST be given after the implement keyword The name MAY be given after
20. 00000 e eee 192 UN teeta Senso Shee echt ee 217 top element 00 0 0 cee eee 217 HOSTING is otal ett A ea eb 34 A a dinsde a pe Made weak 99 Tote dista 99 100 traversing the tre8 ooo ooo oooo 228 MWC io Pet va ok 215 ME A AM ae ce oan eS 225 treestraversals canst pian aon ae eave ease 234 WUC fica rein acct aves acta aoa Raia ees 73 TYPE NAMES s memea ii 144 ty Pe OF TEs ion a ee he Wee eee 42 WE Strategy iia Sea wn Peas 10 unary MINUS ooo ooo 138 unary Operators 0 0 eee ee eee 137 unary plus 0 cece eee 138 A ee hee oie ee eee 213 ungetChar 0 o ooooooooooooooo 199 Unhandled o oooooooooooo oo 49 UNIf Cations Lai gears nen ads 154 unsigned 0 0 0 0 c eee eee ee 76 138 A cd els ein eee ies 73 uppercaseldentifier 135 user interface o oooooooooooooooo 88 Vala ble iia ara dro dea 54 vertical DAT id ata tek 216 vertical bare concisa 168 View Messages Window 193 Visual Prolog Help 194 vpiCommonDialogs 4 27 Wer evs A 193 AA A esate aes 193 Wiles a al ete 213 Weled eae ty gcse Beal oleae E ei ah 200 274 wete a lst oca aiii 170 Write CITO ooooooooooooooomoooo 202 writeByteS nice e ae o 213 WE AA O EEN thaws 213 A o ad a aaa 205 WEN L erha si aare k r A 3 ers 201 Writef iror uriar Ea eee 202 writeQuoted o oooooooooooooo 213 writeQuoted o o oooooooooooooo 200 writeQu
21. CC The inference engine now calls the parent predicate i e it searches the program for a predicate parent and it arrives at the last clause The rule in that clause says that Parent is parent of Child if Parent is mother of Child or Parent is father of Child The question now is can the inference engine use this rule The answer depends on the match between the sub goal and the conclusion of the rule If they match then the rule can be used if they don t match then the rule cannot be used In this case the sub goal parent John CC matches the conclusion parent Child Parent when the engine binds Child to John Using a rule means that the conclusion is replaced by the premisses But before doing so the engine must take care of the bindings that were made when the rule was matched with the sub goal In the conclusion Child was bound to John so in the premisses the engine binds Child also with John Effectively the rule looks now like parent John Parent mother John Parent father John Parent Now the engine is ready to make the substitution in the current goal The current goal was parent John CC It becomes mother John CC father John CC Mind the semicolon 66 The variables in the clause have been replaced with the actual parameters of the call exactly like when you call subroutines in other languages The current goal is an or goal It means that when t
22. Here it placed a line of code you have to scroll almost to the end of the file to find it that connects clicking the menu option File New with the piece of code that you changed The line of code looks like addMenultemListener resourceldentifiers id file new onFileNew In VIP this is called adding a listener as this line of code adds a so called listener to the click on the menu item You may think of a listener as a little demon that sits back and waits until you click the option File New At that moment the demon directs the program control to the clauses in the program that you changed the clauses that start with onFileNew and this code is executed In the previous section you changed the code that was inserted by the IDE What happens when you don t change that code In that case the code acts as a do nothing instruction so you don t have to bother about it In this way the IDE controls which menu items are activated by unchecking disabled and inserting a listener and which are not and next to this ti controls which activated items do something by executing the code that you enter and which items do nothing And maybe more important when 22 you want to change the program the IDE helps you to find the relevant code You can imagine that in a large program it is quite a job to do this kind of bookkeeping But there is a price for this service When you want to change the program code of the Task W
23. Integral Domain Integral domains are used for representing ranges of integral numbers or integers for short There are two predefined domains for integers integer and unsigned they represent signed and unsigned numbers respectively with natural representation length for the processor architecture i e 32bit on a 32bit machine etc Probably you will not bother much about the representation To declare a new integer domain you should declare domains myDomain integer After the word integer you can state two parameters lt sizeDescription gt and lt rangeDescription gt The syntax then becomes There is also the type symbol but I was advised not to mention it 142 domains lt domainName gt lt typeExpression gt lt sizeDescription gt lt rangeDescription gt The lt sizeDescription gt indicates the number of bits that should internally in memory be used for the values in lt domainName gt For lt sizedescription gt you use the word bitsize followed by the integral number of bits The lt rangeDescription gt denotes the value range of the values in lt domainName gt it consists of a minimum and a maximum value The lt rangeDescription gt 1s indicated by two integral numbers between square brackets separated by two dots So to declare that a domain with the Name newDomain contains integer values has a range from 0 to 10 and should be stored in 8 bits we declare and define d
24. SLarge parts of this chapter are inspired by the Wikipedia lemma s on data structure stack queue and tree The sections on trees contains parts of the user manual that came by VIP version 5 0 215 stating the name followed by the sign followed by summing up the possible items that fall under the definition The possible items are separated by a vertical bar that is read as the logical or The concepts are given a name that makes sense and the name is put between angle brackets lt and gt As an example let s look at the definition of an unsigned integer An unsigned integer is a sequence of digits A definition could be lt unsigned integer gt lt digit gt lt digit gt lt digit gt lt digit gt lt digit gt lt digit gt The definition says that the concept lt unsigned integer gt can be a lt digit gt or it can be a lt digit gt followed by a lt digit gt or it can be three lt digits gt in a row or Additionally we can define a lt digit gt by lt digit gt 1 2 3 4 5 6 7 8 9 0 This says that a lt digit gt can be one of the natural digits including zero This definition of lt digit gt works fine It is clear that in any place where lt digit gt appears you can use any of the ten mentioned digits Maybe you would like to place a restriction on leading zeros but that is not necessary But the first definition of lt unsigned integer gt raises a problem Immediately you will se
25. You should take a holistic approach while zeroing in on the functors and other data structures you plan to use in your software Pause and look around at all the corners of your software Only then should you code the necessary functors Just thinking carefully about the data structures is not the only thing that is required You would also need to often simultaneously write modify the various goals and sub goals i e predicates and then use the data developed thus far in those predicates The side effects of attaining those goals and sub goals would make your software work and you can get valuable feedback with which you can further refine your data structures In this chapter we have not fleshed out a software design holistically We have only nibbled at bits and pieces of data which establishes the feel for some kind of real world data concerning relationship between people parents grandparents families ancestors etc In later chapters we will elaborate these topics eventually ending up with useful software that requires such data structures 6 9 Conclusion In this chapter we learnt that data can comprise of simple domains or they can be compound domains represented using functors We found that the arguments of functors must be positionally consistent with the logic they were meant to represent We understood that a functor need not have just simple domains as arguments but it can also have other functors as arguments We then lear
26. as in this case printing is what you want so the writing succeeds by failing And you need an extra clause to prevent the program from failing In version 7 of VIP the foreach construction is introduced It looks like this foreach nondeterm_predicate X do write X nl end foreach The foreach construction is intended as a replacement for fail loops In our example it could look like Write_a_list List foreach member X List do write X nl end foreach 181 This predicate writes the list and succeeds So no extra clause is needed In this case you don t even need an special predicate the for each construction can be inserted as a subgoal in another predicate foreach nondeterm_predicate X do Process X end foreach The foreach construction succeeds when the iteration is completed So computation will continue right after the foreach construction In the clauses with fail you had to take precautions to see that the program continued rightly after the fail Foreach takes care of that Look at this piece of program it continues after the printing is done foreach p_nd X do write X nl end foreach write I will continue here As a result foreach terms can be followed by other terms and they can be properly nested Example foreach member L LL do write lt lt foreach member X L do write X end foreach write gt gt n end foreach LL is supposed to be a list of lists The oute
27. constVIP string Visual Prolog If the domain of the value is one of the standard domains then the name of the domain and the colon can be omitted giving the following abbreviation lt constantName gt lt constantValue gt E g vat 0 19 146 Constants defined in this way can be used in all contexts where a literal of the same kind could be used If the lt constantType gt is omitted then the constant domain must be unambiguously determined by the lt constantValue gt expression Take care of the way VIP distinguishes between integers and reals The number 123 is an integer the number 123 is a real 9 6 Section facts A fact describes the values for one or more attributes of an object or of a class Facts in Prolog are stored in a database Every fact has its own database There are two kinds of facts When the fact represents the value of a single attribute then we call it a fact Variable When a fact describes the value of two or more attributes then we call it a factFunctor A facts section declares the facts that are contained in a fact database The fact database and the facts belong to the current scope As explained before a fact database can exist on a class level as well as on an object level When it exists on an object level then facts will be different for different objects and you have to add change or remove facts by calling an object predicate When a fact exists on class
28. database pro I add this code class facts persons the facts are stored in a database with the name persons person string Name clauses storeNames N N this predicates reads a line and adds a fact to the local database stdIO write nInput Name lt Enter gt 0 to stop 0 stdIO readchar the program looks if the first character is 0 k if it is the program stops storeNames I Number stdIO ungetchar if not the program puts back the first character Term stdIO readline and reads the complete inputline assert person Term and adds the new person to the database N I 1 raises the counter by one 196 Number storeNames N Number counts the number of records fill starts filling the database by calling storeNames 1 Fill local database N storeNames 0 stdIO writef n d terms were stored n Press any key n N stdlO save persons saves the db to the current output stream the screen Wait _Char stdIO readchar DBFile outputStream_file create8 persons txt this creates an outputfile in ANSI text mode We refer to this file with the variable DBFile This variable is now of type Stream stdIO setoutputstream DBFile DBFile becomes the current output stream stdIO save persons write the database to the outputbuffer stdIO flushQ empty the buffer to the file _Char2 stdIO readchar end implement database
29. in words sort the list by taking the following steps 1 remove the Head of the unsorted list 2 sort the Tail 3 insert the Head at the right place in the sorted Tail In Prolog it looks like this insertSort insertSort Head Tail SortedList take an unsorted list and produce the sorted list insertSort Tail SortedTail sort the tail insert Head SortedTail SortedList insert the Head at the right place in the sorted Tail By the way I like to use long names for variables so it will be clear what they represent when you read the program That s why I use SortedList and SortedTail as names But if you want you can of course use names like L1 or L2 Next we need a predicate to insert an element I use the predicate insert with the declaration insert integer integer integer procedure 1 1 0 This predicate uses three arguments The first is the element that must be inserted the second is the list into which the element is inserted and the third is the resulting list This predicate is also a recursive predicate So I start with a simple stopping rule to insert an element into an empty list insert Element Element 183 What to do when the list is not an empty list Well then it depends on the Element_to_insert and the first element the Head of the list Remember that the list into which we insert the Element is already sorted So when Element is less than or equal
30. one to save the database to a file on disk and another one to retrieve the saved data Luckily there are standard predicates for these actions You can find them in the class file The standard predicate to save an internal facts database to disk is save 2 It takes two predicates a filename to write the data to and a name for the internal facts database Until now we haven t given the internal facts database a name That wasn t necessary because there was only one database in the class person But now that we are going to use the predicate save 2 we first must give the facts database an name I chose the name personDB To give that name to the facts database change the line class facts in person pro into class facts personDB Don t forget the in front of the name You don t have to add a period at the end of this line Now that the facts database has a name we can implement the predicates for saving and retrieving First add two new menu items to the TaskMenu I suggest that you give the new options the names SaveDatabase and ConsultDatabase and place them within the option Database Then add standard code for both options in Task Window win and change the standard code into predicates onDatabaseSavedatabase window menultemListener clauses onDatabaseSavedatabase _ Source MenuTag person saveDatabase predicates onDatabaseConsultdatabase window menultemListener c
31. program that you find in this chapter is also available as a standard predicate They are available in the class list When you add this class to the open declaration you get access to all these predicates I give an overview in appendix A2 But I think that when you go through the examples in this chapter your understanding of Prolog will improve Besides it is highly probable that you will use lists and recursion in your own programs 10 1 Recursion Remember what you did in elementary school when you started to learn to count from 1 to 10 You started with the number 1 then added 1 to make 2 then added 1 to make 3 then added 1 et cetera until you reached 10 What you did was that you started with 1 and then over and over again called the same function add 1 to get to the next number This is the basic idea behind recursion In general we say that recursion takes place when a function calls itself thereby generating an endless series of calls As functions in Prolog are predicates in Prolog we say that recursion takes place when a predicate calls itself As an example look at the predicate to count predicates count integer Number procedure clauses count Number stdIO write Number NextNumber Number 1 count Nextnumber When we call this predicate with count 1 it starts counting like you did First it writes the number 1 then it adds 1 to make 2 then it calls itself with count 2 so it
32. removeDuplicates NameList listBox_ctl addList SingletonList Take care to change the dot after generatedInitialize into a comma The three lines of code perform the following person getAllNames NameList Calls the predicate that generates a list of all names in the database e SingletonList removeDuplicates NameList removeDuplicates 1 removes the duplicate names in the list The cleaned list is bound to the variable SingletonList listBox_ctl addList SingletonList This fills the listbox with the names in the list Actually 1t adds the names in the list to the names already in the listbox but as that is empty we only see the names in the list The predicate removeDuplicates 1 is a standard predicate It resides in the class list so we must add 1t to the project Go to deletePerson pro and add at the begin of the code implement deletePerson inherits dialog open core vpiDomains list 103 Actually you only have to add the word list after vpiDomains Mind the comma And don t forget to fill the database before you try the program otherwise you will see nothing that is you see an empty list The listbox should be filled with the names in your database You can select any of them but of course nothing happens We didn t implement the code yet Maybe it is a good idea to try and execute the program What we want to happen is that when you select a name in the listbox the program searches the dat
33. sometimes you have to search the whole tree to find a particular item In that case you have to look at every cell in the whole tree The time taken to search an ordinary tree with N elements is on the average proportional to N A binary search tree is constructed so that you can predict upon looking at any cell which of its subtrees a given item will be in This is done by defining an ordering relation on the data items such as alphabetical or numerical order Items in the left subtree precede the item in the current cell and in the right subtree they follow it Figure 12 3 shows an example Note that the same names added in a different order would produce a somewhat different tree Notice also that although there are ten names in the tree you can find any of them in at most five steps starting from the top and deciding in every node to take the left or right subtree Blackwell Love lace Figure 12 3 Binary Search Tree 232 Searching a binary tree for a specific value is a process that can be performed recursively because of the order in which values are stored E g suppose a tree contains the names and addresses of our friends To search for an address we begin by examining the name in the root node If the value we are searching for equals the root we have found what we were looking for If it is less than the root with names that is the name we are looking for precedes the name in the root then it must be in th
34. stdIO write takes care of lists as well as other types of variables That makes life easy 10 6 Summary These are the important points covered in this chapter 1 Lists can contain an arbitrary number of elements you declare them by adding an asterisk at the end of a previously defined domain A list is a recursive compound object that consists of a head and a tail The head is the first element and the tail is the rest of the list without the first element The tail of a list is always a list the head of a list is an element A list can contain zero or more elements the empty list is written The elements in a list can be anything including other lists all elements in a list must belong to the same domain The domain declaration for the elements must be of this form domains element_list elements elements where elements one of the standard domains integer real etc or a set of alternatives marked with different functors int integer rl real smb string etc separated by semicolons You can only mix types in a list in Visual Prolog by enclosing them in compound objects functors You can use separators commas and to make the head and tail of a list explicit for example the list a b c d can be written as al b c d or a b c d or a b c d or a b c d or al bl c d or even al b el d 111 Note the empty list in the last example List processing consists of recur
35. that s another story So in the next chapter let s turn to programming in Prolog 52 Chapter 5 Fundamental Prolog In this chapter you will learn about the very fundamental ideas of Prolog programming I start with a more or less theoretical introduction and in the next section you will create real Prolog programs and run them Visual Prolog is object oriented strictly typed and mode checked You will of course have to master all this to write Visual Prolog programs But here we will focus on the core of the code i e the code when disregarding classes types and modes and neglecting the graphical user interface You can learn Prolog with pen and paper but it is better to start using a computer as soon as possible That s why we introduced the IDE in chapter 1 In this chapter 5 you will use the IDE to start a project called PIE Project PIE is a program for using the language Prolog in a very classical way more or less the way Prolog was used in the beginning and probably is used still in some courses on artificial intelligence The name PIE stands for Prolog Interpretation Engine The program comes with Visual Prolog you will find it in the examples PIE is a classical Prolog interpreter by using this you can learn and experiment with Prolog without at all being concerned with classes types or GUL 5 1 Horn Clause Logic Visual Prolog and other Prolog dialects are based on Horn Clause logic Horn Clause logic is a
36. the code generator doesn t seem to use the new name Then generate the standard code for the clickresponder of the createObject_ctl Go to the code and change it into predicates 120 onCreateObjectClick button clickResponder clauses onCreateObjectclick Source button defaultAction Name name _ctl getText _NewPerson person new Name To be able to use the form it needs to be displayed Therefore change the code for the menu option CreateObject in the Taskmenu into predicates oncreateobjects window menultemListener clauses oncreateobjects Source MenuTag NewForm createPerson new Source NewForm show When the user chooses the option CreateObject the form createPerson is opened When the user clicks the CreateObject button the variable Name is bound to the text in the edit field Name_ctl is the name of the edit field I gave it to the field name_ctl getText returns the name that the user types in that field The variable Name is then used to call the predicate new in class person and an object is created with whatever name the user entered Unluckely you don t see much of this new object It is created and that s all To see that it is really there change the code in the clause for the constructor predicate new 1 in person pro into clauses new Name name Name stdIO write Hello world I m a new object My name is name stdIO nl Every time you
37. true and the control setUseTabStops 1 predicate IsUseTabStops true controlName setUseTabStops IsUseTabStops to set this flag programmatically VScroll Possible values are True and False They define whether the multi line control gets a vertical scroll bar when it needs one The default value for edit controls is True for other types of controls it is False This property has no effect on single line edit controls You can use the editControl getVScroll 0 gt and listControl getVScroll 0 gt predicates to get whether the control has this flag IsVScrollSet controlName getV Scroll IsVScrollSet true and the editControl setVScroll 1 and listControl setVScroll 0 predicates IsVScrollSet true controlName setVScroll IsVScrollSet to set this flag programmatically WantReturn Possible values are True and False If it is True then a carriage return will be inserted as the part of an edited string when the user presses the Enter key while entering text into a multi line edit control If it is False then pressing the Enter key has the same effect as pressing the default OK push button which has the latest specified Default True property value This property has no effect on single line edit controls The default value is False You can use the editControl getWantReturn 0 gt predicate to get whether the control has this flag IsWantReturn controlName getWantReturn IsWantReturn true and the edi
38. window menultemListener clauses onTreesTraversePreorder Source MenuTag classtree traverse preorder predicates onTreesTraverseInorder window menultemListener clauses onTreesTraverseInorder _ Source MenuTag classtree traverse inorder predicates onTreesTraversePostorder window menultemListener clauses onTreesTraversePostorder Source MenuTag classtree traverse postorder I put the tree and all the predicates for manipulating the tree in the class classTree So every menu option calls a predicate from that class To create the tree I give a listof characters in random order The predicate add_from_list then creates a tree To traverse the tree the predicate traverse is called It has an argument that indicates as a string how to traverse the tree The domains and the predicates for manipulating the tree are declared in classtree cl It looks like this domains stringlist string treetype tree string treetype treetype empty predicates add from list stringlist procedure 1 treeinsert string Newltem treetype OldTree treetype Newtree procedure i 1 0 235 traverse string TraverseOrder procedure 1 preOrderTraverse treetype Tree procedure 1 inOrderTraverse treetype Tree procedure i postOrdertraverse treetype Tree procedure i The tree is kept in a class fact variable that is declared in classtree pro in this way cl
39. 147 factVariable o oooo o 147 oi rales 64 150 151 sa 62 A A Pe ST Be Rk 73 A BG ands By een ay NO 5 73 FIR OR it eet eae 221 File Ne Word aaa 58 File Consult ooooooooooo 58 File NEW tocada area Be 19 44 90 File New in Existing Package 14 46 File NewInExistingPackage 91 96 Pile Saves it ds iaa 58 finda Math tada 62 findAll eersten eeepc 98 findall 3 irie etae sd 178 First In First Out 221 floating literal ooo o o o 138 flow pattern 26 eee ee eee 178 CA ete he Bee 213 PLUSH O A eae Send 199 flush _error O o oo oooooooooooo 201 EOL yes 0s E TN 46 96 form edit window 00005 16 Forms Editor o ooooooo o 48 functor soe a Be 78 82 89 functors 0 0 0 eee eee 144 167 Mitos tat Me Speed it 115 getCRLFconversion 4 212 getErrorStream 0 00 201 getFileName 6 ooo oooooooo oo 36 getInputStream 0 ooo ooooooo 198 PetMOd eer si encase e e ease etd me eles 212 getOutputStream 0 4 200 getPosition 0 cece eee eee 212 GetSelectedItems 104 getString 3 0 eee eee 35 ir a a di ce wa 155 goal clauses o oooooooooooo oo 56 DS ee he he ete Monee 56 58 oreen CU eis crete ioe ova EEE 69 Handled 10 bah a ba 49 Head A renaren 139 153 167 168 Help AbOUt ooooooooooooomoo oo 43 hexadecimal n
40. 28 New Package o ooooooomomoo o 46 NOW PFOJect Lees 40 O O 114 O aude Babbage iat ES Salat ois 213 TOS tas ii i teas 200 pl error Og oi iis Deke need lla tek 201 NODC e a e e eek 226 non deterministic predicate 71 nondeterm 000 148 151 non deterministic predicate 178 Ot ui Rea eh be area wees 31 NOTEZ a aaiae a AAE wt laud e auna 31 Obi bee thd Sia ak Sn ees 126 O A dage ath A 113 Object tacita india 124 Ob CCE PACS a5 at yai Bok ae alan RS 124 object facts isea siers baw tate radia 127 object member 02000000 158 object oriented programming language 113 object predicates 00 0000 121 object predicates 122 124 object states eve ssc decd ala sana 124 ODJCCtS oo edad ee She neo Shee Oe 53 ODE CS ay Sees hid Meet cies eet hy tes 121 ObjectEAttributeEValue 113 octal integers ora iep a pa E E 138 onButtonO0kClick o oo o o 32 Close iii aaah Pea aes 51 onFileNeW 2 0 eee eee tenes 21 onListboxSelectionChanged 104 onMouseDown 2 005 24 ONOKCHEK crio deeds saa aeai sys 99 106 OOP ota cisetlnd nine eines SA 113 E 140 156 OpenFlle iia 213 214 openFile8 ooooo oooomomomoo o 214 Operations ses cio ais bs PRS 113 E scenes a EA aa eed Bs 137 Ole Sa ander it cia ene e 60 153 216 Ouputstream file 208 output arguMent oooocococcoccco o
41. Address Name lt gt 0 stdlO write Name Address has been duplicated stdlO nl onQDuplicate _ Source MenuTag stdIO write Queue is empty nothing to duplicate stdlO nl predicates onQSwap window menultemListener clauses onQSwap _Source MenuTag classQ swap Namel Address1 Name2 Address2 Namel lt gt 0 stdlO write Namel Address1 and Name2 Address2 have been swapped stdIO nl onQSwap _ Source MenuTag 224 stdIO write Queue contains less than two items Nothing to swap stdIO nl Maybe it is a good idea to play a while with stacks and queues to get to grips with their opportunities 12 5 Trees Not only can clauses be recursive so can data structures as we have seen with the list As a matter of fact Prolog is the only widely used programming language that allows you to define recursive data types A data type is recursive if it allows structures to contain other structures like themselves Compare a recursive clause that calls itself within itself The most basic recursive data type is the list although it doesn t immediately look recursively constructed A lot of list processing power is built into Prolog lists were discussed in chapter 10 and in section 12 1 In this section and the following ones I will discuss another recursive data type implement it and use it to show you a very fast sorting program The structure of this recursive
42. Execution terminated No solutions Figure 5 3 1 Solution dal Figure 5 4 then you probably forgot to Reconsult Take care to write the goal exactly as in the figure If e g instance you write GrandFather instead of grandFather you will get an error message A predicate should start with a lower case letter And don t forget the full stop at the end lt Enter gt alone isn t enough Itis a good exercise to also try the other goals from the previous section Take some time to practice 1t is worthwhile the trouble However you need to know a few things about specifying a goal Here are some rules You can use any predicate in the program to formulate a goal In this case you can use father or grandFather With a predicate you must give the appropriate arguments That means the right number and the right type In this case the type is always string so it is hard to make a mistake here As an argument you can enter the name of a specific object or you can use a variable Names of specific objects here are Sue Jim Bill et cetera but in fact any string is acceptable Examples are father Sue Jim father Sammy Simson grandfather Sue Jim When you enter a goal like this the inference engine will answer Yes or No depending on the rules and facts in your program The second option is to use variables as arguments Examples are father
43. For example Cancel_ctl Notice that in this case the variable name is automatically converted to be started with the uppercase character The default value is Fact Variable Title The Title property is where you specify the title label which you want to be displayed as the control caption One character in the Title text may be underlined to indicate an access key To underline a character precede it with the ampersand amp character At runtime this character in the control Title will be underlined Notice that this character is displayed underlined also in the value cell of the Title property Title a Help The used ampersand amp character is seen only when the value cell of the Title property has the focus EI EE o s l If you need to display the ampersand amp character in the Title text then use two of them amp amp Lock to Container Possible values are True and False True defines that the control is locked inside the container selected in the Container Name property Container Name In this list box you can select the Name of one of containers currently only the group box groupBox type of containers is supported defined in the dialog or in the form All controls belonging to the same container are in the same group of related controls The user may use the arrow keys to move from one control in the group to the next one Only the Names of group boxes inside which the control is placed appear in the l
44. IDE opens a new window so you can enter the Project Settings As this window is meant for a dialog with the user that is you a window like this one is called a Dialog Window or shortly a Dialog At the same time this Dialog acts like a form because you will fill in some of its fields That s why a window like this is also called a Form Both Forms and Dialogs can be used as synonyms they are more or less the same You will learn about Forms and Dialogs in later chapters Now fill in the Project Settings dialog as shown in figure 1 3 Project Settings Xx General Directoies Build Options Version Information File Templates Aun Options Project Name Joho pot UI Strategy Object oriented GUI pfc gui y Target Type Exe bl Base Directow x data My Documenis isual Prolog 7 propects IntroYisuaProlog Browse SubDitectory ch01p01 Cancel Help Figure 1 3 Project Settings You only have to fill in the Project Name as most of the fields are already filled by the IDE but it is wise to take a closer look at the content The first field lt Project Name gt will contain the project name For the Project Name I chose ch01p01 Of course you can choose another name but I suggest that you use the same name The next field is the UI Strategy It indicates what kind of user interface you will use The IDE suggests that you will use an Object oriented Graphical User Interface GUI It means that y
45. MenuTag ButtonPressed ask This is the question Button0 Button1 Button2 note You pressed button number tostring ButtonPressed Build and run the program A window should pop up that contains the question and three buttons The user can click This is the question one of the buttons and the number of the clicked button is returned to your Button rogram di Button2 Please take a closer look at the piece of code that says 34 Button0 Button1 Button2 It is the second argument in the procedure call In Prolog this is called a list A list consists of a number of elements between square brackets separated by comma s In this case the list contains the labels for the buttons A label for a button is a string so the names are put between apostrophes I specified three button but you are free to specify one or two buttons You must specify the name of at least one button Specifying four or more buttons is of no use three is the maximum You are free to choose any string for a label Internally the buttons are numbered in the order as they appear in the list The first button gets number 0 zero Take care this is different from what happens with the procedure messagebox The procedure ask 3 is almost the same as ask 2 The only difference is that you also give the messagewindow a title I presume that you can insert the necessary code without problems so I only give the code predicates
46. Once a cut has been passed it is not possible to backtrack to subgoals placed before the cut in the clause being processed and it is not possible to backtrack to other clauses defining the predicate currently in process the predicate containing the cut There are two main uses of the cut When you know in advance that certain clauses will never give rise to meaningful solutions it s a waste of time and storage space to use them in looking for alternate solutions If you use a cut in this situation your resulting program will run quicker and use less memory This is called a green cut When the logic of a program demands the cut to prevent consideration of alternate subgoals This is a red cut How to Use the Cut Here I give examples that show how you can use the cut in your programs In these examples we use schematic Visual Prolog rules r1 r2 and r3 which all describe the same predicate r plus several subgoals a b c etc The content of the rules is not very important as the focus here is on the program flow 1 Prevent Backtracking to a Previous Subgoal in a Rule Take a look at this rule rl a b c Rule rl has three subgoals a b and c Normally the inference engine will first evaluate subgoal a then b and finally c When c fails it would backtrack to b if there were any backtracking points with b or maybe back to subgoal a Now let s put a cut in this clause rl a b c The cut is the exclamation mark
47. Project Tree Now expand the node folder with the name TaskWindow You will see a folder called Toolbars and several files with names like AboutDialog and TaskWindow See figure 4 3 There is a pattern in 42 these files that you ve heard about before Let s take a look at the files with the name Aboutdialog You already know that a Dialog is a kind of form that is used to communicate with the user The About Dialog is the little window that pops up when you click Help About in the main menu of a program The IDE keeps track of this window and its content in four files The file Aboutdialog dlg is the file in which you can edit the About Dialog ch04p01 prj6 X data My Documents isual Prolog 7 ch04p01 53 ProDir C Toolbars AboutDialog cl 5 AboutDialog dig E AboutDialog i AboutDialog pro Taskwindow cl TaskWindow i Task window pack task Window ph Task window pro TaskWindow win 9 TaskM enumnu EA Projectlcon ico main cl i main pack main ph main pres 83 resourceldentifiers Figure 4 3 The contents of folder TaskWindow Double click on Aboutdialog dlg and a graphical editor opens that looks a lot like the forms editor that you already saw in chapter 2 You can change the contents that the IDE places by default in the dialog The file AboutDialog cl contains the declaration Every item in your program must be declared the About Dialog is a class think of it as a m
48. Properties Figure 2 6 Properties window The IDE has already placed three buttons lt OK gt lt Cancel gt lt Help gt as these are very common in Windows windows When you want to change the size of the form click and hold the mouse at lower right corner and drag it as you do when you resize any Windows window We call this a prototype as it shows the general characteristics of a form It has a name on top room to work in and three often used buttons This is the way the IDE works Whenever you create something in the IDE the IDE comes up with a kind of prototype that you can use to modify At this moment the form only contains three buttons Normally you would enter other controls but for now we are satisfied with the form as it is so you can save it by clicking the disk icon on the Task Menu toolbar and close the form prototype by clicking the cross button in the upper right corner of the form prototype As an alternative you can click the cross button without saving The IDE will then ask you if you want to save Please do save the form When you now look at the project tree you see that there is a file added named form1 frm See figure 2 7 This is where the IDE has put the characteristics of the form you created When you double click on the name form1 frm the prototype opens again ch01p01 prj6 X data My Docu You should realize that there is only a prototype of the form there isn t any Prolog code yet
49. TaskMenu mnu Constant Prefix fid filene The IDE opens the TaskMenu Editor It is Item Constant fid file_new shown in figure 2 10 Help Constat fighep Here you can enable disable append and p Accelerator Bo ecked remove menu options F7 M Shift J Chl alt IV Disabled define short cuts et cetera As you are File interested in the option ES 8New tF File New of the menu Eg BO pen tF8 you open the amp File Es amp Savel tF2 option in the lower part ES A of the form by clicking ae E ERA E on the plus sign Ey sEdt Subsequently single 5 Help click on amp New tF7 to select it The IDE shows the Figure 2 9 Editor for the TaskMenu characteristics of this menu option in the upper part of the form There you find the checkbox Disabled Uncheck this box and the menu option will be enabled Save the new menu options and close the TaskMenu Dialog 19 2 3 In CodeExpert add code to Project Tree item The menu option File New is enabled now When you run the ch01p01 prj6 X data My Documents isual Pi 0 xj ch01 pi program you will see that it is no 2 59 ProDir longer grayed But when you click J Taskwindow on it nothing happens That is 33 Toolbars bcause we have not yet added H AboutDialog cl code to the menu option Now is ot B AboutDialog dla the time to add the necessary 83 AboutDialogi Prolog code to this optio
50. The file form1 frm contains the information about the form E la PraD it that you gave when you edited the form in the forms edit H 3 Taskwindow window To generate the Prolog program code you forml fren choose the option Build form the menu Build This is 8 main cl comparable with what we did in chapter 1 There you 93 main pack specified the properties of a program giving it a name main ph and a base directory and then ordered the IDE to B main pro generate the code In this case we designed a form and now we make the IDE generate the Prolog code When you click Build Build you will see that some files are added to the project tree They are form1 cl Figure 2 7 form1 frm added to Project Tree form1 i and form1 pro These files contain the necessary Prolog code and it is the place where you will put your own code You may want to open the file form1 pro to see the code the IDE has generated Simply double click on the name An editor opens with the Prolog code of the form in it You can scroll to see the code but please do not change the code The IDE keeps everything about the form form1 in four files The file form1 frm contains the design prototype the other files contain the Prolog code If you want to change something in form1 you open the design file and change whatever you want Then close and save the design and generate the code again This way of working will come b
51. To stop the visit order mode just click in the dialog outside any controls A1 3 The Control Properties Table When you open a GUI dialog or a form GUI window in the IDE Designer then the Control Properties Table also opens The set of properties displayed in the Control Properties Table depends on the control type and contains two groups of properties General properties and specific properties General properties are common to all types of controls These are Left Anchor Top Anchor Right Anchor Bottom Anchor Container Name Enabled Lock to Container Name Representation TabStop Title X Y Width Height and Visible Specific properties are individual to different types of GUI controls These are 3State AlignBaseline Alignment AllowPartialRows Auto AutoNotify AutoHScroll AutoVScroll Border Case Class Default ExtendedSel HideSel HScroll Icon Name IgnorePrefix LeftText MultiColumn MultiLine MultiSelect Password ReadOnly Rows Sort StaticScrollbar Style UseTabStops VScroll WantReturn Wrap In the Control Properties table you can edit the displayed properties of the selected GUI control Initially this table is empty However as soon as any of GUI controls in the edited GUI dialog form is selected the Control Properties table displays the current set of properties of the selected control If the Control Properties table is closed double click a control to show the table The Control Attributes command from
52. Unicode mode uses Unicode coding The second attribute the position takes care of where you are in an input or output stream It offers e g the possibility to go back in an input stream to a previous read character More on this in the next sections 11 4 Standard Input and Output the class StdIO In VIP there is the class Standard Input and Output It is called class stdIO without the quotes and we have used it already When a program starts the stdIO consists of an input stream an output stream and an error stream The input stream is standard connected to the keyboard When the strategy of the project is console then the output stream and the error stream are connected to the console window the Win32 console When the project strategy is GUTI then the output and error stream are connected to the Message Window To begin with let us take a look at an example program We create strategy console a project with one class called database The program will read some data from the keyboard assert them to a data base and when we end the program it will write the contents of the database to a file I put the predicates in a module database Here is the code including some clarifying comments class database open core predicates classInfo core classInfo storeNames unsigned Counter gt unsigned Number procedure i fill procedure end class database And in the implementation
53. Way your program ch01p01 is a separate program from the IDE When you close ch01p01 the IDE is still open When you close the IDE the program ch01p01 is still running Congratulations You made your first Visual Prolog 7 program Or maybe better said you arranged things and the IDE of Visual Prolog generated the code for the program so you could execute it This is the way you will learn to use the IDE You specify what you want the code is generated occasionally you change the code or add something to it This is the way to create computer programs with an advanced tool like Visual Prolog and its Integrated Development Environment This way of programming seems excessive and laborious The reason for it is that programming in an object oriented programming language environment using a graphic user interface is quite complex It is very difficult to keep track of every detail That s why the IDE does a lot of bookkeeping for you It makes life easier It may be interesting to inspect at this moment the files on your hard disk with Windows Explorer When you inspect the directory where you put ch01p01 you will see that the IDE created not only the files that you see in the Project Tree but a lot more Especially note the two folders Exe and Bin In Exe you will find the executable files the programs that you can start independent from VIP and in Bin you will find other files that the IDE needs to generate your program
54. Window mouseDownListener clauses onMouseDown Source Point ShiftControlAlt Button The IDE understood that you want something to happen at MouseDown so it inserted code for that event This is comparable to what happened in the previous section when you inserted code for File New You may think of the inserted piece of code as the header of a procedure with the name onMouseDown So far so good but of course the IDE has no idea what exactly must happen at MouseDown so you have to insert that code yourself You should replace the piece of code clauses onMouseDown Source Point ShiftControlAlt Button with the following code 24 clauses onMouseDown Source Point ShiftControlAlt Button Window Source getVPIWindow Point pnt X Y vpi drawText Window X Y Hello World Please take care of every dot comma underscore lower and upper fonts as Prolog is just like any other programming language very sensitive for typos Especially take care to remove the underscores in front of Source and Point When you have inserted the code and saved the file build the program and execute it In the program choose the option File New to create a new form Whenever you click at any point within the form the program will write a famous greeting expression The code in onMouseDown takes care of the event MouseDown That s why we call onMouseDown an event handler Windows programming is mostly a mat
55. X Y father Child Father grandfather GrandChild GrandFather Notice that a variable always starts with an upper case letter When you enter a goal with 59 variables the inference engine will try to find values for the variables so that the goal matches a fact When it succeeds it will report the values for the variables The output will be better readable when you use variable names that make sense e You can use object names and variables in the same goal E g father Sue Father In this case the inference engine tries again to find a value for the variable that will make the goal match with one of the facts e A goal may consist of more than one predicate In that case you have to use the logical operators for and or for or An example is the goal father Sue Jim father B111 John Now please practice a little until you get the feeling for the way the inference engine produces answers 5 3 Extending the family theory Adding facts and rules seem a nice way to increase the intellectual power of your program But take care In prolog you are playing with logic and an inference engine The combination of these two sometimes leads to unexpected results In this section we will look at some common problems that you should avoid It is straight forward to extend the family theory above with predicates like mother and srandMother You should try that yourself You should also ad
56. a goal The Dialog Window is used for the dialog with the inference engine Here you enter the goals for the program that is the questions that you want to be answered Also the messages by PIE appear in this window It is important to take care of which window has control is active To give control to a window click its Title Bar Please enter in the Program Window the father and grandFather clauses that is the facts and rules from the previous section The result should look like figure 5 2 x Data My Documents Visual Prolog Exe 6 1 Insert father Bill John father Pam Bil1 j father Sue Jim grandFather GrandChild GrandFather father GrandChild Father father Father GrandFather Figure 5 2 The Program Window The clauses you entered are now in the Program Window If you want the inference engine to use these clauses and rule then you must load them into the inference engine While the Program Window is active choose Engine Reconsult This will load whatever is in the editor into the inference engine so the engine can work with it In the Dialog window you should receive a message like this Reconsulted from pie Exe FILEO PRO Reconsult loads whatever is in the editor without saving the contents to a file If you want to save the contents use File Save A saved file can be brought back to the Editor window by File Consult This will load the contents of the file regardless of whether
57. a recursive procedure can keep track of partial results or counters as it goes along But there s one thing backtracking can do that recursion can t do namely find all the alternative solutions to a goal But it finds them one by one so you may find yourself in a quandary When you need all the solutions to a goal but you need them all at once as part of a single compound data structure What do you do Fortunately Visual Prolog provides a way out of this impasse You have already met the predicate findall 3 It is a standard predicate with the following call template findall ArgumentName predicateCall ValuesList Findall 3 is a procedure with flow i i 0 It s goal is to assemble a list of all solutions returned by a non deterministic predicate You offer as an input the first two arguments and findall 3 returns in the third argument the wanted list The arguments are ArgumentName Specifies which argument in the specified predicate predicateCall is to be collected into a list ValuesList e predicateCall Indicates the predicate from which values will be collected ValuesList Is an output variable that holds the list of values collected through backtracking As an example think of a facts database with the functor item string ItemName string ItemColor It could be the item list of an inventory Each fact contains the name of an item and its color Now when you want to have a list of all the items then the call 178
58. aa ae RE e ORES A At oan a 86 Chapter 7 Using Forms or Dialogs and Controls a minimal database 2 0 0005 88 7 AcA minimal databases nia Pen vdad tank bh bata ta ed 88 T2 The database 1 VIP os cles di AA EE eth ear ehh es 90 7 3 Manipulating the data add a record 0 cece eee n ene nes 96 7 4 Manipulating the data delete a record 1 0 0 cect tee eens 100 7 5 Manipulate the data change the contents of a record o ooooooooooomomomom oo 107 7 6 Saving and consulting the database 2 0 keene ences 110 LF CONCIUSION NO 112 Chapter 8 Object oriented programming classes and objects 2 0 0 ce eee eee eee 113 8 1 An OO view at the world 2 0 0 eee e nent n en eanes 113 8 2 More Oniclasses 1245 4 o O oh Wh Mad bole A LGR Ra 114 8 3 Classes and objects in Visual Prolog 0 0 0 ccc cect ees 115 8 4 Classes and objects are different nna cee ccc tenet eens 121 8 5 Classes and modulos assura eoa ia Mee ee aoe weiss dae eee 124 8 6 Keeping track of the objects a simple OO database ooooooooommommmm o 126 Chapter 9 Declarations in Visual Prolog 0 02 0 cece cece teen teen eee nes 134 9 1 Declarations and compilation 0 0 ene nent ene nee 134 9 2 Basic notions and overview of keywords 0 0 0 c cece eee nen n nee 135 9 3 Overview section keywords n 0 e ene n teen ene nee 139 9 4 Section domains e ape sey ohare A eae eee 141 O S SECHO
59. additional code We will make a predicate in the module person that adds a new person to the database Let us now create the code for this predicate All the code to manipulate the database will be gathered in the module person So go to the file person pro that is the file where the Prolog code is and enter the following code right after the code for listDatabase addPerson Name Weight Length IQ findAlI ID person ID _ _ _ IDList NewID toString toTerm maximum IDList 1 assertz person NewID Name Weight Length IQ stdIO write Person Name has been added with ID NewID stdIO nl The essential part of the predicate is the middle It says assertz person NewID Name Weight Length IQ This is the code to add the record including the newly computed unique ID to the database The standard predicate assertz instructs the computer to place the record at the end of the facts list When you use asserta instead the record is placed at the beginning of the facts list You can see the difference when you list the database after you entered a new record Of course you have to change the code Here I use assertz it is not important here Also you can use assert This predicate works just like asserta Before the record is added we have to find a new unique ID Here I use two standard predicates to compute the new ID The first one is in the first li
60. aligned left You can use the textControl getAlignment 0 gt predicate to get which of these flags the control has AlignmentType textControlName getAlignment AlignmentType alignLeft and the textControl setAlignment 1 predicate AlignmentType alignLeft textControlName setAlignment AlignmentType to set this flag programmatically Auto Possible values are True and False They define whether this control is an automatic an automatic check box or an automatic radio button When you click an automatic control then it change its state in the proper way You can use the checkButton getStyle 2 and radioButton getAuto 0 gt predicates to get whether this flag is set to a control checkButtonName getStyle IsAuto _Is3State IsAuto true and the checkButton setStyle 2 or radioButton setAuto 1 predicates IsAuto true checkButtonName setStyle IsAuto _Is3State to set this flag programmatically The default value is True auto Center Defines that the text in the control will be centered The AlignmentType alignCenter Right Defines that the text in the control will be aligned right The AlignmentType alignRight AllowPartialRows Possible values are True and False If it is True then the size of the list box is exactly the size specified by the application when it creates the list box Normally Windows re sizes list boxes so that a list box does not display partial items The default value is True
61. and one for nonempty trees traverse empty do nothing traverse tree Node Left Right do something with Node process the node traverse Left process the left subtree traverse Right process the right subtree Figure 12 2 depth first tree traversal 228 This tree traversal algorithm is known as depth first search because it goes as far as possible down each branch before backing up and trying another branch Figure 12 2 To see it in action look at following program that traverses a tree and prints all the elements as it encounters them For this program I created a new project target console and entered the declarations in main cl domains treetype tree string treetype treetype empty predicates traverse treetype Then in main pro I added these clauses implement main open core stdIO clauses traverse empty traverse tree Name Left Right write Name n traverse Left traverse Right clauses run console init FamilyTree tree Cathy tree Michael tree Charles empty empty tree Hazel empty empty tree Melody tree Jim empty empty tree Eleanor empty empty traverse FamilyTree _ stdIO readchar succeed place your own code here end implement main Given the tree in Figures 12 1 and 12 2 program ch12e01 prints Cathy Michael Charles Hazel Melody Jim Eleanor Of course you could easily adapt the prog
62. are connected left and right to other things nodes of treetype Compare figure 12 1 But this is not enough This declaration provides no way to end the recursion and in real life the tree does not go on forever Some nodes the leaves don t have links to further subtrees We have to take care of that in the declaration That s why we define two kinds of nodes in a tree ordinary nodes like the ones defined in treetype above that have a left and right subtree and nodes that do not have a left and right subtree This is done by allowing a treetype to have one of two functors tree with three arguments or empty with no arguments So the declaration of a treetype becomes domains treetype tree string treetype treetype or empty Remember that or is used instead of a semicolon Notice that the names tree a functor taking three arguments and empty a functor taking no arguments are created by the programmer neither of them has any pre defined meaning in Prolog You could equally well have used xxx and yyy but of course that would not really improve the understandability of the declaration This is a good moment to realize that in fact we just defined a tree with two kinds of nodes nodes that have a subtree and nodes that have not We ve better paraphrase that in another way the nodes that don t have a subtree are of treetype but with two subtrees empty So node Charles in figure 12 1 could b
63. as determ not only allows succeed but also fail When declaring predicates you must take care that the modes are compliant with each other E g when you declare a predicate to be procedure and in the clauses of this predicate you call another predicate that is nondeterm then in fact you change the mode of the calling predicate Luckily you can change the behavior of a predicate by using the cut and the predicate fail Let me give an example Take a look at the following clauses child Name father Name Father write Father is Father child Name mother Name Mother write Mother is Mother 151 This predicate tells us that someone with the name Name is a child when it has a father or a mother Assume that we have a database with several facts for the functors father and mother Let s call this predicate with a certain name say child John Now this call can obviously fail when no fact is found with the name John When on the other hand it finds a fact for father John Father then it succeeds and it returns the first solution But it also returns a backtrackpoint because there is another clause in the database it can try So this predicate is nondeterm it can come back with one of fail succeed backtrackpoint Now let s add a cut to the first clause child Name father Name Father write Father is Father child Name mother Name Mother
64. back to the first father call in the original goal here we created a backtrack point to the second father clause The engine sets a backtrack point to the third clause and evaluates the second one Using the second clause it finds that AA is Pam and BB is Bill So we now effectively have the goal parent Bill CC When calling parent we now get mother Bill CC father Bill CC Again the engine creates a backtrack point to the second alternative and pursuits the first mother Bill CC This goal succeeds with CC being Lisa So now there is a solution to the goal AA Pam BB Bill CC Lisa When trying to find additional solutions the engine backtracks to the last backtrack point which was the second alternative in the parent predicate father Bill CC This goal will also succeed with CC being John So now we have found one more solution to the goal AA Pam BB Bill CC John The engine continues to try to find solutions and there are some more It will find AA Jack BB Bill CC John AA Jack BB Bill CC Lisa After that the engine will experience that everything will eventually fail leaving no more backtrack points So all in all there are four solutions to the goal Phew Take a short brake and feel good When you understand the work of the inference engine till here you won t have problems with Prolog as a programming language Of course a prog
65. called rules As an example we can extend the father example with a rule about grandfathers Person3 is the grandfather of Person1 if Person2 is the father of Person and Person 3 is the father of Person 2 where Personl Person2 and Person3 indicate individual persons objects In Horn Clause Logic I can formalize this rule as grandFather Person1 Person3 father Person1 Person2 father Person2 Person3 In this rule we use next to the predicates two so called logical operators the words if and the word and In this formalization the word if is replaced by the symbol and the word and is replaced by a comma These are the symbols that are used in Prolog to represent logical operators And clearly in grandFather Person1 Person3 I want to indicate that Person3 is the grandfather of Person1 A formalization like this is called a rule Take a closer look at grammar of the rule The first part grandFather Person1 Person3 is called the conclusion The conclusion is followed by the if sign After the if sign there are one or more fact predicates separated by commas and ended by a period In a rule these fact predicates are called the premisses of the rule Notice also that grandFather starts with a lower case letter This is Prolog syntax a predicate must start with a lower case letter Arguments like Person1 start with an upper case letter In Prolog this indicates that it i
66. clauses from two different points of view declarative and procedural From a declarative viewpoint the clauses say Name is a member of a list if the head is equal to Name if not Name is a member of the list if it is a member of the tail From a procedural viewpoint the two clauses could be interpreted as saying To find a member of a list find its head otherwise find a member of its tail These two points of view correspond to the goals isMember 2 1 2 3 4 and isMember X 1 2 3 4 In effect the first goal asks Visual Prolog to check whether something is true the second asks Visual Prolog to find all members of the list 1 2 3 4 Don t be confused by this The member predicate is the same in both cases but its behavior may be viewed from different angles Recursion from a Procedural Viewpoint The beauty of Prolog is that often when you construct the clauses for a predicate from one point of view they ll also work from the other point of view To see this duality in this next example you ll 176 construct a predicate to append one list to another You ll define the predicate append with three arguments append Listl List2 List3 This combines Listl and List2 to form List3 Once again you are using recursion this time from a procedural point of view If Listl is empty the result of appending Listl and List2 will be the same as List2 In Prolog append List2 List2 If Listl is not e
67. constantName gt 00 00 146 lt constantType gt 0 020 eee 146 lt constantValue gt 00 0005 146 lt Create gt button o oooo o 45 46 Delta add wee gadis seats aed 50 lt Delete gt button o oo oooooooo 48 lt domainName gt 142 143 REI dan 35 lt ESCaPE Ai a 35 lt factFunctorMode gt o oo ooooooo o 148 lt factFunctorName gt 0 148 Lacie a old 147 268 lt factVariableName gt o ooo 147 lt format fleld gt o ooo oooooo o 205 lt format specifier gt 205 lt format string gt 6 0 0 cece ee eee 205 lt formName gt frm 0 00005 48 lt initialvalue gt ooooooooomoo o 147 lt OK gt button 2 0 0 eee eee 51 lt predicateFlow gt 149 152 lt predicateMode gt 149 150 lt predicateName gt 000005 149 lt rangeDescription gt 143 lt realPrecisionDescription gt 143 lt realRangeDescription gt 144 lt retumArgument gt 0 149 lt sizeDescription gt ooooooooooo o 143 E 206 lt typeExpression gt 142 143 O A 205 A AA 134 216 si bach ie ON EEE EE 81 arta a nt aye bts Boas 136 A ace a eae 4 Hew anaes 42 lisse inser O aoe 139 168 216 ai tease tae ents daa lets bat 179 AboutDialog ooooooooom 42 AboutDialog cl
68. counter open core predicates classInfo core classInfo short Class information predicate detail This predicate represents information predicate of this class end count integer Number procedure end class counter Then in counter pro add the clauses for the predicate count 1 It should look like clauses classInfo className classVersion count 11 stdIO write That s all folks count Number stdIO write Number stdIO nl NextNumber Number 1 count NextNumber end implement count I added a line stdIO nl to make the program put every number on a new line Now we are almost ready to run the program Because we use the strategy console we have to insert the right code into the goal First build the project and then go to the file main pro There you will find the clause clauses run console init succeed place your own code here 161 end implement main Change it into clauses run console init counter count 1 _X stdIO readChar end implement main With the clause counter count 1 we Start the counting The clause _X stdIO readChar Forces the program to wait till you hit lt Enter gt after it has counted till 10 If you remove this statement the MsDOS console will vanish before you can read what is written When you run the program an MsDOS window opens and the program shows you that it can count I put the ru
69. create an object it writes a greeting and it s name to the message window 8 4 Classes and objects are different Classes and objects are different things The division of labor in an OO program dictates that a class creates objects and objects perform activities that are specified in the predicates So clearly there are two groups of predicates predicates that are performed by objects and predicates that are performed by the class The first are called object predicates They are focused on a specific object The getName predicate will only give the name of the object that it belongs to it never gives the name of another object The second group the predicates that are performed by the class are called class predicates They are not focused on a particular object but on the class as a whole Both object and class predicates are public predicates when they are declared in file i or cl The difference is that class predicates concern the class as a whole they are focused on the set of all objects created by that class Class predicates can also use facts that are not object attributes These are called class facts and they are attributes of the class as a whole they are shared among all objects of the class In the syntax of VIP you recognize the difference also To call a class predicate you write something like lt className gt lt predicateName gt With the class you use a double colon To call an object predicate you write 121
70. declaration of the type of objects that the class generates That why we sometimes say that an interface defines a named object type All predicates declared in an interface are object members in objects of the interface type When a class cannot generate objects that is up to you to decide then the class interface file is omitted and the code for the class is spread over two files The class declaration file contains the declarations of the predicates that are also public but that are connected to the class as a whole A class declaration file can contain constant and domain definitions and predicate declarations The other parts of the program can see and use exactly those entities mentioned in the class declaration We say that the declaration of a class specifies the public part of the class Finally the class implementation file contains the definitions of the predicates that is the clauses of the predicates and facts that are declared in the class declaration and interface A class implementation is used to provide the definitions of the predicates and constructors declared in the class declaration as well as the definitions of any predicates supported by its constructed objects In each of these files you can enter other sections besides sections for predicates and facts But take good notice of the scope of them See section 9 10 For predicates and facts things are a bit more subtle as there can be predicates class predicates facts and cl
71. f Z Y In the rule we use variables instead of object names A rule is a general relation that holds between any objects As long as there is an object Y that relates as a father to object X and at the same time an object Z relates to Y as a father then in the world of your program object Z is the grandfather of object X Period Summarizing when reading rules you should interpret the symbol as a logical if and the comma that separates the predicates as a logical and In the variable names GrandChild Father and Grandfather the first letters are upper case In Prolog a variable name always starts with upper case In this rule grandFather GrandChild GrandFather is the conclusion and father GrandChild Father father Father GrandFather are the premisses The rule is ended with a period By choosing variable names that help understanding better than X Y and Z I improved the readability of the rule for humans Imagine that you had to decide from grandfather X Y which one of X and Y is the grandfather It could easily lead to confusion Readability was also improved by writing the rule on two lines One line for the conclusion and one line for the premisses Or when the premisses are long you can write every premiss on a new line By adding this rule I have introduced a predicate for the grandfather relation That is efficient Instead of writing a fact for every grandfather and grandchild in my world I only speci
72. file and in output pro enter this code implement output open core constants className classVersion person output clauses classInfo className classVersion clauses myWrite ThingToWrite stdlO write ThingToWrite end implement output As you can see in the class output I use the standard write statement to implement the predicate myWrite In a serious program you would probably never do this I mean create a class to do what is standard available Also notice that in output cl we declare the predicate myWrite 1 twice This gives the possibility to use the predicate to print strings and to print unsigned integers In OOP this is called polymorphism the same predicate can be used in several ways Now go to the file person pro and change the code for new l to use the new predicate myWrite 1 clauses new Name name Name createdCount createdCount 1 output myWrite Hello world I am a new object My name is output my Write getName output myWrite I am person number Number person getCreatedCount output myWrite Number stdIO nl Now build and execute the program It should perform as before 125 All in all you have created a program that resembles a real object oriented system There is a user interface createPerson frm there are objects that perform tasks they tell their name and number and there is a module that takes care output for th
73. for list box controls and False for list button and list edit controls You can use the listControl getAllowPartialRows 0 gt predicate to get whether the control has this flag IsAllowPartialRows controlName getAllowPartialRows 256 IsAllowPartialRows true and the listControl setAllowPartialRows 1 predicate IsAllowPartialRows true controlName setAllowPartialRows IsAllowPartialRows to set this flag programmatically AutoHScroll Possible values are True and False They define whether the control automatically scrolls text in the control in order to display a typed character when the user types the character at the end of the line If it is False then only text that fits within the rectangular boundary is allowed When the user presses the Enter key the control scrolls all text back to the zero position The default value is True scrolls text automatically You can use the editControl getAutoHScroll 0 gt predicate to get whether the control has this flag IsAutoHScrollSet controlName getAutoHScroll IsAutoHScrollSet true and the editControl setAutoHScroll 1 predicate IsAutoHScrollSet true controlName setAutoHScroll IsAutoHScrollSet to set this flag programmatically AutoNotify Possible values are True and False If it is True then the list box control notifies the parent window with an input message whenever the user clicks or double clicks in the list box The default value is True notify
74. for the new form Layout Controls and Properties To understand what is happening here you should know something about the basics behind a Windows program A Windows program consists mainly of windows this will not surprise you in which the user can do something E g in a form you can fill in your name in a field you can click on a radio button you can choose from a dropdown menu you can click an lt OK gt button These things with which the user works are called controls A window like the form we are creating can be filled with many controls The available controls are 15 shown in the window with title Controls You are free to place a control anywhere on a form To do that you need a prototype of the new form That prototype is the window with title form1 You use this window to design and edit the new form I call it the edit window In creating the form you can use the options in the window Layout to change the layout e g to justify a group of buttons Every control in a form has a series of properties E g a button has a place on the form and certain dimensions The properties of a control are shown in the window properties Try it Click on the lt OK gt button in the edit window and watch the content of the properties window change Click anywhere on the form in the edit window but not on a button and you see the general properties of the new form Change the size of the
75. for the triggers SelectionChanged for the listbox_ctl with names and the idbox_ctl with the ID s Please insert the standard code for both refer to the previous section if you forgot how to do that and then change the standard code into predicates onListboxSelectionChanged listControl selectionChangedListener clauses onListboxSelectionChanged Source Name _T Source getSelectedItems person getSomelDs Name IDList idBox_ctl clearAll 1d_ctl setText name_ctl setText weight ctl setText length ctl setText 108 iq_ctl setText idBox_ctl addList IDList onListBoxSelectionChanged _Source predicates onIdBoxSelectionChanged listControl selectionChangedListener clauses onIdboxSelectionChanged Source ID _T Source getSelectedltems person getPerson ID Name Length Weight IQ id_ctl setText ID name_ctl setText Name length_ctl setText toString Length weight ctl setText toString Weight iq_ctl setText toString IQ onIdboxSelectionChanged _Source Then insert the standard code for the lt OK gt button and change in into predicates onOkClick button clickResponder clauses onOkClick _Source button defaultAction ID id ctl getText Name name_ctl gettext Weight toTerm weight_ctl gettext Length toTerm length_ctl gettext IQ toTerm iq_ctl gettext person changePerson ID Name Weight Length IQ The only new
76. form are private to the package implementation and the predicates in them are private You should realize that in the above the name main is more or less accidentally chosen The essential difference is shown in the extensions PH for Package Header and PACK for the package After you fill in all required settings you should press the Create button then the Form editor appears with a quite standard prototype of a form See figure A1 2 ZI ML MC xi Ege T Aegae EH TH a es f T 6 lo e ge itt te Properties 15 xj Foun myForm Y Width 240 Height 120 Foni MS Sene Seal Menu tune Asiigned Toobar tureeb Delauk B utor fureeb Tieda True Clase ox True Mambo True Minime True Enabled Trus Virb True CipSbinge True CipChikien Tra HoieSeroBa Fake YertScrdBar Fake Mininized Fake Bordar Siz Border Figure A1 2 the Forms Editor 240 A1 2 Edit a Dialog To edit a Dialog in the Project Tree in the Project window double click the name of a file with a dialog filename extension dlg To edit a form you have to choose the file with the extension frm An alternative is to highlight the name of the file and then hit Enter The IDE Form Editor also called the Designer appears and you can edit whatever form of a dialog you wish When a new dialog is created it will by default have three push button controls OK Cancel and Help These can freely be rearranged or deleted But take care These b
77. format field E g RealVar 12345 6789 206 stdIO writef 10 4 end of field is here RealVar produces 12345 6789 end of field is here But RealVar 12345 6789 stdIO writef 10 4end of field is here RealVar produces 12346 e 004nd of field is here because the e of end is interpreted to modify the format to exponential notation Note that you can also add escape sequences in the format string like An to go to a new line The escape sequences that you can add here are representing t representing Tab character n representing New Line character r representing carriage return representing double quote UXXXxX here xxxx should be exactly four hexadecimalDigit s representing the Unicode character corresponding to the digits So if you want to insert a backslash in the output put W in the lt format string gt When you put Mt in the lt format string gt the printer will jump to the next tab position Et cetera I suggest that you fool around some time with different formats to get a good feeling for them 11 7 General input and output the class Stream The standard IO class allows you to read from and write to a stream but only one at the time the current input or output stream The class Stream is a more general class I m not sure if what Iam going to say now is correct But you must think of the class Stream a a class that creates an object for every stream that
78. including other lists However all elements in a list must belong to the same domain When the elements in the list are not the standard types integer real string then in addition to the declaration of the list domain there must be a domains declaration for the elements domains element_list elements elements string or domains integerlist integer Here elements must be equated to a single domain type for example integer real or string or to a set of alternatives marked with different functors Visual Prolog does not allow you to mix standard types in a list For example the following declarations would not properly indicate a list made up of integers reals and strings element_list elements elements integer real string Tncorrect The way to declare a list made up of integers reals and strings is to define a single domain comprising all three types with functors to show which type a particular element belongs to For example element_list elements elements i integer r real s string the functors are i r and s For more information about this refer to Compound Lists later in this chapter Heads and Tails A list is really a recursive compound object It consists of two parts the head of a list which is the first element and the tail which is a list comprising all the subsequent elements The tail of a list is always a list the head of a list is always an element from that li
79. is called the root of the tree the bottom nodes are called the leaves of the tree In figure 12 1 every node has exactly two children but that is not a condition for a tree On the other hand in a tree a node has one and only one parent except again the root node When you take a look at the tree in figure 12 1 the recursive character of a tree will be clear Cathy is the root node of the complete tree but Michael is the root node of a subtree consisting of nodes Michael Charles and Hazel This is true for any tree a node with all the nodes below it together form a tree In this section I will restrict myself to a special kind of tree that is widely used the binary tree A binary tree is a tree in which every node has at most two children In a binary tree the children are called the left and the right child Let s define some more concepts The depth of a node n is the length of the path from the root to the node So in figure 12 1 node Charles has depth 2 The set of all nodes at a given depth is sometimes called a level of the tree Nodes Michael and Melody are at the same level e The root node is at depth zero The height of a tree is the depth of its furthest leaf The height of the tree in figure 12 1 is 2 A tree with only a root node has a height of zero e Siblings are nodes that share the same parent node Nodes Michael and Melody are siblings If a path exists from node p to node q where node p is closer t
80. is not needed There are no objects so there are no object predicates publicly available so there is no need for an interface file As there is no interface file this kind of class is not typed You can see that in the cl file The first line doesn t contain a colon and a type Also these classes don t have a constructor And finally as there are no objects these classes don t have object predicates or object facts They only can have class predicates and class facts Let s extend the program with a class that contains code for output Create the class output We are going to use this class only for generating output it will not generate objects So please uncheck the creates objects check box Be sure to put the class in the same pack as person Then lt Create gt the class The IDE now only presents two files output cl and output pro The file output i is not needed for you declared that this class will not generate objects so there is no need to declare objects predicates so there is no need for output i In the file output cl enter this code the IDE already has placed the standard code class output open core predicates classInfo core classInfo Ashort Class information predicate Adetail This predicate represents information predicate of this class vend 124 myWrite string ToWrite myWrite unsigned ToWrite end class output Save and close this
81. it ch08class go to the Project Tree and highlight the root directory ch08class Then click File New and create a new Class Highlight Class in the left pane and enter in the right pane the name of the class person and choose New Package and check Create Objects that is what we are going to do When it looks like figure 8 1 hit lt Create gt The IDE creates three files and presents them to you in three different editors The files are named person 1 person cl and person pro In these three files the class person is specified Everything you want to know about class person is in these files You already are familiar with files like person pro In this file we put the Prolog code for the predicates In the Prolog helpfile this is called the definition of the predicates In the file person cl we give the declarations of the class More on this in the rest of this chapter 115 Lreate Project Item x IE Package Name person COM Package New Package Irterface Parent Directory Sa es Browee gt Dialog C Existing Package main pack Form icy Publi c Z Control Pires Z Draw Cortral Toolbar MV Creates Dbjects New Intelface Existing Interface sboutDislog Cancel Help Figure 8 1 create class person The file that you don t know of is the file person i It is the so called interface to the class person The interface is like a counter
82. item New sub item inserts a new menu option in a sub menu below the selected item New separator inserts a dotted line below the selected item Shift Up shifts the highlighted item one place up Shift Down shifts the highlighted item one place down Test shows the menu in the Task Window To insert a new menu option highlight an existing option and click on one of the icons Which icon depends on what you want to do When you want to insert a new item click the lt New Item gt icon A small edit field is opened below the highlighted option and you can type the name of the new item In the name of a new menu option you can use two special symbols The symbol amp in the name e g in amp File makes the next character F a shortcut key for the option You can place amp anywhere in the name the next character becomes the shortcut Take care that you don t use a shortcut that already exists When you add tF5 to the name the function key F5 becomes a shortcut 28 remen E DOBB s T Constant Prefix lid_messages_writenote Item Constant jic_messages_writenote1 Y Help Constant id_help Y Accelerator TT Checked ecke Py Fo Shit Foch Tar I Disabled File Ey tEdit gt Messages E WriteNotel amp Help m M Figure 3 3 Task Menu with options added For now we want to add a few menu options to generate messages It seems appropriate to put the
83. look at the first Element but don t change stacklist The procedure pop takes the first element away from the stack list pop Element Element Tail stacklist Get the first Element and stacklist Tail take it off the stacklist There are other procedures possible to work with the stack I added the following declaration to stackprocs cl dup determ swap determ showstack The procedure Dup for duplicate peeks the top item and pushes it again onto the stack so that an additional copy of the former top item is now on top with the original below it In Vip it could be done with dup Element _ stacklist Get the first Element and stacklist Element stacklist put it in front of the stacklist Swap or exchange the two topmost items on the stack exchange places In Prolog swap Element1 Element2 Tail stacklist Take the first two elements and stacklist Element2 Elementl Tail change their places 218 For your convenience I added also a procedure to show the stack contents Normally this is not done as you are only interested in the top element If not you wouldn t use a stack showstack stdIO write The stack contains stdIO nl stdIO write stacklist As you can see I use the fact that StdIO can write lists Please note also that the predicates shown here are not complete In any case you should ch
84. meant for human readers of the code the compiler skips it The sign indicates that the rest of a line is comment The first line in person cl says class person person In fact this is the declaration of the class person It declares that there is a class with the name person that s what we wanted and that the type of this class is person What does that mean Well think back of the what is in the interface person 1 In that interface you stated how to access objects of the class person In VIP this is taken as a description of the type of the class Being of type person tells the world that you can access objects generated by classes of this type via the interface person In other words in our program objects from the class person will have two callable procedures getName and setName Next to these two procedures there must be a constructor because when you created the class you checked the box creates objects Objects are created by constructors To declare the constructor change the code in person cl into 117 class person person open core predicates classInfo core classInfo Ashort Class information predicate Adetail This predicate represents information predicate of this class vend constructors new string Name end class person In this class the predicate new 1 is the constructor When you call this predicate it will create a new instance of person
85. onMessagesAsk3 window menultemListener clauses onMessagesAsk3 _ Source MenuTag ButtonPressed ask A convenient Title This is the question Button0 Button1 note You pressed button number tostring ButtonPressed You should know a little bit more about listening to the user as with ask 2 and ask 3 A user is supposed to click one of the available buttons But sometimes she doesn t She may hit the lt Enter gt key she may click the lt close gt button x in the top right corner or she may hit the lt Escape gt key With ask 2 and ask 3 hitting lt Enter gt is equivalent to clicking the first button because the first button is the default with ask 2 and ask 3 When the user clicks the lt close gt button this is equivalent to using lt Enter gt When the user hits the lt Escape gt key Prolog accepts this as if she clicked the last button I don t bother about it here but when you write a program your response to the user input should take this into account Ask 2 and ask 3 are very useful when the question for the user has only a few multiple choice answers Things become different when this is not the case e g when you want to ask for a person s name Then you ask for a string but you don t know what it will look like For input of a string there is the procedure getString 3 Calling this procedure looks like ask 2 and ask 3 This is what the code looks like In my program the code is conne
86. predicate in scopes in which the predicate declaration can be seen When predicates are declared in an interface file this means that the predicate is an object predicate and publicly available and objects of the corresponding class must support these predicates When predicates are declared in a class declaration this means that the class publicly provides the declared predicates And if predicates are declared in a class implementation this means that the predicates are only available locally private In the latter case a predicate can be an object member or a class member When it is an object member then it must be declared in a section that starts with the keyword predicates When it is a class predicate then it must be declared in a section that starts with the keyword class predicates In all cases a corresponding definition of the predicates must exist That is to say that there must be clauses for the declared predicate in the implementation file Table 9 2 gives an overview Predicate declaration is public predicate private predicate for an object member in class interface in implementation predicates predicates for a class member in class declaration in implementation predicates class predicate Table 9 3 declarations of predicates The keyword class can be used only inside a class implementations Outside the implementation it has no use as e g predicates declared in an i
87. project I have chosen that my project should have the name ch04p01 e that the project name is also used as the name of the target that is produced e that in this case the target is an exe file so the target name will be ch04p01 exe that the target should be a GUI program 1 e a program with a graphical user interface The Base Directory is the base of all your projects you should choose a place that is convenient for you This chapter contains many parts from the tutorial Environment Overview by Thomas Linder Puls 39 The new project will be created in a Sub Directory of the base directory By default this directory has the same name as the project itself As usual for the moment you do not need to consider the remaining tabs and options Building Before we make any modifications we will build i e compile and link the project In the Build menu you find commands for building compiling and executing the project If you choose Execute the project is first build so that it is an up to date version that you execute Therefore I will choose Execute If you have not registered Visual Prolog you will be presented to a special screen telling you so I will suggest that you register but you can also choose Continue Evaluation In the Messages Window the IDE writes which files are compiled etc The process is monitored in the status bar at the bottom If the build process succeeds which it should in this case the created pro
88. properties liSt o oo oooooo 47 prototype code 0 0 cee eee 48 p blica oa coms 115 public class predicate 129 public predicates 0 000005 121 Punctuation marks o 137 PUSE oa ds podbean talented e a Sec 217 222 Push Buttons vees eenen cedar ic 47 PushButton cON ooo 47 pushButton Ctl 47 QUESO ti oe heals 56 GQUCUC a dle Wee ath oa 215 221 quicksorb s aae E ries 187 A 212 al Doce wl ga deed Be dial deeds 198 203 readability ooooooooomomo o 55 readBytes 25 5 id 212 readBytes 1 lt fasts di wae 198 read Chars mannaaa aa abi 212 readChar 0 o oooooooooom ooo 199 A ot wha Viet cates ees 73 readLine ooooooooooomoomoooo 213 A eaae Ghk wee ace Fe HS 199 TEAC SUTIN G3 i aa Ses Aa 213 ll AA 199 a td o E 76 reali A A s aa a E 138 feal terale na e a AE EN 138 TECOS Ult air ii A 213 A 2 deen aches abs a aes aaa a ed 88 record TYPC 23 este to 83 recursion 72 160 183 216 YECUPSION 0 eee ee ee 73 85 163 recursive definition 145 216 recursive definition 0 85 recursive predicate o o oo o o 160 recursive types 2 6 eee eee ee eee 226 red G te ica Ane eed SOR aR 69 A heats retained ete Reena 53 relations hacia Beals oe 77 relentless search for solutions 64 removeDuplicates o o o o 103 reserved
89. push an element simply assertz it Take care to use assertz and not assert push Name Amount assertz q Name Amount To peek the first element without removing it simply get it Due to the fact that the inference engine takes by definition the first item in the database it goes like this peek Name Amount q Name Amount peek 0 0 The second clause is to catch the case that the queue is empty In this case the values that are returned have a special meaning I take care of this in the menu To pop the first element simply retract it pop Name Amount retract q Name Amount pop 0 0 Variables Name and Amount will contain the values of the first fact in the database If the queue is empty the returned value for Name will be 0 zero Finally there is a predicate to show every item in the queue When the queue is empty this predicate shows nothing listQO q Name Amount Look if there is something in the queue stdlO write Name Amount Print it and continue with the rest stdlO nl foreach q Name Amount do stdIO write Name Amount stdIO nl end foreach listQQ When the queue is empty say so stdIO write nothing stdIO nl To work with these predicates I created a new menu option in the taskmenu of the taskwindow with the name Q I added four options Push Peek Pop and ListQ Here is the code that I ent
90. put a form with the name Query in it As you have done this before in section 2 1 the steps to do it should be familiar For completeness sake I will give them again Take the following steps In the Project Tree click on the newly created package pack01 not on pack01 pack In the Task Menu click on File New in existing package The Create Project Item dialog opens In the left pane click on Form In the field Name type query At this point you must choose between the two radio buttons New Package and Existing Package When you check New Package the files for your form will be placed in a subdirectory in the directory pack01 When you check Existing Package the files will be placed in the directory pack01 itself I have chosen Existing Package Also you have to choose wether the form will be Public or Private If these words don t mean anything to you choose Public Click the lt Create gt button The IDE shows a new form in the forms editor The IDE shows the forms editor see figure 4 5 Don t close it for the moment We will take the opportunity to add an Edit Field and a button Edit Fields allow the user of the program to enter text a number or another input value or to change a value A click on a button is an event causing the program to execute some activity A es Sry w E e T me gadag SFR H a B E ed a idk 3 de F rr Fom quay az que 50 T 40 Widih 240 Canos Help
91. relation you can ask Which output corresponds to this given input or Which input corresponds to this given output 177 The status of the arguments to a given predicate when you call that predicate is referred to as a flow pattern An argument that is bound or instantiated at the time of the call is an input argument signified by 1 a free argument is an output argument signified by 0 The append predicate has the ability to handle any flow pattern you provide However not all predicates have the capability of being called with different flow patterns When a Prolog clause is able to handle multiple flow patterns 1t is known as an invertible clause 10 4 Special list predicates In the previous sections we created and specified every predicate that we needed That is not necessary As said before there is a class called list in which you find a lot of handy predicates to manipulate lists Take a look in the help file find list and look in class list In this section I should like to introduce to you three predicates that are often used with lists but that are standard predicates in Prolog You don t have to use the class list or any other class as you can call these standard predicates from anywhere in a program Backtracking and recursion are two ways to perform repetitive processes Recursion unlike backtracking can pass information through arguments from one recursive call to the next Because of this
92. s like looking in a telephone guide for a number only one entry will do as long as there is one telephone number per person Here Visual Prolog calls r with a single integer argument Assume that the call is r 1 Visual Prolog searches the program looking for a match to the call it finds one with the first clause defining r Since there is more than one possible solution to the call Visual Prolog places a backtracking point at the clause for r 2 The clause for r 1 fires and Visual Prolog begins to process the body of the rule The first thing that happens is that it passes the cut doing so eliminates the possibility of backtracking to another r clause This eliminates backtracking points and so increases the run time efficiency It also ensures that the error trapping 70 clause is executed only if none of the other conditions match the call to r Note that this type of structure is much like a case structure written in other programming languages Also notice that the test condition is coded into the head of the rules You could just as easily write the clauses like this r X X 1 a b c r X X 2 d r X X 3 c r _ write This is a catchall clause However you should place the testing condition in the head of the rule as much as possible as doing this adds efficiency to the program and makes for easier reading As another example consider the following program Run this program and give it the goal friend
93. same ID to two different persons To avoid that I shall make the program compute a unique ID Back to the Dialog When everything is like in figure 7 5 you save and close the dialog editor Build the project yes again And when the IDE asks you if you want to include something say yes Now you have a form but two things are lacking First we need a menu item to open the form and then we need code so that when you click lt OK gt on the form a new record is added First the menu item that is rather simple and should be familiar to you In TaskMenu mnu enter a new sub option under the option Database I called this option Add Person It is not too critical what you enter here but take care The text of this option is used to generate code so when you enter another text here your code will deviate slightly from mine When the TaskMenu is changed go to Task Window win and open the Code Expert Now choose Menu choose TaskMenu choose id database and enter the standard code for id Database Add Person Then go to the code and change it into predicates onDatabaseAddPerson window menultemListener clauses onDatabaseAddPerson Source MenuTag 97 NewDialog addPerson new Source NewDialog show When you now click on Database AddPerson the Dialog will open and you can enter the data for the new person Mind you that this does not mean that the record is actually entered in the database For that we need
94. seem like a magic formula Things will become more clear in the next chapter For now it suffices to know that in an object oriented environment a computer program consists of so called classes You may think of a class as a module but keep in 90 mind that this is only a partial description A class is much more But for now a class is a fine place to put the database in We will create a class and to keep the structure of the project clear this class will be put in the package person So highlight the symbol for package person and click File NewInExistingPackage In the Create Project Item Dialog that opens choose Class in the leftmost pane and enter the name of the class person It may seem strange to give the same name to package and class but it cannot do any harm class and package are totally different items Choose further the radio button for Existing Package and make sure the line behind the option says person pack Finally uncheck Creates Objects and click lt Create gt See figure 7 2 Create Project Item x Name person C New Package Browse Parent Directory person Interface 3 Dialog Existing Package person pack persons Form e Public uz Control C Private as Draw Control HE Toolbar I Creates Objects ES Menu New Interface FA Bitmap Existing Interface aboutDialog 7 A Icon EA Cursor Text File Cancel Help Figure 7 2 Create c
95. source txt destination txt _ stdlO readchar succeed place your own code here end implement main The only thing we do here is to call the predicate copy source txt destination txt in the class copystream We want to copy the contents of a file with the name source txt to destination txt Be sure that the file source txt is present in the exe folder of your project 208 The code in class copystream looks as follows I comment on the code below it The declaration is class copystream open core predicates copy string InputFileName string OutputFileName procedure 1 1 end class copystream And here is the implementation implement copystream open core constants className copystream classVersion clauses classInfo className classVersion clauses copy InFile OutFile InputFile inputStream_file openFile8 InFile OutputFile outputStream_file create OutFile copyStream InputFile OutputFile InputFile close OutputFile close class predicates copyStream inputStream InputFile outputStream OutputFile procedure 1 1 clauses copyStream InputFile OutputFile repeat String InputFile readLine stdlO write String OutputFile write String n InputFile endOfStream copyStream _InputFile OutputFile class predicates repeat multi clauses repeat 209 repeat repeat end impleme
96. string Name The constructor new has one argument the name of the new object We shall ask the user to provide the name Thereto I created a form with the name createPerson frm It looks like in figure 8 3 It is quite simple It contains some text an edit field name _ ctl and two buttons The button with the word create on it is called createButton_ctl It is newly created and it is clicked to create an object The other button with the word Close is called close_ctl It is the standard button lt OK gt that the IDE provides when a form is newly created By using this former lt OK gt button I produce the effect that when this button is clicked the form closes In this way I don t need to give code for this button I discarded the other buttons lt Cancel gt and lt Help gt as I don t use them Jal Name Create Figure 8 3 form to create a person So there remains only to design some code for the event that the user clicks the lt Create gt button In the form editor highlight the lt Create gt button and in the properties window choose the Events tab Go to the option ClickResponder and choose the value onCreateButtonClick Double click the opton and in createperson pro add the code predicates onCreateButtonClick button clickResponder clauses onCreateButtonClick Source button defaultAction Name name _ctl getText _NewPerson person new Name When the use
97. that any mistakes are purely mine Chapter 1 The Integrated Development Environment In this chapter I introduce the Integrated Development Environment of Visual Prolog Programming nowadays is not only done by typing code in an editor Programming has become very complex that s why the developers of programming languages offer more than a programming language They provide you with programming aids The Integrated Development Environment IDE for short is such an aid When you create a Visual Prolog computer program you will use the IDE The purpose of this chapter is to make you feel at home in the Integrated Development Environment IDE of Visual Prolog VIP 1 1 The Integrated Development Environment IDE When you start Visual Prolog what is really started is the Integrated Development Environment the IDE It looks more or less like figure 1 1 Task Window Task Menu Visual Prolog 7 1 Commercial Edition Licensed to Thomas W de Boer Fie Edt View Insert Project Buld Debug Goto Tools Web Window Help DSAloc O B E TOsSa eSO gt 1 x x i D o a Task Menu Bar amp Visual Prolog 7 1 Commercial Edition Visual Prolog Recent Projects dyslexie pri6 data My Documents Visual Prolog 7 projects dyslexie treeCh1 202 pri data My DocumentsWVisual Prolog 7 projectsWreeCh1202 Biuees pi6 data My Documents Visual Prolog 7 projects nees tree2_pri6 data My Documents Visual Prolog 7 projects tee2 ch8class pri
98. the predicate expects also an end of line but maybe I am mistaken stdio readLine 0 gt readLine gt string String procedure Description The predicate reads a line from the input stream Indicator of the end of the line is a symbol n new line or zero char string termination symbol or end of stream stdio readString 1 gt readString charCount NumberOfChars gt string String procedure i Description The predicate tries to read NumberOfChars characters from the input stream and stores them into the String If the predicate meets end of stream when reading it returns the string with the number of characters actually read stdio setInputStream 1 setInputStream inputStream Stream procedure i Description This predicate sets Stream as current input stream As an input you need a variable of type Stream You can get such a variable e g when you create a new file stdio ungetChar 0 ungetChar procedure Description The predicates returns last read character back to the current input stream Now it is time to look at the predicates for writing to the output stream Here they are stdio flush 0 flush procedure 199 Description This predicate forces the contents of the internal buffer of the current output stream to be written to the resource e g the file that it is connected to The flush is useful when output is directed to a serial port and it becomes necessary to send t
99. the dialog setDefaultButton 1 predicate to set the default button in the dialog 258 Only one button in each dialog can be the default button but any number of push buttons can have the True value of this property IDE makes the default button that of dialog buttons with the True value of this property which value was set the latest one By default this property is True is the default button only for the OK button for all other buttons the default value is False ExtendedSel Possible values are True and False If it is True then the list box control allows selection of multiple items by using the Shift key and the mouse or using the Shift key and the Up or Down Arrow keys The default value is False cannot select multiple items You can use the listBox getExtendedSel 0 gt predicate to get whether the control has this flag IsExtendedSel controlName getExtendedSel IsExtendedSel true and the listBox setExtendedSel 1 predicate IsExtendedSel true controlName setExtendedSel IsExtendedSel to set this flag programmatically HideSel Possible values are True and False If it is True then the edit control hides selection when it loses the input focus and shows selection when it receives the input focus The default value is True hides selection You can use the editControl getHideSel 0 gt predicate to get whether the control has this flag IsHideSel controlName getHideSel IsHideSel true and the edit
100. the file is opened for editing or not To make the inference engine search for a solution you must give it a goal it can pursue This is done in the Dialog Window The procedure is always as follows e Create or load a file in the Program Window with File New or File Consult Save that file when necessary with File Save e Reconsult it by the engine with Engine Reconsult Enter a goal in the Dialog window and hit lt Enter gt 58 Once you have consulted the theory you can use it to answer goals On a blank line in the Dialog window type a goal without the in front Let s ask the question who is grandfather of who In Prolog this becomes a goalwith two variables We want the inference engine to find values names for the variables So in the Dialog Window type on a blank line grandfather X Y 6699 Take care to write a lower case g in grandFather and to i G x write upper case X and Y and don t forget the full stop Df x grandF athert Y at the end It should look like figure 5 3 Now place the cursor at the end of the line and press the Enter key on your keyboard PIE will now consider the text mz yl from the beginning of the line to the cursor as a goal to execute The inference engine does its work and you should see a result as in figure 5 4 If you get instead a message like 101 x grandFather X Y grandFather s v x Pam Y John Unknown clause found grandfather X 0 Y 1
101. the theory and it finds the rule grandFather GrandChild GrandFather father GrandChild Father father Father GrandFather Because the conclusion of this rule matches the first sub goal we use the rule to solve it Thereto we substitute the premisses of the rule in the first sub goal then the resulting current goal will be father X Father father Father Y mother Y Z This may seem stupid as we now have three sub goals instead of two But the idea behind it is that premisses are less complex compound than conclusions and therefore easier to check for truth But don t bother about that too much now Notice also that some variables in the substituted premisses have been replaced by variables from the sub goal as was explained in the previous section With the new formulation of the goal we start looking for facts and rules that will solve the goal When a sub goal is checked against a fact and found to be true it can be removed from the goal 63 When all the sub goals are found to be true the goal is true remember the sub goals are separated by comma s that are logical and s and Prolog will show a message Given this evaluation strategy you can interpret clauses much more procedural Consider this rule grandFather Person GrandFather father Person Father father Father GrandFather Given the strict evaluation we can read this rule like this To solve grandFather Person GrandFather first solve father Pers
102. to Head then it should be placed in front of the Head In Prolog insert Element Head Tail Element Head Tail Element lt Head Here I use the template X Y Tail to indicate the position of the first two elements in list When Element is larger than Head then Head should be skipped and the next element in the Tail should be considered In Prolog insert Element Head Tail Head TailWithElement 29 insert Element Tail TailWithElement This completes the program To run it you need to adjust the run 0 predicate in the main program It should become run console init UnsortedList 4 2 5 3 1 2 6 4 stdIO write The unsorted list is UnsortedList stdlO nl sort insertsort UnsortedList InSortedList stdlO write The sorted list insert sort is InsortedList stdlO nl _X stdIO readchar succeed A second way to sort a list is called selection sorting The algorithm is 1 Take an unsorted list 2 Find the smallest element and take it out 3 Sort the rest of the list 4 Put the element taken_out in front of the sorted rest of the list Let s make a predicate for it Just like we did with insert sort we create a predicate that accepts an unsorted list and produces a sorted list I declared the predicate selectSort selectsort integer integer procedure 1 0 As selection sort is clearly a recursive way to sort I start with a clause to describ
103. to formulate a theory in several different ways It may seem confusing and to some extent it is but it resembles the fact that any theory can be formulated in different ways as can relations between individuals It is up to you to choose the best way But maybe we should warn you As always in science and life there is seldom only one best way Here we specified the data of a certain person as a list of arguments However there is another elegant method to sharpen our focus on the entities being represented We can pack both the Name and the Gender into a package using a formalization known as a compound domain Think of it as a set of values that characterizes a person That has some advantages as Prolog offers you the 77 possibility to represent the entire package using one variable but you can also reach each argument apart from the other As an example in Prolog you can make the statement P person Bill male Then the variable P is bound to represents the whole person Bill But also this statement is possible person Name Gender person Bill male In this case the variable Name is bound to the value Bill and the variable Gender is bound to male The advantages of this will become clearer in the next section 6 3 Compound domains and functors The above facts about persons can be generically represented in a declaration for a compound domain thus person person string Name strin
104. to the event MouseDown So we need to find the event and connect it with the appropriate program code If necessary click somewhere in the form but not on the buttons to bring up the properties of the form In the properties window there are two tabs at the bottom One says Properties the other says Events Click on Events and you see a list of possible events Look for the event MouseDown it is in the middle of the list Click in the field to the right of the text MouseDown The field changes into a listbox Click on the arrow to reveal the options There is only one named onMouseDown Choose this option Then double click on onMouseDown and the IDE opens the code editor In the code there are lines that react to the event MouseDown The cursor is placed next to the code for MouseDown It looks like predicates AE Form formt v ValidateResponder a DestroyListener GetF ocusListener LoseFocusListener MenultemListener MoveL istener Show Listener SizeListener StateListener TimerListener UserListener Context enuResponder CloseR esponder HSerollListener MouseDblListener MouseDownListener MouseMoveListener Mouse pListener WScrollListener CharResponder EraseB ackgroundResponder KeyDownR esponder KeyUpResponder PaintResponder ActivateListener DeactiwateListener Init enuListener onMiouseDown Properties Events Figure 2 13 Events for form1 onMouseDown draw
105. use both kinds of separators in the same list provided the vertical bar is the last separator So if you really want to you can write a b c d as a b c d Here you make a head that consists of two elements but we don t call that a head Table 1 gives more examples 168 Pa b c 1 2 3 2 3 4 D L 2 3 2 3 41 1 Table 1 Heads and Tails of Lists Table 2 gives several examples of list unification Variable Binding for the clause Listl List2 X egbert Y eats Z icecream 7 1 2 3 4 X 1 Y 2 Z 3 4 Table 2 Unification of Lists 10 3 Lists and recursion Because a list is really a recursive compound data structure you need recursive algorithms to process it The most basic way to process a list is to work through it doing something to each element until you reach the end An algorithm of this kind usually needs two clauses One of them takes the head of a list and tells what to do with it This statement is repeated till you reach an empty list The other statements tells what to do with that empty list I hope that you get the feeling that recursion can come in handy here In this and the next section some things that were said in section 10 1 will be repeated If you find that boring skip it In this and the next section a number of small programs are presented I take it for granted that by now you know how to create this program in the IDE To remind you these are th
106. when there are no facts and the new record gets ID number 1 But if you think that the program gets too complicated for your cognitive capabilities at this moment leave it and live with it 7 4 Manipulating the data delete a record Now we will turn to the next database activity delete a record How to change a record will be treated in the next section To delete a record you first have to find it In a person database it is convenient to search for persons by name but there may be a problem when there are persons with the same name 100 in the database So it is wiser to search for a record by ID as this is a unique identifier But most people do not know their IDnumber by heart I shall use a compromise when we want to find a record we first search by name The program then will display the ID s that go with this name You can then click the ID and that determines the record to be deleted So let s start with creating a Dialog that will do the job I want it to look to the user like the form in figure 7 6 deletePerson 2 x Choose a Person Choose an lD To Be Deleted Ea Name Thomas onn ne ID lb OK Cancel Help Figure 7 6 Dialog to delete a record In the box below Choose a Person the list of available names should appear When the user selects a name then in the box next to it under Choose an ID the list of available IDs should appear If the name is unique then there appears one ID wh
107. where you can get information In this case the interface is what the outside world sees of the objects that are generated The public class predicates are in person cl Now you can imagine what that means in the files person cl and person i are the declarations of public methods In Prolog methods are performed by predicates Predicates that are declared here are callable by other objects and classes The IDE has already put some code in the file person i Here it is interface person open core end interface person As you can see there are not yet any public predicates in the class person Next to methods the objects of class person will have attributes Here I shall confine the attributes to a single one the name of the object As said in the previous section when an object has an attribute it can and often will have a get and set method for it that are callable publicly So in the class person we want two predicates getName and setName When you call the first one the object sends its name to you with the second one you can enter or change the name of the object So now change the contents of person i into interface person open core 116 predicates getName gt string Name setName string Name end interface person You now have declared that every object that is generated by the class person will know two predicates that operate as procedures Procedure getName needs no argument
108. will occur setCRLFconversion boolean Enabled procedure 1 Defines whether the stream data will be handled with CRLF lt gt LF conversion before writing to stream LF gt CRLF or after reading from stream CRLF gt L PF setMode mode Mode procedure 1 Sets Mode for read write operations binary ANSI or Unicode setPosition unsigned64 Position procedure i Sets new stream position at which the next read or write operation will occur Class Inputstream Here are some predicates from the class inputstream consult factDB FactDB procedure i Reads facts from a file and asserts them to FactDB endOfStream determ Checks if the stream pointer points at the end of stream read gt _ Value procedure Reads a specified term from the input stream readBytes byteCount Size gt binary Binary procedure 1 Reads Size bytes from the input stream readChar gt char Character procedure Reads character from the input stream 212 readLine gt string Line procedure Reads line from the input stream readString charCount NumberOfChars gt string String procedure i Reads NumberOfChars characters from the input stream reconsult factDB FactDB procedure 1 Reads facts from a stream to a specified facts section reconsult first retracts the fact database ungetChar procedure Returns last read character back to input stream Class Outputstream
109. you open for reading or writing In standard I O a stream is for reading OR for writing In the stream class a stream can be for reading AND writing The class Stream is the root of a hierarchy of classes On top is the class Stream It contains some predicates that are most general that means they can be used with any stream Think of predicates to close the stream to set or get its mode or to set or get the position in the stream The next layer consists of the two classes Inputstream and Outputstream These classes contain predicates to work in general with an inputstream or an outputstream respectively In the class Inputstream there are predicates like read 0 readchar 0 readstring 0 and consult 1 In the class Outputstream there are predicates like flush 0 nl 0 save 1 write writeQuoted and writef These predicates should be familiar by now and you probably are not surprised that they are there But that isn t all There is a third layer of classes below Inputstream and Outputstream They contain predicates for special Here I start the name of a class with a capital This to avoid the extensive use of quote In your code you need to use lower case letters 207 purpose input or output streams Each class is concerned with a special kind of input or output medium Examples are class Outputstream_console for writing to the console class Outputstream_file for writing to files class Inputstream_fil
110. you will see that whenever you choose the File New option a new form is created When you click File New more than once the forms are placed on top of each other To see the underlying form you have to drag the top one 2 4 What is happening in the background At this time you may not be too interested in what is happening in the background Yet I want to explain a few details You will need to know them as the IDE sometimes may seem to do strange things What is happening in the background is twofold When you click the lt Add gt button the IDE places two pieces of code in taskwindow pro Please open that file and try to find the two pieces but take care not to change anything One piece of code you already know It is the code that you changed in the previous section You know by now how to get there with the help of the Code Expert When it was inserted it looked like clauses onFileNew _ Source MenuTag You changed it The second piece of code is put in the part of Task Window pro where you should only come as a visitor but never come to edit It is at the end of the code it starts with the warning in blue This code is maintained automatically do not update it manually 13 14 32 10 7 2007 The sign means that to the compiler this is only a comment A comment is text that is meant for a human reader It is not executed in fact the compiler simply skips it Below this line in the code the IDE takes care of the code
111. 106 from an empty database the program shows simply an empty listbox You may want to change the code for getAllNames to show a message Here s how getAllNames NameList findAll Name person _ Name NameList NameList lt gt getAlINames vpiCommonDialogs error database is empty Note again the exclamation mark 7 5 Manipulate the data change the contents of a record To change the contents of a record in fact you do a number of by now familiar things You retrieve the contents of the wanted record Y ou show the contents to the user e The user changes the contents You discard the old record You insert a record with the new values Concerning these last two steps what to do when the contents are not changed by the user Well Pm a lazy programmer I don t care When nothing has changed I still remove the old record and insert the new but unchanged values The steps we take in this section are very much like the ones we took in the previous sections So I will give the necessary code without much explanation First create a dialog I made it look like the one in figure 7 8 It has the same listboxes as the dialog for deleting a person and I gave them the same names listbox_ctl and idBox_ctl To the right you see five Edit Fields These contain the contents of the record The Field after ID is grayed out We don t want the user to change the ID as it is used for unique identif
112. 152 output stream 6 2 eee eee 199 output StreaM o o oo oooooo oo 192 197 Outputstream o oooooooooom ooo 207 Outputstream _console 208 packag s rubi od 42 90 RA RO 217 Prat a 217 222 a EEE 57 79 PIE Task WindoW o o oooooo o 58 POP sini ads to ahs 217 272 POD panda iii 222 Post order traversal 234 predefined predicates 73 Predicate isch Seta ete 53 54 82 predicates 0 00 00 e eee 149 157 predicates peeraa aos 82 PICMUSSES 325 sauce aa iia aea do 54 Prevent Backtracking 70 Prevent Backtracking 69 PTIV ALC een llas 115 private class predicate 130 procedural interpretation 154 procedural reading 64 procedure 0 0 0 0 cece eee 27 150 program control 00005 62 Program Window 0 0e005 58 Project icon at eRe ees eee bass 44 Project Name neesto na 00 0 cee eee ee 10 Project OvervieW 0 cece eee eee 9 Project Settings 0 0 0 cee eee 9 Project Trees wis teak ai 10 40 Project NeW cios asa ae Vee 39 ProjectToolbar cl 44 ProjectToolbar pro 44 ProjectToolbar tb 44 Prolog facta i ea oani iare onada ina 82 Prolog fileSv s csc ri 42 Prolog Inference Engine 57 79 Prolog prograM o o oooooomomo oo 57 A ev cent eas bees 17 50
113. 185 This is a clause that states that John has a length of 185 You can think of it as a statement that is unconditionally true That s why it is also called a fact When you add a body to the head the head must be separated from the body by the sign E g grandFather GrandChild GrandFather parent GrandChild Person father person GrandFather The sign reads as a logical IF The body can be seen as one or more conditions This is the logical interpretation of a clause When the conditions are met the head is true The body consists of 66 99 statements separated by commas The comma reads as a logical AND When you separate statements with a semicolon this reads as a logical OR The use of the semicolon in clauses is not recommended as it easily leads to confusion So in stead of 153 child Name father Name Father mother Name Mother note the semicolon we prefer to use two separate clauses that have exactly the same effect child Name father Name Father child Name mother Name Mother There is also the procedural interpretation of the clause In this interpretation the head of the clause is the procedure head that is to be called and the statements after will be executed when the predicate is called Which interpretation you need depends on your program and the side effects of the statements When a predicate is called the clauses are tried in turn from
114. 6 SO ir is 148 single quotes ooooooomomoooo 138 SOlUtION eee 57 SOPING e eae Be tebe st he ee ent 182 SACK oreo anan ahcerateh ee 163 215 data Beaton Gee aes Beh ae ar 217 Standard code nes noari fe dees eos 48 Standard Input and Output 192 Static Texter awh ete ed iaa 97 lO a eh ok 192 SIM a tt eas 207 SICA ida 194 SUM is acne ate Ea 76 SUING a ewe hee nt hee 3 eee 138 string literal 2 2 2 0 02 0005 138 string literals oooo oco ooooo ooo 53 SUS ai aia es roe 53 Sub Directory oooooooomomomo 10 tl el ie aoe ea tid 88 taili tees Mics leat Onde a eats e ee fhe 167 PAM dale ween A Beat Sage ath ok 139 168 tail recursion 02 0 e eee eee 163 A i pie nate a ea Na a ea 191 Target Type 0 cece eee 10 Task Menu ec cece eee eens 9 Task Menu Bar 00 0c cece eee 9 Task Window 0 00 ce cece eens 9 Task MOD 35 0 4 i di 27 TaskMenu mnu 0000s eee 19 TaskMenu mnu 0 0 00 eee eee 97 TASKS it a E EE 113 TaskWindow 11 41 42 Taskwindow 0 cece eee eee 93 TaskWindow cl o oooooo o 43 TaskWindow pro o ooooo o 20 43 TaskWindow win 20 43 93 temporary data type 217 DOE a Ea E aN AA 47 97 TextField ci wie eves eae 47 theory hi eid ies a SoA a E A 55 O eb ha 113 OS ia rr ir a 53 A vtec tot nae a ate 8 42 Tools Options
115. 6 data My Documents Visual Prolog 7 projects ch8class chOlHelloWorld pri6 X data My Documents Visual Prolog 7 projects ch01HelloWorld B askDialogDemo pri6 X data My Documents Visual Prolog E xamples askDialogDemo E TLPexampleOR pi6 X data My Documents Visual Prolog 7 projects TLPexampleOR ESTA p data My Documents Visual Prolog 7 ESTA esta B ch11e04Sort priS XatalMy Documents Wisual Prolog 7 projects ch1 1e04S ort BR writeCh11e03 pri6 gt data My Documents Visual Prolog 7 projects wnteCh11e03 E mitelistodi data My Documents Visual Prolog 7 projects wrtelist tem om ome o aloj xi Message Window Figure 1 1 The Integrated Development Environment of Visual Prolog This chapter is a slightly rewritten version of the first chapter of Prolog For Tyros by Eduardo Costa The screenshots are from Visual Prolog Commercial Edition Version 7 1 The IDE shows the usual parts of a Windows program In the rest of this book we will refer to several parts of the IDE These parts are marked in figure 1 1 with the names that are used within Visual Prolog They are The Task Window In the IDE the Task Window is the window that is shown when you start a program You could also call it the Main Window This window usually shows the menu options File Edit View Windows Help on top The Task Menu It is the menu that will be familiar to you as it contains options like File Edit a
116. 6 or 543 with an exponent part like 12345 6789E2 In this case the part E2 denotes that the number 12345 6789 must be multiplied with 1072 so in fact 12345 6789E2 equals 1234567 89 A real may be preceded by a unary plus or unary minus Also the number in the exponential part may be preceded by a unary plus or minus A real literal is also called a floating literal It should not exceed maximum and minimum values for reals Character Literals A character literal or character for short is a single character within single quotes An example ofa character is w A character can be any printable character or an escape sequence String Literals A string literal or string for short is a sequence of characters between double quotes apostrophes Examples are 1234 amp and Hello world List Literals 138 A list literal or list for short denotes a list of elements The elements can be of any type but the elements in the same list must be of the same type A list can be written in many ways but the two most important ones are e write the elements between square brackets and separated by comma s Examples are 1 6 3 2 a list of integers and w d a q w a list of characters partition the list in a Head that is the first element and the Tail the other elements in the list Head and Tail are partitioned by a vertical line The partition is written 3
117. A Beginners Guide to Visual Prolog Version 7 1 Thomas W de Boer Version of this book 1 0 Preface There is a beautiful programming language out there It is called Prolog Originally the language Prolog was created by Calmeraur it was popularized by famous books by Clocksin amp Mellish and by Ivan Bratko and brought to the masses by software producer Borland who sold Turbo Prolog for MsDos systems But the masses didn t buy it and Borland stopped selling Turbo Prolog That was long ago After a few years it became apparent that the makers of Turbo Prolog had returned to Denmark and started the company PDC the Prolog Development Company They put years of effort into their dialect of the language and now there is Visual Prolog It is object oriented it has a Graphical User Interface it has a Integrated Development Environment And it still has the charm and elegance of the language they call Prolog Formerly the great problem in learning Prolog was that you had to master the very different thinking of a declarative programming language Once you ve mastered it it is a great way to write computer programs so the effort was and is worthwhile But nowadays it is not only mastering a declarative language there are these other aspects of a modern programming language object orientation graphical user interface et cetera This book is an effort to tell you all about it This book is an introduction It is meant for people
118. ASCII code table In the ASCII code table there were 256 codes that represented capitals lower case letters control characters newline backspace and punctuation characters like colon semicolon and comma The capital letter A for instance has ASCII code 65 or in bytes 0100 0001 Nowadays other codes are used In VIP two codes are supported ANSI code and Unicode ANSI code is an extension of the older ASCII code It gives the possibility to add control characters that control a o the color of the screen These controls are called ANSI escape sequence An ANSI escape sequence is a sequence of ASCII characters the first two of which are the ASCII Escape character 27 binary 0001 1011 and hexadecimal 1B and the left bracket character binary 0101 1011 and hexadecimal 5B The character or characters following the escape sequence characters specify an alphanumeric code that controls a keyboard or display function You already met this principle when we wrote n in a write statement The backslash works in VIP as an escape sequence the n denotes a new line Unicode is a code that was created in an attempt to deliver a code that can be used for every language in the world As this is rather ambitious the founders of unicode started to base the code on 16 bits in stead of 8 bits You don t have to bother too much about all this as long as you know what code is used for the files that you have Even a simple editor progr
119. Control setHideSel 1 predicate IsHideSel true controlName setHideSel IsHideSel to set this flag programmatically HScroll Possible values are True and False They define whether the multi line edit or list box control gets a horizontal scroll bar when it needs one The default value is False does not scroll This property has no effect on single line edit controls You can use the editControl getHScroll 0 gt or listBox getHScroll 0 gt predicates to get whether the control has this flag isHScrollSet controlName getHScroll IsHScrollSet true and the editControl setHScroll 1 or listBox setHScroll 1 predicates IsHScrollSet true controlName setHScroll IsHScrollSet to set this flag programmatically Icon Name The icon name must be selected from the list of icon names registered in the Project by clicking the F button In order to an icon name appears in the list of registered icon names you should add the 259 icon file IconName ICO into the project files To do this you should use the File Add command the Resource Files should be selected in the Files of Type IgnorePrefix Possible values are True and False Setting True value prevents interpretation of any amp characters in the caption of the static text control as access keys accelerator prefix characters Accelerator characters are displayed underlined and the prefix amp character is deleted The default value is False Notice th
120. DE inserts a line of code that activates the listener and it inserts some standard code for the procedure onClose It opens the Prolog Code editor and places the cursor at the inserted code It looks like predicates onClose frameDecoration closeResponder clauses onClose _ Source frameDecoration defaultCloseHandling Change the clauses part into clauses onClose _ Source frameDecoration defaultCloseHandling vpiCommonDialogs note You hit the lt close gt button n Closing the About window now What is going to happen now is that when you click the close button the program will of course close the About Dialog but first it shows you a note that tells you what you did and what is going to happen By the way do you notice the n in the text It creates a line break in the string Every other control can be given Prolog code in this way Let s add some code for when you click the lt OK gt button Select the lt OK gt button in the form and bring the events to front in the properties window It should look like figure 4 9 oix button ButtonOk f Click Responder onButtonOkClick ValidateResponder DestroyListener GetFocusListener LoseFocusListener MenultemL istener MoveListener ShowListener SizeListener StateListener TimerListener UserListener ContextMenuResponder Properties Events Figure 4 9 Events for the lt OK gt button with ClickResponder Select activated ClickRe
121. Here are some predicates form the class Outputstream flush procedure Forces the contents of the internal stream buffer to be written to resource nl procedure Writes new line symbols to the output stream save factDB FactsSectionName procedure i Save the contents of a named internal facts section in the text stream write procedure Writes specified variables to output stream writeBytes pointer Value byteCount Size procedure i i Writes Size bytes to the output stream writeQuoted procedure Writes specified variables to the output stream If variable has string char domain the prefixed and post fixed double single quotes will be written in the stream too writef string FormatString formatString procedure 1 Produces formatted output to the output stream Class Inputstream_file Here are some of the predicates from class InputStream_File Take note that in fact these predicates are a kind of constructors they create objects that can be referred to openFile string FileName Opens Unicode file for input openFile string FileName stream mode Mode Opens file for input in the specified Mode 213 openFile8 string FileName Opens ANSI file for input Class Outputstream_File Here are some of the predicates from class OutputStream_File Take note that in fact these predicates are a kind of constructors they create objects that can be r
122. Is3State true and the checkButton setStyle 2 predicate Is3State true checkButtonName setStyle _IsAuto Is3State to set this state programmatically A 3 states check box is the same as an ordinary check box except that the box can be grayed dimmed as well as checked The grayed state is used to show that the state of the check box is not determined The default value is False 3State AlignBaseline Possible values are True and False They define whether the control caption text should be positioned on the same horizontal line with caption texts of all other controls which have the same vertical position with this control This property is defined to all controls which can have captions static texts check boxes edit and list edit controls etc 255 You can use the getAlignBaseline predicates like editControl getAlignBaseline 0 gt to get whether the control has this flag IsAllignBaseLine checkButtonName getAllignBaseLine IsAllignBaseLine true and the setAlignBaseline like textControl setAlignBaseline 1 predicates IsAllignBaseLine true checkButtonName setAllignBaseLine IsAllignBaseLine to set this flag programmatically The default value is True align captions horizontally Alignment Possible values are Left Center and Right Radio buttons in this group box determine how the text in the control is aligned The default value is Left align left Left Defines that the text in the control will be
123. It gives you some idea of the efforts that are to be made to create a Windows program Also you can see that your program project consists of several files This is typical for an Object Oriented Language like Visual Prolog Chapter 2 Forms The program you created in chapter is a simple program You can start it minimize and maximize the task windows and you can exit it That s all A real program should at least be able to communicate a little bit more with the user In a Windows environment communication is among others done in special windows There are several kinds of communication windows The ones most used are called Forms and Dialogs Because in VIP Forms and Dialogs look and are very much the same I will not make a difference between them In this chapter you will add a form to the empty project that you have created in chapter 1 So please when project ch01p01 is not open open it To open a project use the option Project Open from the Tassk Menu or choose Ch01p01 from the Project Overview that is shown when you start Visual Prolog 2 1 Create a form A project may become very complex and will consist of a lot of files Therefore it is wise to think beforehand about the folders in the Project Tree where you want to put the various parts or modules of your program In this case we will not bother too much about this we will do that in later chapters Now we simply place the new form in the root of your project Go to the Pr
124. Line 1 80 Father person _ male Line 2 to see if there is a father This predicate is called with Person bound to a specific value and it gives back the Name of the father What happens here is that when the predicate Father Person Father is called the inference engine in Line 1 looks in the database to find the father clause that matches the name and gender bound in Person and then binds Father to the second argument Notice that in this case Father is bound to the whole person In Line 2 the engine checks if the second argument of the newly bound Father is equal to male If it is the predicate succeeds and a new father is found In this version the code in Prolog systematically examines each parent fact asserted into the code and sees if the first logical variable Person matches that passed down from the predicate head If the variable does match it checks if the second argument consists of a person functor whose second argument is the string literal male This example shows one important feature of functors The arguments of a functor can be taken apart and examined using regular Prolog variables and bound values like the string literal in this example If you see Line 2 you will notice that we have used an underscore In Prolog this is an anonymous variable it indicates that there is a value in that place but that we are not interested in it Here we use the anonymous variable for the first argument of t
125. N CONSTA ida a idad 146 O A 147 SA A A 149 9 8 Section ClaUSES 0 ia upre id O dato ona oie Oe eS elaine bk 153 9 9 Section AA TN 154 9 10 Section open and scope access ISSUES ooooooooororrr nenee 155 9 11 Class predicates class facts and where to declare them ooooocoocoococo oo 157 Chapter 10 Recursion lists and SOFtIOg o oooooooooorrrrrrr ete e nena 160 LOA RECursion E eles lene etic de Home A 160 10 2 TAS Sagal ds ort cote ca atte Craton sta Sb ee a eae rhe ial Sent aon Soe eee the 166 10 3 Lists and recursion 0 0 eect e nen ene eee e eee nes 169 10 4 Special list predicates iii sed rd ee gan dea ee Oh Meteo PN Aaa ela he 178 VO SOU A A E E E Sone dese Roe Oe eae das 182 TO G SUMMALY 020 das oes a terse A aia ides Rance A as Sa ALS es acai 189 Chapter 11 Reading writing streams and files 2 0 cece eee eens 191 ITCR As 191 11 2 The Message Window and the Error Window in VIP 0 0020 e eee ee eee 192 A E ted wey Neen PaaS 194 11 4 Standard Input and Output the class StdIO 0 ee eee 196 11 5 The predicate stdIO read 6 cette nen en enn 203 11 6 The predicate writef en the format string 0 0 cece cece eee eee 205 11 7 General input and output the class Stream 1 0 02 eee eee 207 11 8 Files and Directorles cc eee eee eee eee e nee ees 214 Chapter 12 More data structures Stacks Queues and Trees 0 0 0c cee ene 215 12 1 Data Str
126. Package Parent Directorp Browse Existing Package main pack 0 y Public 85 z 25 Draw Cortrol Mu 44 Toolbar 5 Menu A Bitmap EA Icon ES Cursor 4 Text File P Package PA COM Package Class Interface Cancel Heb Figure 2 2 Create Project Item a form Here we want to create a form so in the left pane you highlight Form In the right pane the IDE opens the fields for a few things it needs to know about this form A window like the one in Figure 2 2 with the name Create Project Item is called a dialog window in this case it is the Create Project Item dialog The IDE uses dialogs to hear from you what you want and where to put it For the new form name I chose form1 That is a very bad name as it says nothing about the form In general you should think of meaningful names But I think that here it will do Type the name as shown in figure 2 2 Be sure that you highlighted ch01p01 in the Project Tree to place the form in the project root If necessary you can click on Existing package and accept the name main pack that the IDE suggests When your dialog looks like the one in figure 2 2 everything is complete and you click the lt Create gt button to make the IDE create a new form When you click lt Create gt the IDE opens four small windows that work together to support you in creating the new form These windows are titled form1 that is the name we chose
127. SortedLowerList SplitElem SortedUpperList SortedList then put the sorted lists and the splitelement together in one list As you can see it is the splitting that does the trick We continue splitting lists until each list contains only one element or no element at all For splitting I use the predicate split Here is the declaration split integer integer integer integer procedure 1 1 0 0 It takes an integer and a list for input and produces two lists that can be worked with in the next round This predicate is also recursive So we start with splitting an empty list In that case it doesn t mater what element we use as a splitting element It gives the clause split _ 1 D When the lists are not empty we must compare the first element the Head in the list to be splitted Tail with the splitting element When Head is smaller or equal to splitting element 1t goes into the lower list If not we use the second clause and put it into the upper list In Prolog split SplitElem Head Tail Head LowerList UpperList Head lt SplitElem split SplitElem Tail LowerList UpperList split SplitElem Head Tail LowerList Head UpperList split SplitElem Tail LowerList UpperList In both cases we continue with splitting the Tail Finally we need a predicate to put a list an element and another list together into one list I made the predicate append that takes two li
128. To be able to do so the constructor needs a name You provide that name as an argument for new l The name of the new object is provided as a string Close and save the file person cl and finally take a look at person pro This is the file where the code for the procedures will be placed That s why this file is called the implementation of the class person Here you enter the code that is needed to make the objects behave as expected that is the code for the predicates that were declared in person i and person cl Please change the existing code into implement person open core facts name string constants className classVersion person person clauses classInfo className classVersion clauses new Name name Name clauses getName name clauses setName Name name Name 118 end implement person Let s take a closer look at the code The first line of interest is facts name string This states that the objects of this class have a so called fact of the type string A fact is a variable in this case you will recognize that this fact will contain the name of the object But notice that the variable name begins with lower case Facts like this one belong to an object with every object the fact can and will have a different value Next you find the constructor clauses new Name name Name The constructor new 1 was declared in person cl H
129. WOTAS oooooooooooooo 136 retrieve the Object 126 A Boss au teed Biv ata aaa deeded 54 WMA Gen Ayres nels net ell teeta ated 155 runtime exceptions 04 134 SAV Csi nce AE ee ae ae 213 A Bh diduange el ee Go Beene 16 202 A E 203 SCOPS Si a oc eg abe 139 SCOPE ACCESS tii 155 scope visIbility o o o ooo o 156 A a aa i ea 191 192 screen buffer o oo oooooooo oo o 191 SC al Clits os hanna 15 Ges tcelses 8b a ANAS 62 SECO iia ad ones nd 139 Section clauses o oooooo o 153 Section CONStaM S ooooooooo 146 Section domains 0000 141 Section factS o o ooooooomoooo 147 Section goal o o oooooooooooo 154 Section open 0 0 eee ee eee 155 Section predicates o ooooooo o 149 273 selection sorting 0 0 0 0 ooooooo 184 SelectionChangedListener 104 SelectionChangedListener 105 SOL een pay staat eta alae tee nce noc 115 set of procedures 000 eee eee 27 setCRLFconversion 0 0005 212 setErrorStream 1 2 0 0 0 00 eee 201 setInputStream 1 o oo oooooooo 199 setMode 0 ccc eens 212 setOutputStream 1 00000 200 SetPOsitiOn iii Peas 212 SHOW Jnr Sok aa Alea ee Adee 41 side effect ictal whack hie eee s 95 Side effects ii a BOLI eke kes 73 Side effects o o oooooo o o 62 86 simple domains oooooooommo o o o 7
130. a lowercase letter When you build the project after you create a file the IDE generates several files to handle the form Each of these files will have the Name that you enter here the files will only differ in their extensions These files appear in the Project Tree in the Project window Later you will find the Name in the Title Bar of the Form Package In the Package list button you should select one of the packages known to the project the main pack in figure A1 1 or a new package that will be created The files that are generated for handling the created form will be included into this package In figure A1 1 we choose as existing package the project itself so the files will be added to main pack 238 Create Project Item i xi Mame mpFom C New Package Parent Directory Browse Interface 3 Dialog Existing Package main pack 2 Fom Public 28 Control C Private YS Draw Control 44 Toolbar E Menu A Bitmap FA Icon 8 Cursor Text File Cancel Help Figure A1 1 Creating a Form Public or Private A Form is treated as a class That means that a o two files are generated lt Name gt i and lt Name gt cl where lt Name gt is the name you gave to the Form The file lt Name gt i is called the interface the file lt Name gt cl is called the class declaration Both files contain the predicate declarations that are seen by other parts of the project You should check on
131. a stack In such a stack only the top plate is visible and accessible to the user all other plates remain hidden As new plates are added each new plate becomes the top of the stack hiding each plate below pushing the stack of plates down As the top plate is removed from the stack it can be used and the other plates pop back up and the former second plate becomes the top of the stack Two important principles are illustrated by this metaphor the Last In First Out principle is one the second is that the contents of the stack are hidden Only the top plate is visible or accessible so to see what is on the third plate we have to remove the first and second plate Stacks are used extensively at every level of a modern computer system For example a modern PC uses stacks at the architecture level which are used in the basic design of an operating system for interrupt handling and operating system function calls As an abstract data type the stack is a container of elements and has two basic operations push and pop Push adds a given element to the top of the stack leaving previous elements below Pop removes and returns the current top element of the stack Other operations In modern computer languages the stack is usually implemented with more operations than just push and pop The length of a stack can often be returned as a parameter Another helper operation top also known as peek or peak can return the current top element of the
132. a table is that of a matrix Table 7 1 shows a table with data about some people am Name Weight Length SAE 150 Table 7 1 a table from a database Every row in the table describes the attributes of one person So we see that John weighs 85 kilograms is 185 centimeters tall and has an IQ of 110 A row in a database is called a record Every column is dedicated to one characteristic weight length et cetera In databases such a characteristic is called an attribute When you are used to use statistical programs then you will notice the resemblance with a data matrix This is not hard to understand I think But two remarks need to be made The first is that the attribute PersonID is a special one In the database it is used to identify the record You may think that the name of a person can be used for that purpose like you do with your friends But people do have the same name now and then in that case the identification is no longer unique So in databases we tend to use identification numbers like the social security number The second remark concerns the first row It is a title row meant for human readers In the database this row is omitted from the tables That means that in the database we only register 1 John 85 and the other data It is the responsibility of the database designer and programmer to make sure that the data are presented to the user in a way that makes clear what the data mean 88 A database co
133. abase for records with that name and shows all the ID s that it finds with the selected name These actions should be triggered when you select a name Luckily there is a standard predicate that is helpful here It is one of the predicates that come with the listbox Open the deletePerson dlg in the Dialog Editor and click on the listbox where the names should appear to give it the focus Then open the Events list in the properties window In the Events list select SelectionChangedListener and give it the value onListboxSelectionChanged Then double click on it the standard code is inserted en the editor is opened where the standard code is inserted This code is executed when you change the selection in the listbox Now please change the standard code into predicates onListboxSelectionChanged listControl selectionChangedListener clauses onListboxSelectionChanged Source Name _T Source getSelectedItems person getSomeIDs Name IDList idBox_ctl clearAll 1d_ctl setText name ctl setText idBox_ctl addList IDList onListBoxSelectionChanged _Source Don t forget to remove the underline before Source in the first clause but do insert an underline in the second clause Let me explain in plain words what is happening here Name _T Source getSelectedltems GetSelectedItems is a standard predicate that returns the selected items Because VIP allows you to select more than one item in a l
134. ach node in a tree data structure exactly once in a systematic way Such traversals are classified by the order in which the nodes are visited Linear data structures like lists and one dimensional arrays have only one logical means of traversal element by element in the order they are stored But tree structures can be traversed in many different ways Starting at the root of a binary tree there are three main steps that can be performed and the order in which they are performed defines the traversal type These steps are Performing an action on the current node referred to as visiting the node e Repeating the traversal process with the subtrees rooted at the left child Repeating the traversal process with the subtrees rooted at the right child The three ways are called pre order traversal in order traversal and post order traversal The process is most easily described through recursion To traverse a non empty binary tree in pre order perform the following operations Visit the root Traverse the left subtree Traverse the right subtree This way is called depth first traversal or depth first search and it was described in the previous section When we pre order travers the example tree in figure 12 4 we will visit the nodes and probably process them in the order F B A D C E G I H To traverse a non empty binary tree in in order perform the following operations Traverse the left subtree Visit the root Traverse t
135. ack very often resaurceldentifiers i 2 2 Enable the Task Menu option By now you have a program you ch01p01 prj6 x data My Documents Visual Pras Oj x have a form in it but it still doesn t work When you now E compile and execute the program 8 TaskWwindow nothing happens with the form 7 J Toolbars The reason is that we did not yet AboutDialag cl specify when the form should be ES AboutDialog dla opened In a Windows program it 8 AboutDialog takes a so called event to make E a AbautDialog pro something happen There are E TaskWindow cl many events possible but the one you are probably most familiar if with is the click with the left TaskWindow i 3 Task Window pack taskWindow ph T askw indow pro mouse button A click with that E E ee ee button may open or close a fy TaskMenu mnu window or it may start activate a FA Projecticon ico menu option Your program needs to know when to open the form main el that you created You are free to main pack choose any event that you like EA main oh but here we suggest that you Figure 2 8 Contents of folder TaskWindow choose the event that the user clicks with the left mouse button on the menu option File New in your program When you ran the empty application in chapter 1 you certainly took notice of the fact that the File New option was disabled It was grey that means in Windows that the option cannot be use
136. acktrack point It is the point where the engine should return when it has treated the first fact After creating the backtrack point the engine takes the first fact father Bill John and sees if it matches the goal It clearly doesn t as Bill is different from Sue and John is different from Jim but that is not important anymore The engine now returns to the backtrack point to check the next fact but before it does the check it sets a backtrack point at the third fact father Sue Jim It then checks the second fact that doesn t match the goal either The engine returns to the backtrack point As this is the last fact with predicate father the engine cannot set another backtrack point So it checks if there is a match and yes the fact and the goal match the predicate and the objects match So the engine reports Yes to you Suppose now that you ask the goal father Sue X X is upper case it is a variable that has not yet a value I should liked to have used Father instead of X but it may be confusing when there are two identifiers father the predicate and Father the variable in the same goal You hit lt Enter gt and the story is repeated more or less The inference engine takes the goal and sees that the predicate father in the goal matches with facts in the program It sets a backtrack point at the second fact fat
137. actorial 164 becomes bigger than the limit and you will get a error message that will probably vanish from you screen before you can read it To make things a bit more interesting we can keep track of what is happening Let s introduce a write statement that shows the intermediate steps in the recursion Please go to factorial pro and change the clause for fac Number Factorial into fac Number Factorial NumberMinus1 Number 1 fac NumberMinus1 FactorialMinus1 Factorial Number FactorialMinus1 stdIO write Factorial of Number is Factorial stdlO nl When you now run the program it shows you every factorial up to the given number Notice how fast the factorial grows It grows faster than squares Just as an illustration let us now define a function to compute factorials I give it the name facfunc and the declaration that I give in factorial cl looks as follows facfunc integer Number gt integer Factorial procedure In factorial pro enter the clauses for facfunc clauses facfunc 1 1 facfunc Number Factorial NumberMinus1 Number 1 FactorialMinus1 facfunc NumberMinus1 Factorial Number FactorialMinus1 stdlO write Factorial of Number is Factorial stdlO nl And finally change the code for the run predicate in main pro into clauses run console init stdIO write Type a number 0 to end gt hasdoma
138. ad this statement inside out The innermost expression is maximum IDList Maximum I is a standard predicate that computes the maximum of the specified list in this case IDList The next partial statement is toTerm maximum IDList 1 Toterm 1 changes the type of the maximum of IDList into an integer and I add 1 to it So this value is an integer that is 1 more than the maximum of the database This is a unique number Finally I bind this number to the variable NewID but because ID is of type string I have to change the type from integer to string with the predicate toString 1 NewID toString toTerm maximum IDList 1 It may look complicated and frankly I think it is but it works fine The variable NewID is used in the assert statement The predicate is finished with some output to the message Window that states that the record is added and that shows the new ID stdIO write Person Name has been added with ID NewID stdIO nl We have entered the code for the predicate but we still have to declare this new predicate so other parts of the program can use it Please go to the file person cl and add the code addPerson string Name integer Weight integer Length integer IQ procedure Right under the declaration for listDatabase We are now almost done except for one detail The standard predicate maximum is in a separate module in fact it is a class with the name list We have to tell th
139. age The first says To write an empty list do nothing The second one says Otherwise to write a list write its head which is a single element then write its tail a list And here is how it works The first time through the goal is writelist write _a list 1 2 3 This matches the second clause with H 1 and T 2 3 this writes 1 and then calls write _a list recursively with the tail of the list writelist write _a list 2 3 This is write _a_list T This recursive call matches the second clause this time with H 2 and T 3 so it writes 2 and again calls write_a_list recursively writelist write _a list 3 Now which clause will this goal match Recall that even though the list 3 has only one element it does have a head and tail the head is 3 and the tail is the empty list So again the goal matches the 170 second clause with H 3 and T Hence 3 is written and write_a_list is called recursively like this writelist write_a_list Now you see why this program needs the first clause The second clause will not match this goal because cannot be divided into head and tail So if the first clause were not there the goal would fail As it is the first clause matches and the goal succeeds without doing anything further To see if you understand what happens try to imagine what the result will be if you change the second clause into write_a_list H T write_a_lis
140. akes the least amount of typing but of course you can add nice notes and other popup windows if you like The database is filled but you don t see any record You only get a lousy message but who believes a computer So let s add a predicate to list the records in the database The steps that you need to take are very similar to the ones you took to fill the database so I ll give the necessary actions and code without much explanation First enter the code for a predicate listDatabase in the file person pro In my project I entered the following code right after the code for fillDatabase 94 listDatabase person ID Name Weight Length IQ stdlO write ID ID Name Name Weight Weight Length Length IQ IQ stdlO nl fail listDatabase Here l apply a trick you learned in chapter 5 When we call predicate listDatabase the first record in the facts database is matched and the write predicate in stdIO is called As a side effect the values are written to the Message Window Then the predicate fails The inference engine backtracks and the next record in the database is matched and printed Then the predicates fails again et cetera When there are no more records to match the predicates backtracks to the second clause en there nothing happens so the predicate call ends in silence But in the mean time the records have been written to the Message Window In version 7 1
141. alog with the name addPerson Be sure to place it in the existing package forms pack When you click lt Create gt the IDE opens the Dialog Editor Here you see the familiar grid of the new Dialog or Form with three buttons lt OK gt lt Cancel gt and lt Help gt already placed on the grid Now we are going to enter four Text Fields and four Edit fields so that the Dialog looks like the one in figure 7 5 x Mame Weight 25 Length 180 IQ ho Figure 7 5 the Dialog addPerson To enter a text field like the one with the label Name in the upper left corner go to the Controls toolbar and click on the icon upper case letter T The cursor changes into a plus sign and the word 96 Text Go to the Dialog and click the place where you want to drop the text field The text field gets the focus if not click once on the text field and in the properties list the properties of the text field are shown In the left column on the third line you will find the word Text and in the right column StaticText Change Static Text into Name and when you hit lt Enter gt you will see that the label of the text field will have changed Now enter the other three text fields To enter Edit Fields is a little bit more work To place an Edit Field go to the Controls Toolbar and select the box with the word Edit top row most right symbol The cursor changes in a plus sign and the word
142. am like Notepad nowadays can read and write both codes Now what has all this to do with streams in VIP Well a stream is not just a sequence of binary data 1t has also two attributes One attribute is the mode the other is the position In VIP there are two or three modes depending on how you want to think about it These modes indicate how to interpret the bytes in the stream Here they are Binary mode In this mode your program reads the bytes from the input stream just as they are a sequence of bits What you do with it is purely your business For output goes the same what you write is put in the output stream as binary digits There is no interpretation Text mode In text mode there is an interpretation involved The bytes in the stream are read as letters numbers et cetera This can lead to a conversion of bytes into characters When you read a text in text mode there will be all kinds of special characters like newline When you write to a printer this newline character is converted into two control characters for the printer a linefeed and a carriage return There are other transformations E g in Unicode the number twelve is one code 0x0C When this code is sent to the screen there is an automatic conversion so on your screen you see 12 that is you see the characters 1 and 2 There are two text 195 modes ANSI mode and Unicode mode The difference is that ANSI mode uses ANSI coding and
143. ames So we make a predicate like this getAllNames NameList findAll Name person _ Name NameList I put it in the file person pro right after the predicate addPerson but before end implement person When we call this predicate the standard predicate findall creates the wanted list We use it in the same way as in the preceding section The variable Name is used to collect the strings that appear as the second argument in the functor person and these Names are collected in the list NameList This is the clause for this predicate the declaration 102 getAllNames string NameList procedure o must be placed in the file person cl where the other declarations reside The asterix means that in this case we use a list More on lists in chapter 10 Now we are ready to use this predicate to fill the listbox in dialog deletePerson Dialog deletePerson is created by calling the predicate new in deletePerson pro Please look for that file you may have to build the project first and open it in an editor Then find the following clauses clauses new Parent dialog new Parent generatedInitialize This is the code that is used to create the dialog We will extend the code so that at creation time the listbox will be filled with names Change the existing code into clauses new Parent dialog new Parent generatedInitialize person getAllNames NameList SingletonList
144. an array a list does not require you to declare how big it will be before you use it Not does it have indices to point to elements A list that contains the numbers 1 2 and 3 is written as 1 2 3 The order of the elements in this list matters number 1 is the first element 2 the second 3 the third List 1 2 3 is different from the list 1 3 2 Each item contained in the list is known as an element To form a list data structure you separate the elements of a list with commas and then enclose them in square brackets Here are some other examples dog cat canary mos valerie ann jennifer caitlin benjamin thomas The same element can be present in the list several times for example p p 1 2 1 3 1 BThis section is based on a tutorial by Thomas Linder Puls PDC Prolog 166 A list can contain any positive number of elements A special case is the list that has no elements It is called the empty list and it is represented by Declaring Lists To declare the domain for a list of integers you use the domains declaration like this domains integer_list integer The asterisk means list of that is integer means list of integers Note that the word list has no special meaning in Visual Prolog You could equally well have called your list domain zanzibar It is the asterisk not the name that signifies a list domain The elements in a list can be anything
145. and the parentheses The layout is only for the human eye Make sure you understand that this is not a Prolog clause it is just a complex data structure By the way this looks not really very efficient It is more or less clumsy to represent the tree in the way we do it When using a tree you will probably act differently You will let the computer program create a tree use it and then discard it without you ever really seeing the tree For the computer program the tree is a pointer structure that it can easily maintain and traverse 12 7 Traversing a Tree Before going on to the discussion of how to create trees first consider what you ll do with a tree once you have it One of the most frequent tree operations is to examine all the nodes and process them in some way You could be looking for a particular value say a solution to a problem or you could be collecting all the values in the tree This is known as traversing the tree One basic algorithm for doing so is the following three step algorithm 1 If the tree is empty do nothing In fact here you encountered the empty subtree of a leaf and with an empty tree there is nothing to do 2 Otherwise process the current node then traverse the left subtree 3 When the left subtree is done then traverse the right subtree Like the tree itself the algorithm is recursive it treats the left and right subtrees exactly like the original tree Prolog expresses it with two clauses one for empty
146. another way is introduced instead of using the trick with fail A set of three connected keywords was introduced foreach do end foreach Take a look at the following code listDatabase foreach person ID Name Weight Length IQ do stdlO write ID ID Name Name Weight Weight Length Length IQ IQ stdlO nl end foreach The keyword searches the clauses for person and takes one at a time When it has found one that is used in the lines after the word do When the search of foreach fails because there are no more clauses for person then the program jumps to end foreach and the predicate succeeds The code with foreach do end foreach should prints the data of every person in the database Now go to the file person cl to add a declaration for the predicate listDatabase so other parts of the project may use it Add listDatabase procedure right after the declaration of fillDatabase Now it is time to add a menu item to the TaskMenu Go to the TaskMenu mnu and add the option ListDatabase in the submenu under Database Then add the standard code for this menu item Go to Task Window win right click and open the Code Expert In the Dialog and Window Expert click Menu click TaskMenu click id_database highlight id_database_listDatabase and lt Add gt the code Change it into predicates onDatabaseListdatabase window menultemListe
147. ans a list of numbers and copies it leaving out the negative numbers It uses a module called listmanager predicates discard_negatives Integer integer procedure i o end class clauses discard_negatives discard negatives H T ProcessedTail H lt 0 IfH is negative just skip it the processed tail will not be combined with H discard negatives T ProcessedTail discard negatives H T H ProcessedTail here H will be added to the processed Tail discard negatives T ProcessedTail end implement listmanager 174 run console init listmanager discard_ negatives 2 45 3 468 X stdio write X StdIO readchar For example the goal listmanager discard_negatives 2 45 3 468 X glves 2 3 468 And here s a predicate that copies the elements of a list making each element occur twice doubletalk doubletalk H T H H DoubledTail doubletalk T DoubledTail List Membership Suppose you have a list with the names John Leonard Eric and Frank and would like to use Visual Prolog to investigate if a given name is in this list In other words you must express the relation membership between two arguments a name and a list of names This corresponds to the predicate isMember name name name is a member of name this means that the list name consists of members of the type name In the program below the first clause investigates the hea
148. are lt predicateMode gt The option lt predicateMode gt indicates the behavior of the predicate That is it tells the compiler if the predicate can fail can succeed or can backtrack or both or all three options There are six different modi possible e lt predicateMode gt fail the mode fail indicates that the predicate will always fail There is one standard predicate that has this mode it is the predicate fail 0 e lt predicateMode gt procedure the mode procedure indicates that the predicate will always succeed and that it will succeed with only one solution No alternative solutions are possible no backtracking will take place lt predicateMode gt determ the mode determ indicates that the predicate can fail or succeed but that it never returns with a backtrackpoint That implies that when the Prolog engine backtracks 1t will skip calls to this Clauses are treated in the next section 150 predicate as it can never have a backtrackpoint When it succeeds there will be only one solution e lt predicateMode gt multi the mode multi indicates that the predicate never fails it will always succeed and it may come back with a backtrackpoint so a call to this predicate may result in multiple solutions lt predicateMode gt nondeterm the mode nondeterm indicates that the predicate can fail can succeed and can return with a backtrackpoint e lt predicateMode gt erroneous the mode
149. as explained in chapter 5 that the positions of the arguments of predicates have to be formalized by the programmer who designs the code Once formalized the same positional formalization should be consistently used The same strategy applies to the creation of functors In the case of the couple functor it so happened that we decided to represent the husband as the first argument and the wife as the second one Once this is done then whenever data are created using such a functor then we need to ensure that the husband is always at the first position and the wife in the second Now when we look back on the couple functor someone can correctly argue that if we are so sure that the first argument is always a husband who is always a male and the second one is always a wife who is always a female then what is the need for using the strings male and female in making the predicate call If you are sure that the first argument is always a husband and the second is always a wife then the call could be simplified to myPredicate ACouple ACouple couple Husband Wife But when you are not sure which of the two is on the first position then you probably want to make sure who is who Here is a version for the call that catches it myPredicate Couple Couple couple person PersonsName PersonsGender person SpouseName SpouseGender In the above functor both the following examples make logical sense myPredicate C1 C1 cou
150. ase There are two possibilities 1 You delete the listener but not your own code When you build the project in this case you get a Warning that there are unused predicates in your program The warning points at your own code that is not used anymore as there is no listener to activate it It is a warning because the remaining code doesn t harm the program it is simply not used 2 You delete your own code but not the listener When you build the project in this case you get an Error that there is an unused identifier say variable in your program The listener points to an identifier variable onFileNew that it needs to react to clicking File New But you deleted that identifier so it is not found in the program code so the listener cannot direct the program control to it That s an error you should repair it by entering your own code or by removing via the IDE the listener mE 1 7 66 4 5 Reaching code via events Ding ABANICO You reach the code for the taskwindow via ValidateResponder the Code Expert Other code is reached by Deas pieis fth ind fth GetF ocusListener way of the properties win Ow O u e EATE editor Let s play around a little bit Please MenultemListener open the folder taskwindow and select MoveListener highlight the file AboutDialog dlg ShowListener Right click and choose Edit The IDE SizeListener StateListener opens the editor so you can edit the About Taal ine ar Dialog The pr
151. ase before consulting The best way to do that is to ask the user if she wants to keep or to delete the existing data before adding from the file When you replace the clauses for consultDatabase 0 with the following code the program will behave better In the same code I repaired the bug that the program crashed when there was no file named personDB txt class predicates 111 getData procedure clauses consultDatabase file existFile personDB txt getData consultDatabase vpiCommonDialogs error Sorry can t do what you want No file called personDB txt getData Answer vpiCommonDialogs ask What do you want me to do Do you want to delete the existing records n before consulting Yes No Answer 0 retractFactDB personDB file consult personDB txt personDB stdlO write Existing records have been deleted n New records have been added from file personDB txt stdIO nl getData file consult personDB txt personDB stdIO write New records have been added from file personDB txt stdIO nl The database is now more or less complete But take care There is hardly any security in this program The user is free to do what he wants there is no authorization et cetera But the program works like a database 7 7 Conclusion In this chapter we created a database program or better a datafile program You learned to use a listbox and a few predicates that co
152. ass facts I ll try to explain what to take care about but things may get confusing or confused Facts belong to objects that is with each object from the same class there can and normally will be different facts These kind of facts must be declared and defined in the class interface file But when a fact belongs to the class as a whole then it is a class fact and must be declared and defined in the 157 class declaration file But take care When you declare a fact in the interface file then it is by definition an object fact When you declare a fact in the class declaration file then it is by definition a class fact Only in the class implementation file confusion is possible Declaring facts in the implementation file means that the fact is by definition private It cannot be seen from outside the class implementation But it can be an object fact or a class fact So when you declare a fact in the implementation file you must indicate whether it is an object fact or a class fact When a fact is a class fact you must declare it in a section that begins with the keywords class facts Table 9 2 gives an overview fact declaration is public fact private fact for an object member in class interface in implementation facts facts for a class member in class declaration in implementation facts class facts Table 9 2 declarations of facts The predicate declaration is used to declare the
153. ass facts treefact treetype empty It is a fact variable of the type treetype and it is initialized as empty In classtree pro I also entered the code for the predicates clauses add from list add from list Newltem Tail treeinsert Newltem treefact NewTree treefact Newtree add_from_list Tail treeinsert Newltem empty tree Newltem empty empty treeinsert Newltem tree Element LEft Right tree Element Newleft Right Newltem lt Element treeinsert Newltem Left NewLeft treeinsert Newltem tree Element LEft Right tree Element Left Newright treeinsert Newltem Right NewRight traverse preorder stdIO nl stdIO write the tree in PRE order preOrderTraverse treefact stdlO nl traverse inorder stdIO nl stdIO write the tree in IN order inOrderTraverse treefact stdlO nl traverse postorder stdIO nl stdIO write the tree in POST order postOrderTraverse treefact stdIO nl traverse _ preOrderTraverse empty preOrderTraverse tree Element Left Right stdlO write Element preOrderTraverse Left preOrderTraverse Right inOrderTraverse empty inOrderTraverse tree Element Left Right inOrderTraverse Left stdlO write Element 236 inOrderTraverse Right postOrderTraverse empty postOrderTraverse tree Element Left Right postOrderTraverse Left
154. at is skipped by the compiler You use it to clarify parts of your program code for the human reader A Visual Prolog comment is written in one of the following ways e The a slash followed by an asterisk characters followed by any sequence of characters including new lines and terminated by the asterisk slash characters These comments can be multi lined They can also be nested The percent sign character followed by any sequence of characters Comments that begin with character percent sign continue until the end of the line Therefore they are commonly called single line comments Examples of comments are This is a comment that continues on the next line person Thomas 61 190 the rest of this line is a comment In the editor in VIP the text in comments is colored blue 136 Declaration A declaration is an informative formal specification of the code that you are going to use in your programs E g when you use a predicate you first have to declare it A predicate square that expects an integer as input argument and that returns the square of the number so it is a function is declared square integer Input gt integer Answer procedure i Statements like these are declarations A declaration informs the compiler of the general form of e g predicates They are used at compile time As such they are not executed at run time Declarations will be treated at length in this chap
155. at static text controls cannot be activated Therefore when an access key of a static text control is pressed then the control next to this static text control in the tab stop order of controls in the dialog is activated Most often this flag is used when filenames which can contain the amp character need to be displayed in static text controls You can use the textControl getlgnorePrefix 0 gt predicate to get whether the control has this flag IsIgnorePrefix controlName getlgnorePrefix IsIgnorePrefix true and the textControl setIgnorePrefix 1 predicate IsIgnorePrefix true controlName setIgnorePrefix IsIgnorePrefix to set this flag programmatically LeftText Possible values are True and False They define whether the control title text should be positioned on the left or on the right of the control check box or radio button You can use the checkButton getLeftText 0 gt or radioButton getLeftText 0 gt predicates to get whether the control has this flag IsLeftText checkButtonName getLeftText IsLeftText true and the checkButton setLeftText 1 or radioButton setLeftText 1 predicates IsLeftText true checkButtonName setLeftText IsLeftText to set this flag programmatically The default value is False caption is on the right side MultiColumn Possible values are True and False If it is True then the list box control can have multiple columns The default value is False You can use the listBox g
156. at there are two places to declare public predicates Predicates declared in person cl are called class predicates They focus on the class as a whole The predicates declared in person i are object predicates They focus on individual objects More on this later Now let us try to create objects For that we need an event Create a new option in the Task Menu and call it CreateObjects Go to the Code expert and lt Add gt code for the new menu option Then go to the code and enter the following 119 predicates oncreateobjects window menultemListener clauses oncreateobjects Source MenuTag NewPerson person new John NameGiven NewPerson getName stdlO write New person with name Namegiven stdIO nl NewPerson setName Thomas stdIO write Name is changed New name is NewPerson getName stdIO nl What happens when you click CreateObjects First the variable NewPerson is bound to a new object It is an instance of class person and it is given the name John Remember that the constructor new Name is a predicate that is declared in the class You call a predicate in a class by specifying the name of the class followed by two colons followed by the name of the predicate and the arguments In the next line the name of the created object is asked by calling getName and bound to the variable NameGiven This name is written to the message window Not surprisingly it is John Th
157. bed with the functor tree Node LeftTree RightTree or empty and that we are going to insert new items in this tree We give each clause for insertion the procedure name insert and use three arguments the item_to_be_inserted the node that we look at and the resulting new tree with the item inserted in it The first clause is insert Newltem empty tree Newltem empty empty 1 0 Translated into natural language this code says The result of inserting NewItem into a node that is an empty tree is tree Newltem empty empty The cut ensures that if this clause can be used successfully no other clauses will be tried for insertion of the element Newltem The second and third clauses take care of insertion into nonempty trees Both have the flow pattern 1 1 0 insert Newltem tree Element Left Right tree Element NewLeft Right Newltem lt Element insert Newltem Left NewLeft insert Newltem tree Element Left Right tree Element Left NewRight insert Newltem Right NewRight If Newltem lt Element you insert it into the left subtree otherwise you insert it into the right subtree Notice that because of the cuts you get to the third clause only if neither of the preceding clauses has succeeded Also notice how much of the work is done by matching arguments in the head of the rule 233 12 10 Other tree traversals In computer science tree traversal refers to the process of visiting e
158. bjects In the other predicates I gave this list as an argument Here I use the fact that it is a fact database Thaat means that I can use the list from anywhere in the file person pro Here is how this works I take the full list createdObjects peel the head off If this object has the right name then createdObjects gets the value of the tail So the named object is deleted from the list If the name is not the one wanted then in the second clause I again peel off the head but this time that object is retained in the variable Object Then deleteperson 1 is recursively called it uses again the list createdObjects but that is now the list without the previous first element The wanted object is deleted and then the list is reunited with its former first element If this sounds or looks like magic to you read more about lists in the next chapter The program can be built and executed now It should work properly But maybe you are worried about a detail When we delete an object we remove it from the list But when it was created there was some memory reserved for it We did not remove somehow that piece of memory You might say that the object is still there but we cannot reach it anymore So next to the constructor one would expect something like a destructor But there are no destructors in Visual Prolog There is 132 something else it is called the garbage collector This is a program that works almost like a demon It checks
159. bles in different clauses or different invocations of the same clause are distinct Listlength length_of 1 2 3 Length Listlength length_of 2 3 Length Listlength length_of 3 Length Listlength length_of 0 Length 0 1 1 Length Length 1 2 Length Length 1 3 Tail Recursion You probably noticed that length _of is not and can t be tail recursive because the recursive call is not the last step in its clause Can you create a tail recursive list length predicate Yes but it will take some effort The problem with length of is that you can t compute the length of a list until you ve already computed the length of the tail It turns out there s a way around this You ll need a list length predicate with three arguments Argument one is the list which the computer will whittle away on each call until it eventually becomes empty just as before The second argument is a free argument that will ultimately contain 172 the result the length The third argument is a counter that starts out as 0 and increments on each call When the list is finally empty you ll unify the counter with the up to then unbound result What this predicate is doing is it takes each element from the list and counts it when it is removed predicates length of integer List integer Length integer Counter procedure 1 o 1 end class clauses length of Result Result length _of _ T Result Co
160. but always in e format if exponent of the value is less than 4 or greater than or equal to the precision Trailing zeros are truncated dorD Format as a signed decimal number uor U Format as an unsigned integer xor X Format as a hexadecimal number o or O Format as an octal number c Format as a char B Format as the Visual Prolog binary type R Format as a database reference number P Format as a procedure parameter s Format as a string Note when converting real values into a text representation they are truncated and rounded to 17 digits unless another quantity is specified Here is an example clauses run console init stdIO writef a lt formatstring gt without variables n IntegerVar 12345 stdIO writef Numbers 5 n wider 10 n left justified 10 lt end of field is here IntegerVar IntegerVar IntegerVar RealVar 12345 6789 stdIO writef nRealNumbers 5 4 n wider 10 10 n left justified 10 4 lt end of field is here n rounded 5 2 RealVar RealVar RealVar RealVar String This is a string stdIO writef nString n 20lend is here n 20 end is here n 5 end is here n 20 7 end is here n String String String String stdIO write n hit lt Enter gt _ stdIO readchar succeed place your own code here end implement main Take care that when you add text in the format string next to a format field then the first letter is interpreted as part of the
161. by a comma and a lt list gt As a list can be the empty list the latter could mean lt element gt and as a list can also be lt element gt it could also mean lt element gt lt element gt and finally it could even mean lt element gt lt element gt lt list gt In the latter case we can extend the lt list gt recursively into lt element gt lt element gt lt element gt lt element gt 216 and make the list as long as we want This is what we mean when we say that the list is a recursive structure We define it with a recursive definition that creates lists that can have any length that we want or need It is only a matter of convenience that we do not write lt element gt lt element gt lt element gt lt element gt but write lt element gt lt element gt lt element gt lt element gt in stead 12 3 The Stack In computer science a stack is a temporary data type That means that in a computer program the stack is used at run time but when the program ends it is not kept like you do with the data in a database The stack is based on the principle of Last In First Out LIFO You can think of a stack as a pile of papers on your desk Whenever an element is put on the stack it is put on top Whenever you need an element from the stack you take the top element A frequently used metaphor is the idea of a stack of plates in a spring loaded cafeteri
162. called a function The number of the clicked button is returned and put in variable Answer In the last line of the code this variable is used in a note for the user Because Answer is a number and the procedure note expects a string we have to transform Answer into a string with the function tostring Buttons are numbered from left to right starting with the number 1 At this point I suggest that you start playing with the possibilities of the procedure messagebox Change the icon and the buttons It will give you a feeling for using these procedures They are very useful tools 3 3 Getting the user response With messagebox you could receive input given by the user Messagebox is one possibility but there are other ones Their procedure names are ask 2 ask 3 getString 3 listSelect 5 and getFileName 6 To show how they work I shall use options in the TaskMenu to trigger them and as they are meant to receive input from the user I shall add some code to show the input that was received Just like I did with messagebox 6 where you saw the answer that the user clicked First let s take a look at ask 2 and ask 3 Their use looks like the use of note and error As triggering events for ask 2 and ask 3 add two menu items in the submenu below Messages in the Task Menu I called the options ask2 and ask3 Generate the code and change it into predicates onMessagesAsk2 window menultemListener clauses onMessagesAsk2 Source
163. case This refers to the object that is created by the constructor The code puts the newly created object at the head of the list of created objects The only thing left to do is to add code to the option in the TaskMenu Use the Code Expert to reach the standard code for CreatePerson and change it into predicates oncreateperson window menultemListener clauses oncreateperson Source MenuTag NewForm createperson new Source NewForm show This completes the code for creating person objects It is a good moment to build and execute the project and to generate some objects If you want to make things a bit more interesting you may add some more attributes Now that we can create objects it is time to show them to the world To achieve that I created a predicate showAllPersons 0 and placed it in class person As this predicate is to work on all objects it is a class predicate Besides it is to be called from outside the class person so it is a public class predicate That why I declare it in person cl This is the declaration showAllPersons procedure Next I entered the definition in person pro Here is it showAllPersons stdlO write These are the persons in the database n 129 showAllObjects createdObjects This predicates announces that the objects will be printed and then calls another predicate showAllObjects 1 to do the job ShowAllObjects 1 takes the list of ob
164. cate fails This may seem stupid but in combination with the so called backtracking it will turn out to be a very powerful technique The next section will explain how failing is treated in the general case i e when it is not the goal that fails 5 5 4 Backtracking By now you know that the inference engine tries to solve the goal by matching facts The engine searches the rules and the facts from left to right and from top to bottom In its relentless search for solutions the inference engine uses something called backtracking You should understand what it means and how it is applied Backtracking is the subject of this section Suppose you have written a program with the following facts and rule father Bill John father Pam Bill 64 father Sue Jim grandFather GrandChild GrandFather father GrandChild Father father Father GrandFather Rather familiar isn t it Suppose now that you ask the goal 2 father Sue Jim You hit lt Enter gt the inference engine takes the goal and sees that the predicate father in the goal matches with three facts in the program So it has three chances to find a match for the goal Because it works from top to bottom it is going to start with the first fact father Buill John to see if it matches the goal But before it does so the engine puts a pointer at the second fact father Pam Bill This pointer is called a b
165. cate outside its scope then you can indicate where to find it by preceding it with the name of the class or the object 9 3 Overview section keywords In this section you will find a short introduction to some of the keywords for sections in VIP We give a short overview the next sections give a comprehensive description Not every keyword is described here there are other keywords in Visual Prolog but the ones mentioned here are for now the most important ones No matter how I write them here I have a stubborn word processor that loves to use UpperCase keywords in your program must always be lowerCaseldentifiers Constants The keyword constants is used to mark a section of the code that defines commonly used values in the program code For example if the string literal PDC Prolog is to be used in multiple locations throughout the code then you can declare and define a mnemonic a short easily remembered word for this string thus constants pdc PDC Prolog Note that the definition of a constant ends in a period Unlike a Prolog variable the name of a constant should be a lowerCaseldentifier that is a word starting with a lower case letter 139 Domains The keyword domains is used to mark a section declaring the domains that will be used in the code A domain describes e g a range of values for variables or a kind of predicates Well known domains are integer real and string There are many variations for the
166. cates ondeleteperson window menultemListener clauses ondeleteperson Source MenuTag person getAllNames Nameslist b_true vpiCommonDialogs listSelect Select a name NamesList 0 Name Index person deletePerson Name ondeleteperson _ The pattern looks like the way we coded onShowSingle First we get the names then show it to the user She chooses a name and the object is removed from the list Deletion is performed by the predicate deletePerson 1 in the class person This predicate is a public class predicate so I declare it in person cl deletePerson string Name procedure 1 The code for this predicate looks like this deletePerson Name Object Tail createdObjects Name Object getName createdObjects Tail stdlO write Person Name has been deleted n deletePerson Name Object Tail createdObjects createdobjects Tail deleteperson Name createdObjects Object createdObjects deletePerson _ The pattern is like other predicates you take the head of the list of objects compare the name of it with the name chosen by the user and when they match you discard it If the name is not the one that is given then in the second clause we retain the object continue with the tail of the list and upon returning from the that call combine the retained object with the tail without the deleted object Take a closer look at the way I use the list createdO
167. chael empty empty empty This gives a way to build up trees step by step The next program below demonstrates this technique In the program the items to be inserted are given in the clause run In real life the items to be inserted into the tree will come from some external input like a list of items or from a database I created a new project target console en declared the domain and the predicates in main cl domains treetype tree string treetype treetype empty predicates create tree string treetype procedure 1 0 insert left treetype treetype treetype determ 1 1 0 insert_right treetype treetype treetype determ 1 1 0 Then in main pro I added the following clauses clauses create_tree A tree A empty empty insert _left X tree A B tree A X B insert_right X tree A B _ tree A B X clauses run console init First create some one cell trees create_tree Charles CharlesTree create_tree Hazel HazelTree create_tree Michael MichaelTree create tree Jim JimTree create_tree Eleanor EleanorTree create_tree Melody MelodyTree create_tree Cathy CathyTree then link them up insert left CharlesTree MichaelTree Mi2 insert_right HazelTree Mi2Tree Mi3 insert _left JimTree MelodyTree Me2 insert_right EleanorTree Me2 Me3 insert_left Mi3 CathyTree Ca2 insert_right Me3 Ca2 Ca3 and print the result
168. changed the property ReadOnly of both from False into True You will find this property in the Properties List Please complete the dialog save it and build the project 101 deletePerson Choose a Person Chose an ID To Be Deleted Mame Edit ID fed it Figure 7 7 Dialog to delete a person Now we will take care of the menu item that triggers this form I added a menu item in the Task Menu right under the option Add Person I gave it not surprisingly the name Delete Person Clicking this menu item should trigger the appropriate dialog So please go to Task Window win right click and open the Code Expert Click Menu click TaskMenu click id_database and lt Add gt the standard code for the new menu item id_delete person Then go to the code and change the standard code into predicates onDatabaseDeletePerson window menultemListener clauses onDatabaseDeletePerson Source MenuTag NewDialog deletePerson new Source NewDialog show When you now execute the program the form should appear when you click Database DeletePerson But nothing can be done yet with this dialog because we didn t add functionality The first thing we should take care of is to fill the listbox with the names of the people in the database Remember that we want to keep all the predicates that do something with the database in class module person To fill the listbox so the user can choose a name we need a list of all the n
169. chor is active In the picture below you see three arrows which specify that the Left Top and Right anchors are active to the Push Button control 2 Push Button Numbers near such arrows show distances in Dialog Base Units between the correspondent boundaries of the control and the dialog Normally each control has one active horizontal and one active vertical anchor When the dialog is resized then such control is always positioned on the specified distances from the specified boundaries of the dialog The size of the control is not changed When a control has both horizontal or and both vertical anchors active then both horizontal or and both vertical boundaries of the control should be placed on some specified distances from the 29 248 correspondent horizontal vertical boundaries of the dialog Therefore when the dialog is resized then such control is also resized accordingly to keep the specified distances When no one of the horizontal or vertical anchors of a control are active then no one of the horizontal or vertical boundaries of the control are bounded to the correspondent horizontal or vertical boundaries of the dialog Coordinates of such controls are handled on the proportional basis For example when the dialog width is increased onto Delta dialog base units then the X coordinate of the control is also increased but it is increased two times smaller That is the X coordinate of the control is increased on
170. ck on id_ file New gt onFileNew The Figure2 11 Dialog and Window Expert TaskW indow win Code Expert opens an editor with the Prolog code in TaskWindow pro at the place where it put the code The window is a text editor the cursor is placed at the following piece of code predicates onFileNew window menultemListener clauses onFileNew _Source MenuTag Now it is time to build the application Close the editor with TaskWindow pro click on Build Build in the Task Menu When the IDE asks you if you want to insert a certain pack simply say yes When the message Project has been built appears in the Message Window go to the Project Tree right click on TaskWindow win and use the Code Expert to go to the inserted code for onFileNew This may seem Too Much Work and to be honest it is but it makes you feel at home in the IDE and it is wise to build the project frequently In the editor you modify the snippet to clauses onFileNew Source MenuTag NewForm form1 new Source NewForm show Please take care to type the modification exactly as it is shown here Take care of upper and lower case letters and of the underscores commas and full stops Did you notice that you have to remove the underscore in front of Source 21 Close the editor and save the changes Then build the program again by choosing the option Build Build from the Task Menu as in figure 1 4 After that execute the program and
171. code showSinglePerson Name getData Name createdObjects Number stdIO write You have selected Name with number Number stdIO nl showSinglePerson Name class predicates getData string Name person ObjectList unsigned Number determ 1 1 0 clauses getData Name ObjectsList Number Head Tail ObjectsList Name lt gt Head getName getData Name Tail Number getData _Name ObjectList Number Head _ ObjectList Number Head getNumber The predicate showSinglePersson 1 gets the name and then calls another predicate getData 3 to retrieve the data Predicate getdata 3 shows a pattern that should be familiar by now It takes the list retrieves the name of the first object in the list and compares it with the name given as argument If the two names differ getData 3 continues with the rest of the list If the names are the same then the second clause is invoked Here we take the first object in the list that is the one we were looking for the names are equal and we retrieve the other data i c the number Then showSinglePerson 1 writes these data to the Messages Window Build and execute the program to verify that it really does what it says Finally we need code to delete a person I assume that by now you are getting familiar with the program so I will give the code without much explanation Change the standard code for the TaskMenu item DeletePerson into 131 predi
172. controls An Edit Control reserves an area for text editing e g names numbers text constants etc Editing text strings with multiple lines is possible A List Box allows you to view a list of elements and to select one or several elements from this list A List Button control serves for choosing one from the pop out set of the alternatives revealed by pressing the button Pressing the button again pops the list back in again A List Edit control allows single line text editing like an Edit control or selection from a list of elements The Vertical Scroll Bar and the Horizontal Scroll Bar controls serve to select a value within a scale of values A Group Box control serves for assembling together a number of controls in a functional group with a Group Name Its only function is a visual one An Icon control reserves an area for an icon image A Custom Control can be either IDE Controls created in the project it can be VPI user defined control with a window class defined by vpi classCreate or it can be controls imported as COM packages from DLLs VBXs or OCXs 242 Inserting Controls To insert a new control in a Dialog youfirst select it and then click on the place in the Dialog where you want the control to be There are three ways to select a control click on the icon in the Controls Toolbar select the Control in the Controls menu of the IDE select the Control in the popup menu that appears when you righ
173. ct Item dialog opens figure 4 4 In the left pane click on Package In the field Name enter pack01 The field Parent Directory should be empty The form should look like in figure 4 4 Click on the lt Create gt button to create the new package Create Project Item d xj Name pack i COM Package Parent Directory 3 Class Browse Interface pa Dialog Used Packages J Public Add Control 5 ProDir pFc Draw Control Private _Delete 21 Toolbar 53 ProDir pfc exception D Cancel Help Figure 4 4 Create a new package Take care where you place the package If you want the package to be in the project root that is the main folder for your project leave the parent directory empty as in figure 4 4 One way to start with an empty parent directory is to select the project tree root before clicking File New in existing 45 package as shown in figure 2 1 It is also possible to search for the right directory after you click File New In that case you click the Browse button at the right and the Create Project Item dialog shows you the directory structure on your computer From here you may choose any directory that you want But take care Preferably you should choose a directory within the sub directory of your project unless you have good reasons not to do so Step 2 Create a form and store it in the package Now that you have created a package pack01 you will
174. cted to the menu option GetString in the submenu of Messages in the TaskMenu predicates onMessagesGetstring window menultemListener clauses onMessagesGetstring Source MenuTag AnswerString getString Title Question Preset Answer note Your answer is AnswerString onMessagesGetstring Source MenuTag note You clicked the lt Cancel gt button 35 GetString 3 takes three strings as an input a Title a Question and a Suggested Answer GetString 3 uses these to show a window with Title Question an edit box filled with Suggested Answer and two buttons labeled lt OK gt and lt Cancel gt The user can accept the Suggested Answer or change it The window closes when the user clicks lt OK gt or lt Cancel gt Getstring 3 returns the string that the user entered There are two possibilities When the user clicks the lt OK gt button the answer string is returned When the user clicks the lt Cancel gt button no string is returned That s why the code contains two lines for the answer It is too early to explain how this works in Prolog this will be dealt with later By the way when you don t have a suggested answer simply use the empty string When the user closes the window in another way the results are as follows When she uses lt Enter gt it is like she hits lt OK gt When she uses the lt close gt button or lt Escape gt the result is like lt Cancel gt This is different from
175. cts that can be in the database Obviously a factVariable can have only one value The mode can be determ nondeterm or single e If the mode single then a fact always has one and only one value If you assert a new value to the facts database then the old values are overwritten You cannot retract a single fact If a factFunctor has mode single then a call of this fact will always succeed If the mode determ then the fact can have zero or one value If a fact has zero values then any read access to it gives fail If a factFunctor has mode determ then a call of this fact can fail when there are no facts or it can succeed If the mode nondeterm then the fact can have zero one or any other number of values If a factFunctor has mode nondetrm then a call of this fact can fail succeed and it can return with a backtrackpoint that can give more solutions When you omit the lt factFunctorMode gt the mode nondeterm is assumed A factFunctor is initialized via the clauses section When you declare the factFunctor facts person string Name integer Length then in the clauses section see below you can add clauses like person John 185 Initially the data base is filled with these facts In this case take care that the expressions in the clauses can be evaluated at compile time Facts declared in the section that begins with the keyword facts are by definition object facts The section in which you dec
176. ctsSectionName An input file must first be created with the save 1 predicate or created by hand in a text editor or another program The file should have each fact on a separate line it must not contain extra spaces and the functors must be written in lower case letters When you use a file that was previously saved with save 1 there should be no problem Comments are allowed both single and multi line comments stdio save 1 save factDB FactsSectionName procedure i Description 202 The save predicate saves the contents of the facts section with the name specified by FactsSectionName into the current output stream stdio save_error 1 save error factDB FactsSectionName procedure i Description The save predicate saves the contents of the facts section with the name specified by FactsSectionName into the current error stream stdio endOfStream 0 endOfStream determ Description Checks if current input stream position is the end of the stream Predicate fails when stream pointer does not point to the stream s end This predicate always fails for undetermined streams 11 5 The predicate stdIO read The predicate read 0 is a general predicate It has as a characteristic that the predicate determines itself what type of input it should read I think it is best to show you a little program I made a new project target console and added these clauses to main pro clauses run console init readExa
177. d If we want to use it it must be enabled To find the place to enable File New go to the Project Tree In the Project Tree you see a folder icon labeled Taskwindow This folder contains everything that is needed to create the Task Window or Main Window when you start the program Please open the folder TaskWindow You open it by clicking on the plus sign in front of the name or by double clicking on the name In the folder you find typical parts of the Task Window of the program that is also called the Main Window See figure 2 8 There are several files and maybe you already see a familiar pattern E g there are four files with the name AboutDialog but with different extensions The file AboutDialog dlg contains the design of the window that pops up when you click the option Help About when you run your program Double click on AboutDialog dlg and the IDE opens a dialog editor that looks very similar to the one that we used to create form1 The other files contain the Prolog code for the AboutDialog window For now we are interested in the menu bar of the program because that is where the option File New resides So you click on TaskMenu mnu This is a kind of prototype of the Task Menu You can compare it to form1 frm that is a prototype of a form If you want to change something in the main menu use the file TaskMenu mnu eS WEE option in the menu so you double click on the ab Ejt oT name
178. d left rotate and right rotate A right rotate will move the first element to the third position the second 219 to the first and the third to the second A left rotate goes the other way around Here are two equivalent visualizations of these processes apple banana banana right rotate gt cucumber cucumber apple apple cucumber banana left rotate gt apple cucumber banana In Prolog rotation can be done analog to swapping elements Take a look rotateRight El E2 E3 Tail stackList stacklist E2 E3 El Tail I assume in this case that the stacklist is a facts variable If not the stacklist can be given as an argument You call the pradicate with a Stacklist that must be right rotated rotateRight OldStackList RotatedStacklist Then the predicate rotateRight 2 may look like rotateRight El E2 E3 Tail E2 E3 El Tail Calculators employing reverse Polish notation use a stack structure to hold values and give a nice last example Expressions can be represented in prefix postfix or infix notations Conversion from one form of the expression to another form needs a stack Many compilers use a stack for parsing the syntax of expressions program blocks etc before translating into low level code For example The calculation 1 2 4 3 can be written down in postfix notation with the advantage of no precedence rules and parentheses needed In postfix notation 1 2 4 3 is writ
179. d a library with pieces of code and refer in your program to the code that you need when necessary Such a piece of code is called a procedure It is like a little program that performs a task and that can be used whenever you need it A procedure has a name and you activate it by naming the procedure in your code This is called calling a procedure Let me give an example Suppose you have to do some arithmetic in your program say finding the sum of two numbers Your program could look like this this is not Prolog or any other program code it is only an example Number 2 Number2 3 Answer Number Number2 In this code we give the variables Number and Number2 a value and then add the two variables and give the resulting value to Answer The variable Answer will get the value 5 Now suppose that there is a procedure called sum that performs additions of two numbers Now your program could look like Numberl 2 Number2 3 sum Number1 Number2 Answer In this code we give the variables Number and Number2 a value and then call the procedure sum After the name sum there is a list of variables between brackets These are called the arguments of the procedure In this case there are three arguments The first two Numberl and Number2 are used to tell the procedure which numbers it should add The third argument is the variable in which the procedure returns the result In this case the variable Answer will hav
180. d how you can should use them In this chapter I have to be quite formal That s the way computers work To make the formal syntax better readable I shall use now and then a kind of notation that places special words between hook brackets lt and gt A word between these brackets has a special meaning I shall try to make these words easy understandable So I shall use the word lt className gt to indicate the name of a class This opens the possibility to describe the name of the file with the code of a class as lt className gt pro When I use a word between lt and gt I shall explain its meaning But most of the time the meaning will be obvious I think I think that it is possible to skip this chapter at first reading But I m not sure about it and reading this chapter gives you a good feeling about the richness of Visual Prolog So chew it 9 1 Declarations and compilation In classic Prolog e g in PIE when we use a predicate we simply do so without any prior intimation to the Prolog engine about our intention For example in the earlier chapters we used PIE and a program about a family The clause for the grandFather predicate was directly written down using the traditional Prolog predicate head and body construction We did not bother to inform the engine explicitly in the code that such a predicate clause was to be expected nor did we tell anything about what it would look like Similarly when a functor i
181. d more persons I suggest that you use persons from your own family because that makes it lot easier to validate whether some person 1s indeed the grandmother of some other person etc Given mother and father we can also define a parent predicate You are a parent if you are a mother you are also a parent if you are a father Therefore we can define parent using two clauses like this parent Person Parent mother Person Parent parent Person Parent father Person Parent In these rules I have chosen that the second argument is the mother father parent of the first argument Now the first rule reads Parent is the parent of Person if Parent is the mother of Person The second rule reads that Parent is the parent of Person if Parent is the father of Person Together these rules state that there are two possibilities either you are the mother or you are the father in both cases you are a parent It is as if the facts offer you two chances If you are not the mother then try and look if you are the father Y ou can also define the parent relation using the semicolon which means logically or Then the rule will look like this parent Person Parent mother Person Parent father Person Parent This rule reads Parent is the parent of Person if Parent is the mother of Person or Parent is the father of Person 60 I will however advise you to use semicolons as little as possible or better not at all There ar
182. d of the list If the head of the list is equal to the name you re searching for then you can conclude that Name is a member of the list Since the tail of the list is of no interest it is indicated by the anonymous variable Thanks to this first clause the goal listMan isMember john john leonard eric frank is satisfied Here is the program class listMan predicates isMember string string determ end class implement listMan clauses isMember Name Name _ isMember Name _ Tail isMember Name Tail end implement 175 clauses run console init listMan isMember john john leonard eric frank le stdio write Success stdlO readChar stdio write No solution _ stdJO readChar If the head of the list is not equal to Name you need to investigate whether Name can be found in the tail of the list In English Name is a member of the list if Name is the first element of the list or Name is a member of the list if Name is a member of the tail The second clause of is Member relates to this relationship In Visual Prolog isMember Name _ Tail isMember Name Tail Appending One List to Another Declarative and Procedural Programming As given the isMember predicate of the previous program works in two ways Consider its clauses once again isMember Name Namel _ IsMember Name _ Tail isMember Name Tail You can look at these
183. d output channel is a different stream Standard there are three streams the input stream the output stream and the error stream Streams have two advantages above the older IO systems where the programmer connects his program directly with input and output devices Streams are automatically connected you don t have to do anything And secondly streams are covered from your program You simply use something like stdIO write and the system takes care of writing your output to the device That means that you can concentrate on the problem that you want to solve with your computer program en can forget about technical details of input or output Besides in this way it is easier to transport programs to other computer with probably other implementations 194 A stream is thought of as a sequence of bytes A byte is a sequence of eight binary digits so a sequence of bytes looks like 11001010 01010101 01010100 11010101 01010010 10110101 11010101 10101010 11111100 In a text you can think of a byte as a code that stands for one character but that is not always true The spaces that I typed are only for human eyes In a real stream they are not there A stream like this one looks impressive but to read it you need some interpretation What does 11001010 mean in plain English To solve questions like these and to create a universal standard for interpretation several solutions have been proposed The first one as far as I know was the
184. d predicates look almost the same but they are very different Technically a functor represents a logical function that binds several domains together In simpler words a functor is a mechanism that teaches the Prolog Engine how to put together data from its component parts It effectively puts parts of the data into a common box That s why I said before that a functor is for storing and retrieving data values You can also retrieve the parts subsequently whenever required like seen in the previous examples It may look like a Prolog fact or a predicate call but it isn t It s just a piece of data which you can handle in much the same way as a string or a number Please note that a functor has nothing to do with a function in other programming languages It does not stand for some computation to be performed It simply identifies the compound domain and holds 82 its arguments together When you know the programming language Pascal then you may compare a compound domain to the record type in Pascal When you study the above example you will notice that nothing special needs to be done when logical variables are used to stand in for data represented by functors The logical variable that is used to stand in for such data is written just like any other logical variable a word starting with a capital letter Thus if we take a look at the grandFather predicate in the example in this tutorial you see that nothing has changed when you co
185. d the computer can then add 1 to it for the Head and bind Length to 3 So how is the middle step executed That step was to find the length of 2 3 by satisfying the subgoal listlength length_of 2 3 TailLength In other words length_of calls itself recursively This goal matches the second clause binding 3 in the goal to Tail in the clause and TailLength in the goal to Length in the clause Recall that TailLength in the goal will not interfere with TailLength in the clause because each recursive invocation of a clause has its own set of variables So now the problem is to find the length of 3 which will be 1 and then add 1 to that to get the length of 2 3 which will be 2 So far so good Likewise length_of will call itself recursively again to get the length of 3 The tail of 3 is so T is bound to and the problem is to get the length of then add 1 to it giving the length of 3 This time it s easy The goal listLength length_of TailLength matches the first clause binding TailLength to 0 So now the computer can add 1 to that giving the length of 3 and return to the calling clause This in turn will add 1 again giving the length of 2 3 and return to the clause that called it this original clause will add 1 again giving the length of 1 2 31 Confused yet I hope not In the following brief illustration we ll summarize the calls We ve used subscripts to indicate that similarly named varia
186. data type is a tree Figure 12 1 Crucially each branch of the tree is itself a tree that s why the structure is recursive Figure 12 1 shows an example of a tree relating members of a family You may read this tree as Cathy is a child of Michael and Melody Michael is a child of Charles and Hazel et cetera Maybe you are used to draw a diagram like this with the parents above the children but upside like in figure 12 1 is equally possible Melody Trees are important for sorting but are also widely used in problem solving e g in Artificial Intelligence where you use a tree like structure to find a solution in a specific problem domain A tree consists of so called nodes and links In figure 12 1 the rectangles represent nodes and the arrows represent links Nodes have names to identify them So you can say that node Cathy is linked to node Michael and node Melody When we talk about trees in treetalk we say that Michael is the left link of Cathy and Melody is the right link of Cathy Cathy is also called the parent node of Michael and Melody and in reverse they are the child nodes of node Cathy On its turn node Michael is the parent node of nodes Charles and Hazel and they are the child nodes of node Michael In A tree every node has at least a parent node and a child node Exceptions are the topmost node of the tree it has no Figure 12 1 Part of a family tree 225 parent and the bottom nodes they have no children The top node
187. data of all persons Although the program doesn t do much it only generates objects 126 e ShowSingle to show the data of a specific person DeletePerson to delete a person Please add these options to the Taskmenu and have the Code Expert generate the standard code for each of them We ll come back to these menu items later on The data will be stored in the objects that are generated by the class person In the definition file person pro I add two kinds of facts There are the object facts name and number that contain the data of each object Next to these there are the class facts createdCount and createdObjects The class fact createdCount takes care of counting the number of objects that have been created The class fact createdObjects is a list that contains the created objects Here are the declarations class facts createdCount unsigned 0 createdObjects person facts name string number unsigned 0 Pay attention to the class fact createdObject The declaration says that the type of it is a list of person but how does the compiler know what type that is The answer is shown in the class declaration The first line in person cl says class person person It tells the compiler that this class person is of type person This means that each object that is created in this class can and will react when you call one of the object predicat
188. declaration then you can reach them as part of an object lt objectIdentifier gt lt constantName gt with a colon between lt objectIdentifier gt and lt constantName gt But when you declare constants and domains in the interface file they are not seen as object members In this case the interface is also a global scope in which constants and domains can be defined but these constants and domains defined in an interface are not part of the type that the interface denotes or of objects having this type Such domains and constants can be referenced from other scopes by qualification with the interface name lt interfaceName gt lt constantName gt or by using an open qualification See section 9 10 159 Chapter 10 Recursion lists and sorting In this chapter I introduce the concept of recursion and the list Both are very much used in Prolog programming As a matter of fact you have seen both in the previous chapters but here I will bring out the details that you should know In the first section I turn to recursion The second section is on lists and in the third section the two will meet to show some powerful possibilities In section 10 4 I introduce the class list that contains many handy predicates for list manipulation The fifth section is on sorting as sorting shows you some nice applications of recursion and lists I should warn you beforehand though that in a certain way this chapter is superfluous Every example
189. dicate father Pam BilP in the program The second sub goal was evaluated and failed as the mother predicate in the program did not match The third sub goal is under evaluation It is about to evaluate the first father predicate in the program so it set a backtrack point at the second father predicate The partial program look like mother Bill Lisa father Bill John Backtrack point 1 sub goal gt father Pam Bill lt backtrack point third sub goal father Jack Bill By now it may seem very confusing so I suggest that you use two colored ball pens to represent the backtrack points and move them according to the creation of backtrack points The engine now tries to use the first father clause to solve the third sub goal but that fails because the first arguments do not match i e John does not match Bill Therefore it backtracks to the second clause but before it uses this clause it creates a backtrack point to the third clause Move your ball pen The second clause also fails since John does not match Pam so the engine backtracks to the third clause No new backtrack point remove the ball pen This also fails since John does not match Jack So the two premisses of the rule did not bring a solution Does that mean there is no solution at all No there are still some chances to find a solution because there is still a backtrack point left The 67 engine backtracks all the way
190. dth will automatically be set to the number of places needed Nor has a hyphen effect when the number of characters in the actual value is greater than width value lt 0 gt The zero before width means for values that zeros will be added until the minimum width is reached If lt 0 gt zero and lt gt hyphen appear the 0 is ignored lt width gt The lt width gt is a natural number that is it is a positive decimal integral number specifying a minimum field width If the number of characters in the actual value is less than width value then the required number of space characters will be added before the value or after it if the lt gt field was set No changes occurs if number of characters in the actual value is greater than the width value 205 lt precision gt The point with the following unsigned decimal integral number can specify either the precision of a floating point value or the maximum number of characters to be printed from a string It depends on the matching variable lt type gt Specifies other formats then the default for the given variable For example in the type field you can give a specifier that says an integer must be formatted as an unsigned The possible values are f Format reals in fixed decimal notation such as 123 4 or 0 004321 This is the default for reals e Format reals in exponential notation such as 1 234e 002 or 4 321e 003 g Format reals in the shortest of f and e format
191. e empty function will return true only if there is nothing in the waiting line In Prolog the list is not really a convenient way to implement a stack The trouble is in the push The program needs to travel all along the waiting elements to the end of the line before the element can be put in place That s why Pll use a fact database to implement a queue There is one restriction in VIP the fact database cannot be empty as a queue can So if you implement a queue in a fact database be sure to enter a dummy element at the start of the program When there is a fact database you can use the standard procedures that work on a database for simulating or implementing a queue Let s assume that we need a queue with customers that are going to pay some amount of money I created a project wit name ch12Queue In it I created a class no objects with the name classQ In classQ pro I declared a database with the fact functor q class facts q string Name real Amount nondeterm 221 Nondeterm means that the fact database can contain zero one or more terms To manipulate the queue I declared four predicates in classQ cl predicates push string Name real Amount procedure 1 1 peek string Name real Amount procedure 0 0 pop string Name real Amount procedure 0 0 listQ 0 Then I added code for these predicates in classQ pro The predicate push is for adding an item at the end of the queue To
192. e several reasons for this e The typographical difference between and is very small but the semantic difference is n no rather big is often a source of confusion since it is easily misinterpreted as especially when it is on the end of a long line e Visual Prolog only allows you to use semicolon on the outermost level PIE will allow arbitrarily deep nesting so it is not very useful When you combine and in one rule it soon becomes confusing which premiss has priority in evaluating the rule Of course this is well defined but with normal human cognition it leads easily to mistakes e There is a simple better way to represent a logical or Take the rule parent Person Parent mother Person Parent Do you see the semicolon father Person Parent You can rewrite this rule as two separate rules parent Person Parent mother Person Parent Do you see the full stop parent Person Parent father Person Parent These two rules should be read as before to see if Parent is parent of Person we first see if Parent is the mother If that is not the case then we see if Parent is the father So in fact the two rules are combined with a logical or Try creating a sibling predicate And ask the inference engine to find siblings Did that give problems You may find that siblings are found twice At least if you say two persons are siblings if they have the same mother two
193. e execution here As in the previous chapter we shall continue to use the PIE environment to develop and learn Prolog We shall get into the Visual Prolog IDE in the later chapters 6 1 Domains In chapter 5 all the people were represented as Bill John and Pam etc To human beings Bill John and Pam are just the names of the individuals In Prolog we take a different look at data To a programming language as Prolog Bill is a sequence of letters and maybe other signs delimited by double quotes Such a sequence is called a string A string is a way to represent a value In this case Bill is the value that could be assigned to the variable Name Such a way of representation is called a data type There are several data types available in Prolog To mention a few char this type allows values that consist of one character between single quotes Examples are a b string a string consists of a series of characters between double quotes Examples are this is a long string and This is a string with amp strange characters integer an integer is an integral signed number When the sign is omitted it is assumed to be positive e unsigned values of the type unsigned are integral unsigned numbers real values of the type real are signed numbers that may contain a decimal part A data type describes the values that may occur You may think of a data type as a set of possible valu
194. e left subtree so we recursively search the left subtree in the same manner Similarly if it is greater than the root then it must be in the right subtree so we recursively search the right subtree If we reach a leaf and have not found what we were looking for then the address is not where it would be if it were present so it is not in the tree at all Every time you look at a cell in a binary search tree during a search you eliminate half the remaining cells from consideration and the search proceeds very quickly If the size of the tree that is the number of nodes were doubled then typically only one extra step would be needed to search it The time taken to find an item in a binary search tree is on the average proportional to log N or in fact proportional to log N with logarithms to any base To build the tree you start with an empty tree and add items one by one The procedure for adding an item is the same as for finding one you simply search for the place where it ought to be and insert it there The algorithm is as follows 1 If the current node is an empty tree insert the item there 2 Otherwise compare the item to be inserted and the item stored in the current node If the item is less than the item in the current node insert it into the left subtree Otherwise insert it in the right subtree In Prolog this requires three clauses one for each situation We suppose that there is already a tree that can be descri
195. e cece eens 48 4 5 Reaching code via events sedas e e eee cette a ene a 50 Chapter 5 Fundamental Prolog 0 0 ee ee teen eben teen ene ia 53 RI A E Ses 53 5 2 PIE Prolog Inference Engine o0o ooooooororrrrr nett rnare 57 5 3 Extending the family theory 0 0 0 cece eee e tenn nents 60 5 4 Prolog is a programming language 0 ccc ent e eens 62 5 5 Program Controls risa Seis Batre eaters cea Pere Lae a Rage whe giles weak pace CAPE A 62 5 5 1 Einding a Mat A Oe alate Dares DES 62 2 Solving Theodor a ae 63 SS AAA O OS 2 aa eines alas 64 5 54 Backtrackin ges nc eo ices a que 64 S OVRECULSI ON eee ores Atte Pera bate eels hte Staelin Yh te Segre aaa hates 72 5 1 Side A euied iana i a i bids BUA dug Maat aunhee a ele Aner has 73 5 8 Conclusions a a ale ett abate ania aust Maa A 75 Chapter 6 Data modeling in Prolog ooo ooooocorrorrrrrr eee n teen e eens 76 OA DOMAINS A nb alee ele bays ee ane TION RAAT Ee Se HE OSS 76 6 2 Improving the Family Theory 0 unauna nearne TT 6 3 Compound domains and functors 0 0 cette enna 78 6 4 Using TINO eta need Set RAS GAAS eer Se va ed ee a ee ek Ades 80 6 5 Functors and Predicates 0 00 teen tenn ene eens 82 6 6 Functors as arguments 6 cence ene nen eens 83 6 7 Recursion Using Functorss jcc aoa vied binge Sa te Pata a Pande we Ales RE 85 6 8 Strategies for Using Functors 0 0 rr 85 6 9 Conclusio ea
196. e code Build the project and when the project is built select Task Window win right click and choose the Code Expert In the Code Expert open the folder Menu open TaskMenu open id messages and lt Add gt the code for id_messages_writenotel See figure 3 4 29 a ProDir o TaskwWindow HHE Toolbars Show AL C Handed Unhanded By AboutDialog dig pHa Window BB AboutDialogi mE Menu B AboutDialog pro 8 3 TaskMeru 3 Taskwindow d HEH id fil Task indow i H id_edt D Taskwindow pack aa E taskWindow ph D ll All B Tee ei id_help E Taskwindow win 3 Sciolbar Eg TaskMenu mnu i Control 2 Projectleon ico H Key 33 main cl H Mouse mann pack 4 09 Miscellaneous 3 man ph 3 man pro resourcel dentfiers i id_messages_wntenole1 Delete fonMessagesWrileratel Set Figure 3 4 Adding code to a menu option Then double click on 1d messages writenotel gt onMessagesWriteNotel and the IDE opens the editor where you can enter the code to produce a message to the user In the editor you see the code that is inserted by the IDE It looks like predicates onMessagesWritenotel window menultemL istener clauses onMessagesWritenotel _ Source MenuTag Change it into predicates onMessagesWritenotel window menultemlL istener clauses onMessagesWritenotel _ Source MenuTag vpiCommonDialogs note This is a message The last line tells Prolog to use the procedure with the name note fro
197. e for reading from files class Inputstream_console for reading from the console As this is an introductory text I shall restrict myself to the classes Inputstream_file and Outputstream_file The class Inputstream_file contains predicates to open a file in one of the three modes binary ANSI Unicode the class Outputstream_file contains predicates to create a new file in a certain mode to open an existing file in a certain mode and to append to a file So generally speaking the predicates to handle input and output are in a hierarchy that is depicted in figure 11 3 When you are looking for a predicate look in the class or its interface that is as high as possible in the hierarchy given the type of predicate you are looking for Inputstream Inputstream Inputstream _console _file Figure 11 3 The class hierarchy of Stream Ouputstream Ouptstream _console Before I go into the details of the predicates here s an example program It will give me the opportunity to indicate some characteristics The program is from the VIP helpfile but is slightly adjusted The program will copy the contents of a file to another file The project has target console and consists of a class copystream that contains predicates to copy the contents of one file to another In the file main pro we call the predicates from copystream as you can see here clauses run console init copystream copy
198. e of the Public or Private radio buttons to specify whether the generated interface and class which handle the form will be public or private to the package in which the form is included If you check the Public radio button then the include directives for the generated interface and class will be added to the header file in figure Al 1 main ph of the specified package In this file these lines will be inserted exported interfaces include myForm i exported classes include myForm cl Since the package header file will be inserted by the include main ph directive in all places where the package is used the result is that in all these places also the include myForm i and include myForm cl directives will be seen This means that the predicates that are declared in the 239 interface and class that handle the myForm form are callable from any place in the project and hence they are public If you check the Private radio button then the include directives will be added to the package implementation file in figure A 1 main pack of the specified package In that file these lines will be added private interfaces include myForm i private classes include myForm cl These include statements are only seen within the package itself Therefore the interface and the class declarations will not be seen outside of the package implementation Hence the interface and class that handle the myForm
199. e predicate with the anonymousIdentifier in the place of the Age You met the anonymousldentifier already in chapter 7 in the simple database As this is a book for beginners I shall leave out the ellipsis Keyword Keywords are special words that have a specific meaning They are used by the compiler You can only use a keyword in the way and with the meaning as it is intended by the makers of VIP You cannot use a keyword in any other way e g as a variable name VIP knows the following keywords The meaning of some of them will be explained in this chapter align and anyflow as nondeterm bitsize or class clauses constants constructors open div delegate domains determ digits do predicates procedure else erroneous externally end quot facts from failure foreach reference rem resolve goal guards supports single implement inherits interface if then to language multi mod monitor For now don t bother too much about the meaning of these keywords just remember never to create classes predicates or variables that have one of these keywords as their name All keywords except as and language are reserved words Notice that div mod rem and quot are also reserved words but these are not keywords but arithmetic operators See below The keywords guards and monitor are at the moment not used in the language but are reserved for future usage Comment A comment is a part of your program code th
200. e program to open this module Please go to the file person pro and change the code at the beginning into implement person open core list This completes the code for person But the program misses yet some code to connect the form that gathers the data with the predicate that adds the record to the database It seems a good idea to relate that code to clicking the lt OK gt button in the form So first insert some standard code for clicking the lt OK gt button Go to the Project Tree and double click addPerson dlg or addPerson frm if you made a Form The Dialog editor is opened Select the OK button and in the properties window click the lt Events gt tab In the list of events choose Clickresponder and in the value field choose onOkClick Now double click on onOkClick to go to the code editor Change the inserted standard code into predicates onOkClick button clickResponder clauses 99 onOkClick _ Source button defaultAction Name name_ctl gettext Weight toTerm weight_ctl gettext Length toTerm length_ctl gettext IQ toTerm iq_ctl gettext person addPerson Name Weight Length IQ What is happening here is in fact quite simple I extract in four subsequent statements the values that are in the four Edit Fields in the form and put the values in four variables With these four variables I call the predicate person addPerson Name Weight Length IQ The
201. e steps 1 Create a new project lt ProjectName gt with strategy console Add a class lt ClassName gt where the clauses the code can reside Uncheck creates objects Enter the clauses the definition for the predicate s in the file lt ClassName gt pro Enter the declaration of the predicate s in the file lt ClassName gt cl Change the run predicate in the file main pro so that the predicate in lt ClassName gt pro is called i This section is from the same tutorial as section 10 2 169 6 If necessary add the needed write statements Don t forget the statement _ stdIO readChar at the end otherwise you won t be able to read the results Writing Lists If you just want to print the elements of a list here is a program that does the job This is the declaration predicates write_a_list integer end class These are the clauses clauses write_a_list Ifthe list is empty do nothing more write_a_list H T Match the head to H and the tail to T then stdio write H stdio nl Write the element that is the head of the list write a_list T and continue with the tail end implement run console init writelist write_a_list 1 2 3 I gave the class the name writelist _ stdlO readChar The program neatly writes the elements of the list to the console Here are the two write_a_list clauses described in natural langu
202. e text you want There is another field in the properties list that you should pay attention to It is the first field the Name Field Here you find the name that will be used in the program to refer to this Push Button By default the button gets the Name pushButton ctl because it is a control with the name Push Button But you will want to change that to make your program better readable You could change the name into the same name as you entered in the Text field In that case don t forget to add the extension ctl to indicate that it is a control You may want to look at the other options in this list Later on when the program is finished go back to the query frm and change some other attributes See e g what happens when you change the attribute Visible into false Hix button pushButton_ctl z Representation Fact Variable Text Push Button Lack to Container False Container none x 116 N 16 Width 48 Height 12 Left Anchor True Top Anchor True Right Anchor False Bottom Anchor False Enabled True Visible True TabStop True Style Regular Properties Figure 4 6 properties of PushButton_ctl In order to put the form inside a package be sure that the package folder is selected before going to File New For the name of the package and the form choose names that have some meaning For instance if your package holds diagrams name it plots if it holds computer graphics
203. e that the first definition of lt unsigned integer gt doesn t work very well An unsigned integer number can have any number of digits so this definition will never show every possibility That would take an infinite number of expressions The solution for this problem is recursion In BNF you can create recursive definitions For an integer the definition becomes lt unsigned integer gt lt digit gt lt digit gt lt unsigned integer gt The definition now says that an unsigned integer can be one lt digit gt or it can be a lt digit gt followed by an lt unsigned integer gt In the first option lt unsigned integer gt is defined as one lt digit gt so this second option can be read as saying that an lt unsigned integer gt is a lt digit gt followed by a lt digit gt Thus an unsigned integer can be two lt digits gt at a row But then the second option can also be read as saying that an lt unsigned integer gt is a lt digit gt followed by two lt digits gt You can repeat this infinitely This is called a recursive definition and in fact it says that an lt unsigned integer gt can be a sequence of any number of lt digits gt Now let s go back to the list and try to give a recursive definition lt list gt lt element gt lt element gt lt list gt This says that a list can be the empty list or it can be a single lt element gt between square brackets or it can be an lt element gt followed
204. e the sorting of an empty list selectsort When the list_to_be_sorted is not empty we 1 take the smallest element out 2 sort the remaining elements and 3 put the smallest element in front of the sorted list In Prolog 184 selectsort UnsortedList SmallestElement SortedRestOfList smallest UnsortedList SmallestElement find the smallest element remove SmallestElement UnsortedList UnsortedRestOfList take the smallest element out selectSort UnsortedRestOfList SortedRestOfList sort the rest of the list To find the smallest element in a list I use the recursive predicate smallest The declaration is smallest integer integer procedure 1 0 It takes a list and returns the smallest element I use three clauses The first is when the list consists of one element smallest Element Element When there are more elements I take the Head of the list then find the smallest element in the Tail and compare the Head with it When the Head is smaller it clearly is the smallest element if not the other one is the smallest In Prolog smallest Head Tail Head smallest Tail Element Head lt Element smallest _ Tail Element smallest Tail Element smallest _ 0 The last clause is a catchall Probably I m doing something wrong because that clause will never be reached But when I leave it out the compiler complains that smallest 2 is not a procedure F
205. e the value 5 after the procedure has been called Of course this is a silly example because there are better ways to perform an addition but I hope you get the idea Groups of procedures are kept together especially when they are related in some way Groups of procedures are brought together in so called classes You may think of a class as a set of procedures that you can call from inside your program In this chapter you will meet the class vpiCommonDialogs that contains procedures for as the name says common dialogs for communication with the user Visual Prolog offers many of these classes They are made available when you install VIP and the IDE uses them in compiling and linking You don t have to indicate where these classes can be found the IDE knows where to look One final remark In Prolog there is a shorthand notation for procedures For the procedure sum in the example you can write sum 3 indicating that the procedure sum uses three arguments The name and the number of arguments are a unique identification of a procedure so everybody will know exactly which procedure you mean This is necessary because in the libraries that come with Prolog sometimes the same name is used for different procedures These procedures then have the same name but a different number of arguments 3 2 Writing messages Messages can be generated anywhere in your program Here I will generate them by using the Taskmenu Of course th
206. e user What we did here is a general principle that is applied in OO programming The information about the name is stored in the object That is the place to get it when you want to write the name the object is the one that should provide it So you ask the name with a call to getName The value of the number of objects is stored in a class fact so the class is the one to ask the number of objects created with a call to person getCreatedCount note the double colon By the way of course you don t have to use the code output my Write getName You can use output myWrite Name but I thought it to be more elegant to use consistently the get predicate to retrieve a value 8 6 Keeping track of the objects a simple OO database In the previous sections the programs created objects and that was it In this section I like to extend this We are going to use objects to store data and retrieve these data when we need it by asking the objects This is in short the principle behind an object oriented database By now you know how to create objects and how to assign data to their attributes The next question is how to retrieve these data For retrieving the data you need to retrieve the object How to retrieve an object There are many answers to this question E g one could use the fact that when an object is created it is bound to a variable This variable acts as a pointer to the object One way to find the object is to
207. e written as tree Charles empty empty This indicates that the node has name Charles and that its left and right subtree are empty This gives a way to recognize a leave a node with two empty subtrees is a leave The subtree starting with Michael can be written as tree Michael the name of the root node tree Charles empty empty the left subtree tree Hazel empty empty the right subtree the finishing bracket Of course we can see that the left and right subtrees are only leaves but the structure is the same as for the other nodes The integral tree from figure 12 1 can be written as tree Cathy tree Michael the node of the left subtree tree Charles empty empty the left subtree of the left subtree tree Hazel empty empty the right subtree of the left subtree tree Melody the node of the right subtree tree Jim empty empty the left subtree of the right subtree tree Eleanor empty empty the right subtree of the right subtree This is indented here for readability but Prolog does not require indentation nor are trees indented when you print them out normally Another way of writing this same data structure is tree Cathy tree Michael tree Charles empty empty tree Hazel empty empty tree Melody tree Jim empty empty tree Eleanor empty empty 227 You can see that the structure is defined by the functors the commas
208. eck if the stack is empty before you peek or pop and take appropriate action To use these predicates I made a new menu called stack in the taskmenu in the taskwindow and added several options like Push Peek Pop The code for these options I used the code expert to generate the standard code is predicates onStackPush window menultemListener clauses onStackPush Source MenuTag Element vpiCommonDialogs getString Give a string input a string DefaultString stackprocs push Element stdIO write Element Element has been pushed stdIO nl onStackPush Source MenuTag predicates onStackPeek window menultemListener clauses onStackPeek Source MenuTag stackprocs peek Element stdIO write the peeked element is Element stdIO nl onStackPeek Source MenuTag predicates onStackPop window menultemListener clauses onStackPop _ Source MenuTag stackprocs pop Element stdIO write the popped element is Element stdIO nl onStackPop Source MenuTag I guess that by now you can create the code for the other options yourself It looks very much alike There are more possibilities Rotate then the topmost items are moved on the stack in a rotating fashion For example if n 3 items 1 2 and 3 on the stack are moved to positions 2 3 and 1 on the stack respectively Many variants of this operation are possible with the most common being calle
209. ed control It is called the selection frame To create an extended selection that contains more than one Control there are two ways that will be familiar to you The first way is to move the mouse pointer to the point in the dialog where you want one corner of the selected area to be and press and hold the left mouse button While holding the left mouse button move the mouse until all the desired controls in the dialog are inside the selected area Then release the mouse button All controls inside the specified rectangle will be selected The second way is when one or more controls are already selected You can include one extra control in the selection by holding down the Ctrl key and click the control In this way you can also deselect individual Controls To deselect the control or group of controls click outside the selection frame Deselected controls lose their frame Resizing Controls If you simply drop a control in the Dialog it has a default size To re size a control first click inside the control area to select it Then move the cursor to the sizing handles on the selection frame A new shape of cursor indicates the direction in which you can re size the control Press and hold the mouse button drag until the selected control has the size you want then release the mouse button You can also change the size of a control by changing the values in the Control Properties List Select the Control and then change the values for X and or Y to c
210. edicates are special because e they always succeed e they do more than simply evaluate to true or false In Prolog every predicate is evaluated In the evaluation the predicate may come true that we call succeed or it may turn out to be false untrue in that case we say it fails Now any predicate can in 73 principle succeed or fail But some predicates can only succeed or fail You already encountered the predicate fai that always fails There are predicates in Prolog that are used for their side effects These predicates always succeed so that aspect is not interesting What is interesting is what they do more than succeeding This is called their side effect Important examples of predicates that always succeed and are used for their side effect are predicates for reading and writing The following goal will write the found ancestors of Pam ancestor Pam AA write Ancestor of Pam AA nl How does it work Well the ancestor call will bind an ancestor of Pam in AA The write call succeeds and as a side effect will write the string literal Ancestor of Pam and then it will write the value of AA Because write succeeded the next sub goal nl is executed The nl call will as a side effect shift to a new line in the output When running programs in PIE PIE itself writes solutions so the overall effect is that your output and PIE s own output will be mixed This might of course not be desirable A very
211. edit window by pulling a side and watch the dimensions in the properties window change The four windows work together but you are free to place them anywhere on your screen Let s take a closer look at the four windows The window titled Controls figure 2 3 is in fact a toolbar It controls E shows icons for every control that can be put on the form On the top row you see from left to right icons for a button a check box T mr a radio button a static text field and an edit field When you move a the cursor over the icons little text boxes show their meaning To fe gt NS el e add a control to the form you click the icon in the Controls Toolbar and then click in the forms editor at the place where you want the control to be Figure 2 3 Controls Toolbar The window titled Layout figure 2 4 is also a toolbar The icons represent different ways to adjust the layout of the controls on the form On the top row you see icons for left center and right justifying When you select a group of controls on the form and then click one of these icons the positions of the controls are justified When you move the cursor over an icon its meaning is displayed Especial handy are the icons in the bottom row they are for equalizing the spacing between controls and for equalizing the dimensions Figure 2 4 Layout Toolbar The window titled form1 figure 2 5 is the form edit window It is a prot
212. efault values to the controls The test mode is activated and deactivated from the Resource menu or from the pop up menu MyDialog 4 Xx Radio Button 2 Radio Button Edit Cancel Help The Dialog in Test Mode Tab Stops When activating the menu or the pop up menu entry Resource Tabstops it is possible to specify to which controls you can Tab to in the dialog When this command is activated a small button with the or appears on controls depending upon whether the control has a tab stop or not By clicking the small buttons it is possible to toggle the setting 250 MyDialog Xi i GroupBox YS ethadio Button hadio Button ethadio Button Specifying Tab Stops for a Dialog To exit the tab stop mode just click in the dialog but outside of any controls Visit Order When the tab stops have been specified for a dialog then it is possible to specify the order in which controls will receive focus when tabbing MyDialog xi GroupBox Zhadio Button 1 Shadio Button 2 tAhadio Button Shush Button Displaying the Visit Order To change the visit order click a small rectangle displaying the visit order number for a control that has an incorrect sequence number and this will bring up another dialog to change this sequence 251 isit Order SS Type Edit Control Constant ide edit Cancel Help EER Order 1 sl Changing the Visit Order
213. eferred to append string FileName Opens Unicode file for output setting the stream position to the end of file append string FileName stream mode Mode Opens file for output setting the stream position to the end of file appends string FileName Opens ANSI file output setting the stream position to the end of file create string FileName Creates a new file for output in Unicode mode create string FileName stream mode Mode Creates Unicode file for output and set data output mode to Mode create8 string FileName Creates ANSI file for output openFile string FileName Opens Unicode file for output openFile string FileName stream mode Mode Opens file for output specifying initial stream mode openFile8 string FileName Opens ANSI file for output 11 8 Files and Directories Next to the predicates now for input and output there is a class with numerous other useful predicates It is the class file with among others the classes directory and file These classes and their interfaces contain predicates for manipulating directories and files You may want to take a look in the help file but for now I leave these classes out of this book 214 Chapter 12 More data structures Stacks Queues and Trees In programming we use data structures to store data so they can be used efficiently That means that for different uses of data we need different structu
214. en beginners can use it But if you think that some things are not understandable please let me know You can reach me at t w de boer rug nl Groningen Autumn 2008 Thomas W De Boer Contents Mntroductiony 23 4 ce A ie ek A eee bette eae DS 7 Chapter 1 The Integrated Development Environment 0 0 c eee eee eens 8 1 1 The Integrated Development Environment IDE 00 0 cece eee eee 8 1 2 Creatingya project VIPS a occa dee aed ata ee bo da aa eed Gna ace Gp acd a Dla 9 Chapter 2 FOrmsys oes A ala hid A Sella Sot 14 DV ICTS ate A LOTTI baci Gb dee Peeks ect Beth vege ros 14 2 2 Enable the Task Menu option 0 ccc cece tent nen teens 18 2 3 In CodeExpert add code to Project Tree item 0 cece eee 20 2 4 What is happening in the background 0 c ccc cents 22 257A MOUSE eVet te teehee nie tee A OS ete ta ae 23 Chapter 3 Simple user interfacing 0 0 0 0 eee nen ene n een eae 26 3 1 About procedures ninia di aed iaa Sine Dit dices 26 3 2 Writing Messages se wus candice E 27 3 3 Getting the user response 6 keene nen tenn eas 34 Chapter 4 A closer look at the IDE 20 nnana 39 41 The IDE in Generals son eating tena eaten ie Wadia Re et ead ed a o eae 39 4 2 TaskWindow in Project Tree 6 0 eee eet ene n ene n eens 42 4 3 Creating a new item in the Project Tree 61 cette eens 44 4 4 The Code Expert and the Dialog and Window Expert 0 0 0 ce
215. en the name is not unique then all the IDs that go with the name should appear When the user selects an ID the corresponding ID and Name appear in the fields at right The user then can click lt OK gt or lt Cancel gt When he clicks lt OK gt the record is deleted When he clicks lt Cancel gt the record is not deleted In both cases the window is closed I shall not use the lt help gt button but you may connect it to an appropriate help text in a message box Now create a new Dialog called deletePerson in the Forms package We will fill it with controls as is shown in figure 7 7 Most controls should be familiar to you except the two boxes beneath Choose a Person and Choose an ID Both are a listbox you will find the symbol in the Controls Toolbar at the second row on the most left place The Name of the Listbox under Choose a Name is simply listbox_ctl that is by default given by the IDE The Name of the Listbox under Choose an ID cannot be the same so I renamed that one to idBox_ctl I also changed the name of the two edit fields at right The one after ID will contain the ID of the chosen record so I changed the Name of that edit field into id_ctl The other edit field will contain the name from the record so I changed the Name of it into name_ctl Both edit fields are only to check the data in the record_to be deleted The user should not be able to change the contents So I
216. en we change the name of Newperson into Thomas Again we ask the name with getName and now it turns out to be Thomas Please notice that a predicate in an object is called by naming the object followed by a single colon followed by the name of the predicate and its arguments This is a possible way to use the constructor and getName and setName although it is a bit stupid to create only objects with the same name and to change the name immediately As you will understand this is only a simple example to show the use of the creator and the other predicates In a serious application you will probably ask the user for a name and then create the object Let s do that Please create a dialog or a form like the one in figure 8 2 enter erson E xj Type a name Create abject Figure 8 2 a form to enter a name and create an object I named this form createPerson Next to the familiar buttons it contains an edit field and a button with the label Create Object The idea is that the user enters a name in the edit field and then clicks the CreateObject button and then the program creates an object with that name To make the form work like that you have to enter code for the CreateObject button In the property list I changed the name of the button into createObject_ctl The edit field I gave the name name_ctl Do the same and close the form editor Now reopen the form in the editor this seems silly but if you don t
217. enu Open that one too and you see the standard menus that the IDE placed in your program The options File Edit and Help are represented here by id_file id_edit and id_help Open the id_file folder and single click id_file_new 20 At the bottom of the dialog a button appears Dialog and Window Expert laskWindow xi labeled Add When you single click this Show E Al Handed Urhandied Heb button the Code Expert adds to your program the Prolog code that will be executed when this option is clicked by the user You don t see this code yet but you will see that the Code Expert gt adds the name of something to id_File New 0 i ie id file_open It added onFileNew and you may think of it 0 id filo_rava oOo d he y as the name of a procedure that will be id_file_vawe_as id_file_est gt onFieE wit executed when you click on the option id_edit File New Or in more technical words 6 2 id_help y 4 13 Scrolbar onFileNew is excuted when the event happens fig Contol that you click the option Flle New H Key As a sign that something was added to your H Mouse 4 43 Miscellaneous program the label on the Add button changes into Delete You can delete the code 1f you want to but please don t delete it now In the program the menu option File New is now e sas connected to the procedure with the name onFileNew To see the added code double JonFileNew cli
218. er defined comparator predicate list sortBy 3 gt sortBy comparator Elem Comparator Elem List sortOrder Order gt Elem SortedList SortedList is the sorted version of List Comparator is an user defined comparator predicate Order specifies the sort order it can be ascending and descending Default is ascending list tryGetIndex 2 gt tryGetIndex Elem Item Elem List gt core positive Index determ Gets the index number of Item in the List This predicates fails if Item is not found 267 e Oat Sl ee i a Sie 54 60 153 este teat tt ieee atin tanks Aone Nana Neat 54 ee ee oe ne a 119 147 Bs ce Pace race E OE 153 SNe erate E E 60 153 Draco doa erase cn athe whiten ach asa aa 68 151 ProDir sewed 2 eae ant Salt wring 40 A IATE IN eset seh Bataan Uae 147 dd Nake oo 149 E 167 DM ad ac 44 A 4g Seale dion ati seta 42 BCU hak IN 44 MCU ised Bee TN Ape SA Rane ee Bakes 44 dl ias 44 AM de a da 44 e e doo ee bo odo 42 A A EES 44 MD RS e ae a 44 A pas ates lente E s 44 DAK cuts cast seat a ech at bade baad aaah ae 42 PMA A cee ay ack wasvamte aire 42 MDL OS sa lavas to Magis 42 DAA A O 44 x Wiha ra gach na eds A dete ek 44 Mola da edo ESA 136 UB sie Aes E otha ues ate 136 a O a a 134 216 AA Pe ee hed Bae 206 ee O 205 lt Add gt button 0 0 0 0 00000 e ee 48 lt argumentlist gt 4 148 149 lt close gt button oooooooo o 35 50 lt
219. ere you find the code for this predicate It says that when new 1 is called the fact name must get the value of the variable Name notice upper and lower case that is implied in the call from outside When you call the predicate new you should give a name for the new object Notice the assingment symbol The expression a B means in words assign to a the value of B The next clause is for the predicate getName The code says clauses getName name When this predicate is called it should return the name that is contained in the fact name And finally there is the predicate setName clauses setName Name name Name When this predicate is called internally in the object the value of the fact name is changed into whatever Name is provided by the call Now the picture is complete We have declared a class person with one attribute that is put in the fact name We have declared a constructor for this class and in the interface person i two predicates to get and set the name Finally we have put the necessary code into the implementation file person pro Please take some time to digest the following We did declare a class but when your program runs the activities are performed by objects When you don t generate an object nothing will happen Also the fact name is a distinct fact in each object and it has a distinct value within each object that is created Also you may have noticced th
220. ered changed after the code expert had generated the standard code 222 predicates onQPush window menultemListener clauses onQPush Source MenuTag NewForm enterdata new Source NewForm show predicates onQPeek window menultemListener clauses onQPeek Source MenuTag classQ peek Name Amount Name lt gt 0 stdIO write q Name Amount is first in the queue stdIO nl onQPeek Source MenuTag stdIO write Queue is empty stdIO nl predicates onQPop window menultemListener clauses onQPop _Source MenuTag classQ pop Name Amount Name lt gt 0 stdlO write q Name Amount is first in the queue and is retracted stdIO nl onQPop _ Source MenuTag stdIO write Queue is empty stdIO nl predicates onQListq window menultemListener clauses onQListq _ Source MenuTag stdIO write The queue contains stdlO nl classQ listQ To enter a new item to the queue I use a form It contains two edit controls named name_ctl and amount ctl When the user clicks the lt OK gt button the content of these two controls are added to the queue Here is the code that you should add to the code of the form predicates onOkClick button clickResponder clauses onOkClick _ Source button defaultAction Name name_ctl gettext Amount toterm amount_ctl gettext classQ push Name Amount 223 stdlO write
221. es In Prolog such a set of values is called a domain The domains mentioned here are called simple data types or simple domains In the case of the names of the people the simple domain was a string A domain offers a set of values for a variable The domain numbers offers a range of numeric values Most of these domains will be familiar to you and offer traditional values that you expect from it Simple domains offer values that are used to describe one characteristic However individuals are represented by more characteristics than just their names What if we need to represent all those characteristics together instead of representing only one In that case we look for a way to represent a This chapter is a slight remake of the tutorial Fundamental Prolog Part 2 by Sabu Francis Tangential Solutions and Thomas Linder Puls PDC 76 set of characteristics It means we need some mechanism to represent compound domains a collection of simpler domains held together in one package 6 2 Improving the Family Theory If you continue to work with the family relation from chapter 5 you will probably find out that you have problems with relations like brother and sister because it is rather difficult to determine the sex of a person unless the person is a father or mother The problem is that we have chosen a bad way to formalize our theory The reason that we arrived at this theory is because we started by considering the relations li
222. es are True and False If it is True then the list control sorts strings alphabetically The default value is True sort You can use the listControl getSort 0 gt predicate to get whether the control has this flag IsSort controlName getSort IsSort true and the listControl setSort 1 predicate IsSort true controlName setSort IsSort to set this flag programmatically StaticScrollbar Possible values are True and False If it is True then the list control shows a disabled vertical scroll bar for the list box when the box does not contain enough items to scroll If it is False then the scroll bar is hidden when the list box does not contain enough items The default value is True shows vertical scroll bar You can use the listControl getStaticScrollbar 0 gt predicate to get whether the control has this flag IsStaticScrollbar controlName getStaticScrollbar IsStaticScrollbar true and the listControl setStaticScrollbar 1 predicate IsStaticScrollbar true controlName setStaticScrollbar IsStaticScrollbar to set this flag programmatically Style Push Buttons and group boxes can have several styles The style determines some of the activities of the control For Push Buttons the style can have the values Cancel OK and Regular Cancel Push buttons with the Cancel style are created by the button newCancel l constructor When a dialog form contains a push button with the Cancel style then this dialo
223. es it should show In MSWindows most of the time we use the file extension the part of the name after the dot for this The first argument indicates that when the window opens it should show files with extension txt The second argument is a list of paired strings Every pair consists of a file type and the filter that is needed to select that file type E g text files have in general the extension txt so the first pair is Text Files txt In this list you specify every kind of file that the user may choose to select from It is wise to offer also All Files After this list you should specify the Title String for the window The next argument the fourth specifies a list of special properties for the window There you see the empty list it indicates that I don t want any special property Argument five is a string with the name of the folder that should be used when the window opens I put an empty string in that place it means that Windows will decide which folder to show On my computer it is the folder My Documents The last argument is _SelecFiles Here getFileName 6 returns the list of selected files The underscore in front means that I don t want to use this variable GetFileName 6 also returns the selected file name as a function and I grab that name in the variable SelectedFiles Normally the user will end with OK an exception is when she hits the Cancel button This is taken care of i
224. es that are mentioned in the interface person 1 In this interface we declare in general the predicates among others to set and get the attributes of an object In this case the object has two attributes facts name and number The name is given by the user so there need to be two predicates setName to set the attribute and getName to retrieve it The number will be calculated by the program so for this attribute we only need a predicate getNumber These are the three predicates to declare in person i So please go there and add this code predicates getName gt string Name setName string Name procedure 1 getNumber gt unsigned Number Next we need to add the code for these predicates in person pro Here it is clauses getName name setName Name name Name getNumber number To create an object we need a so called constructor It is a class predicate that creates the objects that is it reserves some memory for the newly created object and generates a kind of pointer to the object This pointer is bound to a variable and we can reach the object by way of this variable For the 127 construction of the objects I declared the constructor new As the constructor is by definition a class predicate the class creates objects objects cannot directly create objects I put the declaration in the file person cl The declaration looks like this constructors new
225. esents the father in the relation 6This chapter is a remake of the tutorial Fundamental Prolog Part 1 by Thomas Linder Puls PDC 53 I have chosen to represent the persons by their first names and to represent the names by strings In a more complex world this would not be sufficient because many people have the same first name But for now we will be content with this simple formalization With formalizations like the one above I can state any kind of family relation between any persons Or more general with a predicate and arguments I can specify any relation between objects Take a look at some examples lives_in Thomas Groningen is married_to John Claire are examples of relations between two objects Any number of arguments is possible E g one argument is_dumb John But also three objects are possible has_degree Jeannet shorthand Spanish The number of arguments is only limited by your imagination Specifications like these are called facts A fact consists of a predicate a number of arguments between brackets and separated by comma s and it is ended by a full stop a period A fact describes a static relation between objects Facts are useful to know but they get boring as you find out when you watch quizzes on TV To make facts more interesting you need more general relations between objects or groups of objects General relations between groups of objects are
226. eset answer The argument 0 after the list indicates that in the window the first item should be the preset answer If another answer from the list should be the preset answer you should give its index If you don t have a preset answer you should use 1 The variables SelectedString and SelectedIndex receive the answer of the user That is to say when the user finishes with lt OK gt When she uses lt Cancel gt or the lt close gt button no string is returned and the returned index is 1 A last way of communication with the user is the familiar question to select a file In VIP there is a useful procedure getFileName 6 to do this It may seem complex so if you want to skip it for now that is OK In my program I connected the code to the menu option GetFileName and inserted the following code in the program predicates onMessagesGetfilename window menultemListener 36 clauses onMessagesGetfilename Source MenuTag SelectedFile getFileName txt Text Files txt Prolog Files pro All Files Title Please select a file LI woe gt _SelecFiles note You selected File SelectedFile onMessagesGetfilename Source MenuTag note You pressed the Cancel button Getfilename 6 needs the following input arguments For the sake of readability I placed the arguments on separate lines As you are going to select a file getFileName wants a so called filter that indicates what fil
227. etMultiColumn 0 gt predicate to get whether the control has this flag IsMultiColumn controlName getMultiColumn IsMultiColumn true and the listBox setMultiColumn 1 predicate IsMultiColumn true controlName setMultiColumn IsMultiColumn to set this flag programmatically MultiLine Possible values are True and False When this property is True then the edit control allows multiple lines The default value is True allows multiple lines Notice that the VScroll HScroll AutoVScroll and WantReturn properties do not work for single line False edit controls You can use the editControl getMultiLine 0 gt predicate to get whether the control has this flag 260 IsMultiLine controlName getMultiLine IsMultiLine true and the editControl setMultiLine 1 predicate IsMultiLine true controlName setMultiLine IsMultiLine to set this flag programmatically MultiSelect Possible values are True and False If it is True then the list box control allows selection of multiple items using the mouse The default value is False cannot select multiple items You can use the listBox getMultiSelect 0 gt predicate to get whether the control has this flag IsMultiSelect controlName getMultiSelect IsMultiSelect true and the listBox setMultiSelect 1 predicate IsMultiSelect true controlName setMultiSelect IsMultiSelect to set this flag programmatically Password Possible values are True and Fa
228. ew resources both execution time and memory space as possible Data structures are implemented using the data types references and operations on them provided by a programming language There are many formalized design methods and programming languages in which data structures rather than algorithms are the key organizing factor Most languages feature some sort of module system allowing data structures to be safely reused in different applications by hiding their verified implementation details behind controlled interfaces Object oriented programming languages such as C Java and VIP in particular use classes for this purpose Later on you will use classes like these as built in procedures in your program via their interfaces But for now we will program the data structures as if there are no standard classes This will give you more insight into the different structures In this chapter we will take a closer look on lists stacks queues and trees Stacks queues and trees are new you have seen lists before The reason to bring it up again is that I want you to understand that the list is a recursive structure 12 2 Again the list We ve seen that a list is a series of elements between square brackets and separated by comma s That is a good working definition but it is only partial true To give another definition of a list I shall use a way of defining that is called Backus Naur Form or BNF for short In BNF a concept is defined by
229. findall ItemName item ItemName _ NameList returns in the variable NameList a list of all the items If you only want to have a list of the say red items then the call is findall ItemName item ItemName red NameList Finally if you want to see a list of the colors of the coats in your inventory then the call should be findall Color item coat Color ColorList Findall 3 returns every value that it finds So most of the time there are multiples of the same color But in the class list you will find a predicate to remove duplicates when that is necessary Some remarks that may be useful when you use findall 3 The findall predicate builds the list ValuesList by backtracking into the predicate predicateCall until 1t eventually fails ValuesList consists of all instances of the variable ArgumentName which must be an output argument of the specified predicate predicateCall In order for findall to work there must be a declaration of the list domain for ValuesList in the user domain declarations That means that when e g you want a colorlist this must have been declared beforehand Findall 3 generates a list alright but there is a more general way to generate lists It is called list comprehension Let s first take a formal look at list comprehension It is a function that is called in the following way ResultList Exp Gen Mind the double in the expression on the right side List comprehension
230. fines a domain in the current scope that is in the current file class module that is the file in which the domain is defined You cannot use the domain outside the current scope unless you make an explicit reference to the scope where the domain is declared and defined The syntax of a domain declaration in its simplest form is domains lt domainName gt lt typeExpression gt A lt domainName gt is simply the name of the new domain You are free to choose a meaningful name but it must be a lowercaseldentifier The lt typeExpression gt is the name of a standard type e g real or a domain that you declared defined before In general the lt domainName gt is called the child domain and the lt typeExpression gt is the parent domain But take care there are exceptions to this rule lt typeExpression gt There are a few basic types predefined in VIP They are integer unsigned real character and string These basic types are used to create any type that you want to use in your program The wanted types are declared in the domains section and defined by one or more lt typeExpression gt A lt typeExpression gt is an expression that denotes a type A lt typeExpression gt can be integralDomain realDomain typeName compoundDomain listDomain e predicateDomain e typeVariable typeApplication Because this is a book for beginners I shall skip the predicateDomain the typeVariable and the typeApplication
231. formal system for reasoning about things and the way these things are related to each other An example in natural language I can express a statement like John is the father of Bill Here I have two things John and Bill and a relation between these namely that one is the father of the other In Horn Clause Logic I can formalize this statement in the following way father Bill John In the formalization I put the name of the relation in front and put the two things between brackets separated by a comma The names of the two things are in computer slang known as string literals or strings The apostrophes are used to delimit the strings To add some more concepts we say that father is a predicate or a relation taking two arguments Bill and John The predicate describes the relation between two objects in this case Bill and John In the interpretation that I give here the second argument John is the father of the first argument Bill Notice that J have chosen that the second object should be the father of the first I might as well have chosen it the other way around The order of the arguments is the choice of the designer of the formalization However once you have chosen you must be consistent So in my formalization the father must always be the second person You can say that in a computer program you create a world and that in this world the second argument of the predicate father repr
232. fy one rule and use implicitly the facts about fathers and children Again I have chosen that the grandfather should be the second argument It is wise to be consistent like this i e that the arguments of the different predicates follow some common principle In Prolog facts and rules are called clauses With facts and rules we are ready to formulate theories In general a theory is a collection of facts and rules in Prolog we would say a theory is a set of clauses Let me state a little theory that consists of three facts and one rule father Bill John 55 father Pam Bill father Sue Jim grandFather GrandChild GrandFather father GrandChild Father father Father GrandFather With this theory you could answer questions like Is Jim the father of Sue This question can be answered by looking at the facts The third fact acknowledges that Jim is the father of Sue so the answer is yes Is John the father of Sue There is no fact stating that John is the father of Sue so the answer is no Notice however that there is a fact that says that Jim is the father of Sue This implies that John cannot be the father of Sue Smart humans will notice this but this is not the way Prolog reasons In Prolog the answer is no because there is no fact stating that it should be yes Who is the father of Pam This question can be answered by looking at the second fact It says that Bill is the fat
233. g frm file contains a form e win file contains a window task window or conventional PFC GUI window mnu file contains a menu e ico file contains an icon If you continue to explore you might also meet ctl files containing controls tb files containing toolbars e cur files containing cursors bmp files containing bitmaps pictures lib that are libraries All these types are treated in different editors The IDE recognizes the type of file and chooses the right editor That s why it is important to choose the right extension and never change an existing extension If you right click on a file name a context menu will appear with commands that have relevance for that particular node If you double click on a node the corresponding entity will be brought up in a suitable editor All Prolog code is edited in a text editor but windows resources like dialogs and menus are edited in graphical editors As an example open the folder Toolbars and take a look at the first three items ProjectToolbar cl ProjectToolbar pro and ProjectToolbar tb From the list above you know that a tb file is a toolbar When you right click on the name ProjectToolbar tb a menu opens with four options Edit This opens the graphical editor to edit the toolbar It is equivalent to double click the file name Attributes Opens a dialog to change attributes of the toolbar E g you can change the position
234. g Gender This declaration says that with person you can expect two arguments both of type string one is the Name the other one is the Gender Note that for the compiler the string is important as a type declaration the words Name and Gender are there for human readers Note also that this is neither a fact nor a predicate A fact contains only one value while this person contains two values It is also not a predicate because a predicate is used to manipulate data while in person we only store data of one person The declaration states that there is some compound domain called person in the system and that each entity from that domain has two characteristics represented by the logical variables Name and Gender Remark that we talk about values from a simple domain and about entities from a compound domain That is because an entity from a compound domain consists of more than one value Each of those values comes from a simple domain The word person is known as a functor and the variables are its arguments We shall now package our facts using these functors As a compound domain always has a functor we shall henceforth use the term functor in this chapter to represent the respective compound domain For now let us modify the first example of the previous chapter so that we use our newly defined functor person So now we thnk of a person as a set of two values It means that we specify Bill as per
235. g form will provide standard handling of the Esc key the dialog form is simply closed without any additional actions OK Push buttons with the OK style are created by the button newOk 1 constructor When a dialog form contains a push button with the Ok style then this dialog form will execute dialog form validation check when this button is clicked See Dialog Form Validation tutorial on the WEB for more information Regular Push buttons with the Regular style are created by the ordinary button new 1 constructor Such buttons do not initiate any special default handling For group boxes possible values for Style are Group Box Simple Border Horizontal Separator No Border These values define the border type of the group box control The default is Simple Border You can use the groupBox getBorderStyle 0 gt predicate to get the border style 262 GroupBoxBorderStyle controlName getBorderStyle and the groupBox setBorderStyle l predicate GroupBoxBorderStyle groupBox controlName setBorderStyle GroupBoxBorderStyle to set the border style programmatically UseTabStops Possible values are True and False If it is True then the list box control recognizes and expands TAB characters when drawing strings The default value is False does not expand You can use the control getUseTabStops 0 gt predicate to get whether the control has this flag IsUseTabStops controlName getUseTabStops IsUseTabStops
236. gram is executed As aresult you will see a little doing nothing GUI program You might notice that the program looks a bit like the IDE itself This is no coincidence since the IDE is actually a Visual Prolog program Later in this tutorial we will also see what happens if the compiler or the linker detects errors in the build process Browsing Right now we will turn our eyes to the Project Tree in the Project Window and explore that a bit The Project Tree itself is presented in the way that you know from the Windows Explorer This is a standard Windows tree control so you should already be familiar with its usage The Project Tree can be browsed in the same way as you browse folders in the Explorer Now let s take a closer look at the contents of the tree Right now the project tree will look like figure 4 1 ch04p01 prj6 data My Documents 1ProDir 3 Taskwindow E main el 95 main pack main ph H main pro resourceldentifiers i F fife feo Figure 4 1 The Project Tree Because the various folders constitute a tree structure we will talk about the folders as nodes in a tree The top node with the name ch04p01 represents the project and the project directory It is the name you gave in the creation dialog Right below that is the logical node folder ProDir This directory contains as we shall shortly see libraries and library code from the Visual Prolog system The
237. gs One difference is that a Dialog is not resizable at run time With a Form you can specify the border and this specifies if the Form can be resized In this appendix I shall use the words Dialog and Form as if there are no differences A1 1 Create a Dialog or a Form New Form Creating Forms can be created in any project with target GUI Graphical User Interface They can be created both in projects using the Conventional GUI pfc vpi and the Object GUI pfc gui types of UI Strategy Notice that windows can be created only in Graphical User Interface projects which use the Conventional GUI pfc vpi type of UI Strategy See description of forms in the GUI package overview in the PFC part of Help To create and register a new form in a project you need to click the File New menu command The IDE opens the Create Project Item Dialog see figure A 1 that has on the left side a list of items that can be created with this dialog You should select the type Form to create a Form or the type Dialog to create a Dialog When you select type Form the Create Project Item dialog takes the shape as shown in figure Al 1 After selecting the type Form the following fields are shown that you can edit Name In the Name field you should type the name which will be associated with this form in the automatically generated code This name should be a correct Visual Prolog name It should be any sequence of letters digits and underscores beginning with
238. h to change the class of students Only when you want to allow the user to use these new attributes then you 1l have to change the user interface and the report generator accordingly Another advantage is that a module only has to know how to ask for an operation by another module It doesn t have to know how the other module does the job When you ask an object how old it is you don t need to know how the age is calculated by the object So when you decide that calculating the age should be changed you can simply change it within the objects in a class without changing other classes This is known as encapsulation and it is one of the magic points of OOP In the example I used the words modules and classes interchangeable In fact that is not correct but it won t harm you To understand OOP you should think of your computer program as a simulation model of the real world To understand that we have to take a closer look on classes 8 2 More on classes Classes are the core concept of OOP A class is more than a set of objects To begin a class is a declaration When you create a class you give a specification of the attributes and methods that the objects in the class will posses When you declare a class there are no objects The objects are created when necessary when you execute the program The class generates its own new objects Objects of a class are called instances of that class Let us elaborate a little bit more on the exam
239. had to give it the right interpretation So maybe I am telling you things that are not true or only half true 11 1 The console The console is the entry and display device for system administration messages for messages during booting starting the computer and from the system logger that lets you logon It is a physical device you can touch it for simple text entry and text display consisting of a keyboard and a screen On traditional computers the console was a dumb terminal physically consisting indeed of a keyboard and a non graphics screen By way of a serial connection it was connected to the computer This terminal was usually kept in a secured room since it could be used for certain privileged functions such as halting the system or selecting which media to boot from Large midrange systems e g those from Sun Microsystems Hewlett Packard and IBM still use serial consoles On PCs the keyboard the mouse and the screen are connected to the computer or maybe you like to say that they are an integrated part of it and they act as the console But this console is more complicated Not only a mouse is added but the screen is capable of showing graphics as you will notice when you logon to Windows It is more than a simple place to show characters When we talk of a console you should keep in mind that we think of it as a simple device that receives input from the keyboard and shows output a s series of characters on a simple
240. hall only use lists of integers but that should not be a real restriction I created a program with the name ch10SortList with strategy console for sorting In the project I created a class without objects a module with the name sort This module contains all the predicates The declarations are as usual in sort cl and the clauses for the predicates are in sort pro The first method takes the elements of an unsorted list and one by one puts them on the right place in a new list When all the elements have been put in the new list that list will be sorted In Prolog we can do that by taking the Head of the unsorted list put it at the right place in a list that was empty at the start and then recursively take the next element that is the first element indeed the Head of the Tail of the unsorted list This way of sorting is called insert sorting I shall use the predicate insertSort It clearly has two arguments it needs an unsorted list as input and then returns as output the sorted list So my declaration becomes insertsort integer integer procedure 1 0 There are two clauses for this predicate The first one is the sorting of an empty list insertSort That s an easy one when you sort an empty list you get a sorted empty list An alternative could be the trivial sorting of a list with one element insertSort X X When there are more than one elements in the list we can
241. hange it into predicates onOkClick button clickResponder clauses onOkClick _ Source button defaultAction ID id_ctl getText person deletePerson ID What we do is that when the user clicks lt OK gt we get the ID from the ID Edit Field and tell person to execute the predicate deletePerson This predicate should delete the indicated record Let s see how it works In person pro add the following code just before end implementation deletePerson ID retract person ID Name _ stdlO write Person Name with ID ID has been deleted stdIO nl deletePerson _ This predicates tells the program to retract the record with the indicated ID To retract a record means to discard it To make sure the Name in the retracted record is retrieved and shown to the user Finally enter the declaration into the file person cl It is deletePerson string ID procedure i Now save build and execute the program Fill the database add some persons preferably with the same name or with existing names and delete persons Check the results now and then with List Database Don t bother too much about the possibility that the user clicks lt Cancel gt We didn t insert code for that option so when the user hits that button nothing happens That s exactly what the user wants VIP simply closes the Dialog Also there is the bug that when you want to delete a record
242. hange the position Change the values for Width and Height to change the size There is one exception you cannot change the size of icons You can only change their location Moving Controls To move a control or a group of controls first select them Then move the cursor inside the selection frame press and hold the mouse button drag the mouse pointer to a new location and release the mouse Also when one or more controls are selected it is possible to use the cursor keys to move the selection in small steps corresponding to the Grid settings Arranging the layout of the Controls There are three ways to arrange the layout of Controls You can use 245 the Layout Toolbar the Layout menu item in the IDE menu the popup menu that appears when you right click in the Dialog Designer To arrange the layout of a group of controls first select the group Then click an appropriate button in the Layout toolbar or a menu item in the Layout menu of the IDE menu or in the pop up menu The Layout Toolbar There are the following Layout commands for justifying and resizing controls EEE a l Align Left Align Center Align Right Align Top Align Middle Align Bottom Even Horizontal Spacing Grid Even Vertical Spacing Make Same Size Align the selected controls along their left sides Center the selected controls vertically Align the selected controls along their right sides Align the selected contro
243. have found two ancestors of Pam Bill and John If we use the backtrack point to the second ancestor clause we get the following goal parent Bill P1 ancestor P1 AA Here we will again find that John is the parent of Bill and thus that P1 is John This gives the goal ancestor John AA If you pursuit this goal you will find that it will not have any solution So all in all we can only find two ancestors of Pam Recursion is very powerful but it can also be a bit hard to control Two things are important to remember the recursion must make progress It is no use as recursion leads to an endless loop in one clause That would produce the same answer over and over the recursion must terminate when recursion doesn t terminate your program will keep on creating new backtrack points and will never end Even when you go back to Adam and Eve as ancestors this is not what you want In the code above the first clause ensures that the recursion can terminate because this clause is not recursive i e it makes no calls to the predicate itself In the second clause which is recursive we have made sure that we go one ancestor step further back before making the recursive call Le we have ensured that we make some progress in the problem 5 7 Side Effects Besides a strict evaluation order Prolog also has side effects For example Prolog has a number of predefined predicates for reading and writing These pr
244. he Code Expert opens the Dialog and Window Expert The Dialog and Window Expert has a tree shape that is shown in figure 4 7 This Expert shows the events that can take place and indicates between brackets in the title bar for which window the events are shown In figure 4 6 the events of the Task Window are shown It is up to you to choose an event for which you want to insert or change the code The Dialog and Window Expert is in fact a tree A tree consists of branches and leaves a leave being the end of a branch At a leave you can manipulate the program code To navigate the tree and reach the point where you want to insert code left click on the appropriate branches of the tree until you arrive at the leave you want The Dialog an Windows Expert uses a simple procedure You go to a leave in the tree and click on it Then there are three possibilities e There is no code inserted for this leave Then you click the lt Add gt button and the Expert inserts some standard code also known as prototype code e There is already code inserted Then you can delete it by clicking the lt Delete gt button There is code inserted and you go there by double clicking on the leave So if you want the Dialog and Window Expert to add standard code on a leaf click on the leaf and press the button lt Add gt that will appear at the bottom of the dialog Then double click on the leaf again to reach the code See figure 4 7 If you want to add in CodeE
245. he IDE and execute the program You can execute PIE as a normal project as described in part 1 You don t have to change anything so in fact you only use the IDE to execute the program lala File Edit Engine Window Heb ODSaBDBIS BARES an AT A There is no ND ta My Documents Visual Prolog Example pies Exespie32 ini file Delaults been sel Use OnineHelp to see descriptions To bad example fles choose File Coreult menu entny Type pour predicate colk here like wiite Hello workd lt Set Caret here and press Enter Figure 5 1 PIE and the dialog window 57 If everything goes well PIE will start by showing the PIE Task Window and in it a dialog window It looks like in figure 5 1 The Dialog Window shows that a file pie32 in1 was created with some default values It tells you to use the Online Help for descriptions and a few other things Here I want to execute the program that we developed in the previous section To create that program select File New from the Menu bar PIE opens a second window the Program Window that we will use to enter the program There are now two windows open the Program Window and the Dialog Window The Program Window is used for the program its title bar shows the name File0 pro that PIE gives by default In the Program Window you place the facts and the rules of your theory Sometimes we call these also the program but remember that a program is not complete without
246. he TaskWindow Normally you will approach the code via the Code Expert but by opening the file TaskWindow pro you get directly access to the code Please be careful not to damage the code At the beginning of the file Task Window pro you find these lines of code implement task Window inherits applicationWindow open core vpiDomains Pay attention to the third line that says open core vpiDomains This line tells the compiler that when it encounters a procedure name like note that it doesn t see in your code it should look in the classes core and vpiDomains We know that we are going to use the procedure note We know that this procedure is in class vpiCommonDialogs So we can tell the compiler to look in class vpiCommonDialogs by adding that name to this line Change the line into open core vpiDomains vpiCommonDialogs When you add the name vpiCommonDialogs to this line you can remove it from the other lines Change those lines into clauses onMessagesWritenotel _ Source MenuTag note This is a message And clauses onMessagesWritenote2 _ Source MenuTag note This is a title This is a message 31 respectively You will find the lines when you scroll down to the end of TaskWindow pro above the code that is maintained by the IDE Please make sure that you don t change that code As an alternative you can of course also find the lines of code with the help of the Code Ex
247. he data to the port before the buffer is full It is also used to ensure that another process will get latest data from the stream stdio getOutputStream 0 gt getOutputStream gt outputStream Stream procedure Description This predicate returns the name of the current output stream It should be set previously by setOutputStream 1 predicate stdio nl 0 nl procedure Description The predicate writes a new line symbol to the current output stream stdio setOutputStream 1 setOutputStream outputStream Stream procedure i Description This predicate sets Stream as current output stream All output will be directed to this new current output stream The variable Stream is of type stream That type is invoked when you open new file for output That is done with e g A clause like DBFile outputStream_file create8 persons txt as we did in the last program By definition the variable DBFile gets type stream and can be used everywhere where you use a variable of this type stdio write write procedure Description The predicate writes an arbitrary number of variables to the current output stream The variables are separated by comma s and you can include strings to clarify the output You can use any type of variable stdio writeQuoted writeQuoted procedure Description 200 The predicates writes specified variables to the current output st
248. he directory where it should reside the IDE knows that it is a Windows program so it generates the code to produce a Windows program with a Main Window that is called the Task Window and the menu items File Edit et cetera that you know from other Windows applications So now it is time for step 2 Build compile and execute the program ch01p01 prj6 Yisual Prolog 7 1 Commercial Edition Licensed to Thomas Y de Boer File Edit view Insert Project Build Debug Goto Resource Tools Web Window Help Gompile GEE Build Ctrl4 Shift B r Rebuild All Ctrl Alt ShiFt B a Stop Building Gtrl Break 3 Execute Ctrl F5 Ea ProDi Rumin Window AlHES E Taskwi main pa Script Preview Figure 1 5 Build the project 11 To generate the Prolog code choose the option Build Build from the Task Menu as shown in figure 1 5 When you click this option the IDE generates the Prolog Code It generates several messages that you see in the Message Window Building the project takes several seconds Watching the messages in the Message Window gives you an idea of how much code is necessary to create an simple Windows program The IDE generates the code compiles it and finally links the parts of the program When all is done the message Project has been built is shown When you have built the project that is you made the IDE generate the Prolog source code take a look at the Project Tree It has grown Building the project means gene
249. he first premiss is not met we have another chance at the second one So the engine creates a backtrack point to the second alternative and evaluates the first You should realize that there are now two active backtrack points one in the sub goal to the second alternative father John CC and one in the program to the second clause in the father predicate After the creation of this backtrack point the engine takes care of the sub goal mother John CC So it calls the mother predicate The mother predicate fails when the first argument is John because it has no clauses that match this value in the first argument In case of failure the engine backtracks to the last backtrack point it created That is the point in the goal so it will now pursuit the goal father John CC When calling father this time we will again first create a backtrack point to the second father clause Recall that we also still have a backtrack point to the second clause of the father predicate which corresponds to the first call in the original goal Let s make a sketch of the situation at this moment There is a goal to solve It was father AA BB parent BB CC But it has changed because the engine found a solution for the father predicate and a rule for the parent predicate So it now looks like father Bill John mother John CC father John CC The first sub goal is solved it set a backtrack point at the pre
250. he first three are input and the last two are output then the lt predicateFlow gt looks like i i 1 0 0 So a predicate declaration can take the form myPredicate string Title integer NrOfPages procedure 1 0 The declaration says that myPredicate has two arguments one string and one integer the mode is procedure that means it always succeeds no backtrackpoints that you have to input the string and that it will return the number of pages in the second argument It is possible to declare more than one lt predicateFlow gt The declared lt predicateMode gt applies to all flows that follow it When different flows have diffferent modes you must declare it It can lead to a sequence like ere procedure i o nondeterm o 0 152 This declaration says that with the flow 1 0 the predicate has mode procedure and with the flow 0 0 the mode nondeterm When declaring a predicate the flow can be omitted Inside an implementation i e for a private predicate the needed flows are derived from the usage of the predicate Inside an interface or a class declaration i e for a public predicate omitting flows means that all arguments are input The special flow pattern anyflow can be stated only in declarations of local predicates i e in predicate declarations inside the implementation of a class It means that the exact flow pattern will be evaluated during the compilation If the optional preceding predicate mode is
251. he operators mod and rem give the remainder with an integer division Please read the helpfile on the difference Literals Literals are to be taken literally It means that they do not represent a variable or a predicate or whatever they are just what they are The number 12345 is just this number nothing else Literals fall into following categories integer character floating point string binary and list Integer literal An integer literal or integer for short is a number that has no decimal part it is an integral value It may be preceded by the unary plus or minus sign You can use octal integers in your program They must be preceded by 00 zero and lowercase o So 0012 octal 12 is decimal 10 Also hexadecimal numbers are possible They must be preceded by Ox zero and lowercase x So 0x22 hexadecimal 22 is decimal 34 There are two standard domains for the integer literal It can belong the domain integer or to the domain unsigned Take care that an integer does not exceed the maximum and minimum values for the domain See the next section on domains Real Literal A real literal or real for short is a number that has a decimal part or at least a decimal dot or that contains an exponential part If a number has no decimal dot a decimal part or an exponential part then VIP takes it to be a integer There are two ways to write a real with a decimal dot or part like 1234 5
252. he person functor as we are in that predicate not interested in the name of the father We only want to know if there is a father Notice also the use of the sign It is an indication that the rest of the line is a comment and should be skipped by the inference engine The other way is this 2nd version father Person person Name male parent Person person Name male This predicate is also called with Person bound to a variable and gives back the Name of the father The logic behind both versions is the same but the manner in which the logic is indicated to the Prolog engine is different In this second version the set of parent facts is also examined On arriving at the correct value for Person the code halts and returns the correct functor data back to the predicate head provided the second argument of that functor is the string literal male If you note the functor was NOT bound to any intermediate Prolog variable as it was done in the first version The second version is much terser than the first one and sometimes this method of writing the code can be less legible to beginners So feel free to use the first version as it is correct Prolog code Let us now use this code in a complete example where we will give a suitable set of person facts Please enter into the Program Editor of PIE the following clauses parent person Bill male person John male parent person Pam female person Bill ma
253. he right subtree In order traversing the tree in figure 12 4 gives the sequence A B C D E F G H I Note that the in order traversal of a binary search tree yields an ordered list This is because of the way the tree is built To traverse a non empty binary tree in post order perform the following operations Traverse the left subtree Traverse the right subtree Visit the root Post order traversal sequence is A C E D B H I G F Finally trees can also be traversed in level order where we Visit every node on a certain level before going to a lower level This is also called breadth first traversal Level order traversal sequence is F B G A D I C E H Figure 12 4 an example tree 234 12 11 A program for tree traversal Now let s make the program that creates the tree from figure 12 4 and that traverses this tree in several ways I started a new project with the name ch12Trees I added an extra option to the taskmenu in the task window called Trees and I added four options create traverse PRE order traverse IN order and traverse POST order I made the IDE generate the standard code and then changed it into this code predicates onTreesCreate window menultemListener clauses onTreesCreate _ Source MenuTag classtree add from _list F B G A D I C E H stdIO write Tree has been created stdIO nl predicates onTreesTraversePreorder
254. her Pam Bill and then takes the first fact father Bill John and sees if it matches the goal It clearly doesn t as Bill is different from Sue The engine now returns to the backtrack point to check the next fact but before it does the check it sets a backtrack point at the third fact father Sue Jim It then checks the second fact that doesn t match the goal either The engine returns to the backtrack point and checks if there is a match between the goal and father Sue Jim The predicate and object Sue match but how about the variable Well in this case the variable X does not yet have a value So the inference engine is free to make a match by giving the value Jim to X In Prolog this is called to bind a variable Now that X is bound to Jim the goal and fact match The engine reports this by reporting X Jim We call that the solution of the goal When the inference engine has found a solution it doesn t stop When there are more facts to check it continues till it runs out of facts This is sometimes called the relentless search for solutions In the examples so far I only used a simple goal and facts Now let s take a look at an example with a compound goal and facts and rules Consider the clauses in this program 65 mother Bill Lisa father Bill John father Pam Bill father Jack Bil
255. her of Pam Is John the father of Ann As there are no facts about Ann we cannot answer this question In Prolog we then assume that the answer is no as we cannot state that the answer is yes Is John the grandfather of Pam To answer this question we have to use the rule and the facts The rule tells us that we need to find two facts The first should say that Someone is the father of Pam and the other fact should state that John is the father of the same Someone Looking at the facts you see that the facts are precisely saying this when we take Bill for Someone So yes indeed John is the grandfather of Pam In Prolog you can asks these questions to the inference engine that is built in Prolog In Prolog such questions are called goals maybe because the inference engine pursues them Goals can be formalized like this respectively The question Is Jim the father of Sue becomes 2 father Sue Jim The question Is John the father of Sue becomes father Sue John The question What is the name of the father of Pam becomes father Pam X The question Is John the father of Ann becomes father Ann John The question Is John the grandfather of Pam becomes grandFather Pam John Such questions are called goal clauses or simply goals Together facts rules and goals are called Horn clauses hence the name Horn Clause Logic 56 Some goals like the firs
256. his clauses run Kwad func squareFunc 3 I added this line for my program stdIO write K wad I added this line for my program _X stdIO readchar I added this line for my program succeed place your own code here end implement procfunc You don t have to bother much about this as the IDE takes care of it But one thing may embarrass you Normally the predicate run 0 in the goal must have procedure mode It means that the predicate run 0 has mode procedure Now when you insert code as I did in the example above it may happen that you call a predicate that has a less strong mode e g nondeterm Then the compiler will complain that by calling this predicate you in fact change the mode of run 0 to nondeterm This is annoying but you will have to solve this problem in some way As a last remark the goal section defines its own scope therefore all calls to other predicates invocations should contain class qualifications That is you must give the class name where the called predicate resides 9 10 Section open and scope access issues Visual Prolog divides the total program code into separate parts each part defining one class In object oriented languages a class is a package of code and its associated data which are put together Classes were introduced in chapter 8 As noted earlier for those of you who are not familiar with object oriented programs you can think of a class loosely as being synonymous t
257. his code is now known inside the class person but not outside That is a problem because we want to connect the menu item in the Taskwindow with this piece of code in class person So we need to state the existence of this code to the other parts of the project The place to do this is the file person cl Think of it as a counter or a window where you can see what is available in the module person Please open the file person cl and after the lines predicates classInfo core classInfo short Class information predicate detail This predicate represents information predicate of this class end add the line fillDatabase procedure Take note of the dot at the end of the line This line in person cl informs the the other parts of the program outside class person of the existence of a predicate fillDatabase so it can be used Now let us connect the menu item FillDatabase with the predicate fillDatabase We ll do it in the usual way Go to the TaskWindow win right click and choose the Code Expert In the Code Expert actually the Dialog and Window Expert open Menu TaskMenu id_ database highlight id_database_fillDatabase and lt Add gt code for this option See figure 7 4 Then go to the code by double clicking the line with id_database_filldatabase gt and change the inserted code into predicates onDatabaseFilldatabase window menultemListener clauses o
258. how things are handled with ask 2 and ask 3 Another way to input strings is listSelect 5 You can use this procedure when there are a restricted number of possible answers An example is when the user must enter the country where she lives With listSelect S you show the user the list of possible answers and she selects one of them When you use this procedure you enter a title a list of choices and the index of the preset selection The results are returned in arguments four and five these should be variables Take a look at the code that in my program is connected to the menu option Listselect predicates onMessagesListselect window menultemListener clauses onMessagesListselect _ Source MenuTag b_true listSelect Title Choice1 Choice2 Choice3 0 SelectedString SelectedIndex 1 note Y our selection is SelectedString note with index tostring SelectedIndex onMessagesListselect Source MenuTag The IO pattern of listSelect 5 is 1 i i o 0 the first three arguments are input the last two arguments are output As an input you give as the first argument the title of the window As the second argument the list of possible choices are put together in a list of strings Choice1 Choice2 Choice3 The first one Choicel has index 0 zero the next one Choice2 has index 1 one and so on The third argument indicates which of the possible choices should be shown as the pr
259. iatian Fact Variable Text OK Lock to Container Palas Contaner none 72 1 102 Width 48 Heighl 12 Left Anchor False Top Anchor False Right Anchor Tiue Bottom Anchor Tiue Enabled Tiue Visible Tiue Tab5top Tue Defaut False Style Ok 247 Please noticew that in fact the properties table also contains the events that are processed by the control You can switch between the properties and the events by clicking the tabs at the bottom Initially when no Control is selected the Control Properties table contains the properties and events of the form As soon as one of the Controls in the edited Dialog or Form is selected the Properties Table displays the properties of the selected control The displayed set of properties depends on the control type The Control Properties table contains two types of properties The General group contains attributes which are common to all types of controls the Text determines the control title the Constant name is used as the control identifier and the Control Size group determines position and size of the control Notice that the Text property attribute is not used for some controls for instance it is not used for scroll bars Notice also that in the Text fields the ampersand symbol amp is reserved for indicating that the next to it character is to be underlined For example if you use the text string E amp xit as a control name then this control will be displayed with the title Exit at runti
260. ic clauses because of the possible demands made on memory resources at run time However Visual Prolog makes internal checks for non deterministic clauses reducing the burden on you the programmer 71 You can change a non deterministic predicate into a deterministic predicate by inserting cuts into the body of the rules defining the predicate For example placing cuts in the clauses defining the friend predicate causes that predicate to be deterministic because with the cuts in place a call to friend can return one and only one solution More on this in chapter 9 5 6 Recursion Most family relations are easy to construct given the principles above But when it comes to infinite relations like ancestor we need something more If we follow the principle above we should define ancestor like this ancestor Person Ancestor parent Person Ancestor ancestor Person Ancestor parent Person P1 parent P1 Ancestor ancestor Person Ancestor parent Person P1 parent P1 P2 parent P2 Ancestor The problem with this formalization is that this line of clauses never ends The way to overcome this problem is to use a principle that is called recursion Recursion is created by a recursive definition i e a definition that is defined in terms of itself like this ancestor Person Ancestor parent Person Ancestor ancestor Person Ancestor parent Person P1 ancestor P1 Ancestor The first clause states that y
261. ication So I set the property read only of the ID field to True Besides I changed the names of the five edit fields Not surprisingly I called them id_ctl name_ctl length_ctl weight ctl and iq ctl respectively Now please go ahead and create this dialog I named it changePe rson changePerson ID 5 Choose a name Choose an ID Name Thomas A 2 J ph Weight 180 Lid 6 Length 75 fi DO 10 OK Cancel Help Figure 7 8 Dialog for change person 107 You see that the dialog is filled wit the names from the database I filled the database with the four records from section 7 1 and then added two records with the name Thomas In the dialog I selected the name Thomas the program responded with showing the three ID s I selected ID 5 and the program shows the contents of the record with ID 5 The code for this dialog will be quite familiar if you understood the previous section First when the dialog is generated we need to fill the listbox with names Build the project go to the file changePerson pro find the code clauses new Parent dialog new Parent generatedInitialize Change it into clauses new Parent dialog new Parent generatedInitialize person getAllNames NameList SingletonList removeDuplicates NameList listBox_ctl addList SingletonList Then go to the top of the file and change open core vpiDomains into open core vpiDomains list We need code
262. ience unexpected output coming from failing parts of the program Perhaps this little advice can help you Separate the calculating code from the code that performs input output In our examples above all the stated predicates are calculating predicates They all calculate some family relation If you need to write out for example parents create a separate predicate for writing parents and let that predicate call the calculating parent predicate 74 5 8 Conclusion In this tutorial we have looked at some of the basic features of Prolog You have seen facts rules and goals You learned about the execution strategy for Prolog including the notion of failing and backtracking You have also seen that backtracking can give many results to a single question And finally you have been introduced to side effects 75 Chapter 6 Data modeling in Prolog Loosely speaking a computer program or wider an information system consists of data and procedures to manipulate the data and a user interface In the previous chapter we paid attention to the basics of how Prolog manipulates data to find answers for the user In this chapter we will dig deeper into the way the data in a Prolog Program are being structured You will find out that there are various and elegant ways to create data structures We are going to concentrate on how data is modeled in Prolog before we can perform actions on them Hence there are not much examples concerning cod
263. ike a bubble goes up in the water I think that explains the name I will use the code proposed by Ivan Bratko It is quite compact it took me some time to understand what is happening and how the program works As usual I start with the predicate that is called to perform the sort I call it bubbleSort and the declaration is bubblesort integer integer procedure i o Bubblesort 2 takes the list and calls bubble 2 In bubble 2 we decide if there has to be a change If so bubble 2 changes the place of two elements and starts bubblesorting the new list When there is no change then clearly the whole list is sorted Bubble 2 fails and the second clause for bubblesort 2 sees to it that the sortedlist is returned to the calling predicate bubbleSort List SortedList bubble List List1 bubbleSort Listl SortedList bubbleSort SortedList SortedList So the trick is in the predicate bubble 2 The declaration is bubble integer integer determ 1 0 The clauses are bubble X Y Rest Y X Rest X gt Y bubble Z Rest Z Rest1 bubble Rest Rest1 The first clause swaps the first two elements when the first one is larger than the second one When that is not the case then in the second clause we keep the first element in its place and bubble the rest of the list Finally add to the run 0 predicate the clause to produce the bubble sort sort bubblesort UnsortedList BubSortedList
264. in If there is no parent domain then the largest possible range for the declared precision will be used Type Names A type name is a lt domainName gt that is defined in the current scope or elsewhere When you have defined a lt domainName gt you can use that name in other declarations This is called using a typeName Using typeNames for declarations is fairly straightforward An example domains lt domainName gt lt aPreviousDeclaredDomain gt with optional parameters for e g bitsize precision and or range If you want to use a range from another scope say another class then you must precede lt aPreviousDeclaredDomain gt with the name of the class and a double colon An example domains myNewDomain anotherClass myOld Domain In this example myNewDomain will be a subtype of myOldDomain and you must take care that the properties bitsize range precision of myNewDomain do not violate the properties imposed by myOldDomain E g the range of myNewDomain must not be greater than the range of myOldDomain Also the bitsize or precision cannot be greater Compound Domains The domains mentioned till here are simple domains There are also compound domains also known as algebraic data types They are used to represent lists trees and other tree structured values In its simple forms compound domains are used to represent structures that you have seen before as functors and enumeration value
265. in the constructor which increments the createdCount Notice that in the constructor new there are two assignment statements name Name createdCount createdCount 1 They have the same shape but the first one assigns a value to the object fact name the other one assigns a value to the class fact createdCount In OO language this is called one updates the object state whereas the other updates the class state A state of an object is the set of values that are assigned to its attributes When one or more of the values change the state is changed The same goes for the class When you create a new object the value of createdCount changes and in OO language we say that the state of class person has changed The other clauses in the constructor new 1 are to show you what is happening in the Message Window Now build and execute the program and enjoy the birth of new objects 8 5 Classes and modules You can think of a class or of objects as a set of predicates that perform tasks that are closely related Sometimes you want to group some predicates but don t need a class with them that generates objects In Prolog you can declare a class as non object constructing Such a class that is like a module is created with with File New option class In the Create Project Item Dialog you uncheck the creates objects check box A class that doesn t create objects is described in two files The interface file
266. in in PIE by simply using it in your predicates This is different from Visual Prolog where you have to declare what you are going to use I will come back to that in chapter 9 If you study the facts for the father relationship you will notice that the persons described are richer than what was done before This time each person is described with both the person s name and his her gender using the person functor whereas in chapter 5 Fundamental Prolog Part1 we were only using the person s name After you ve written the new code ensure that the PIE engine is reset Use Engine Reset Then you re consult the code using Engine Reconsult As before on a blank line in the Dialog Window type a goal without the sign in front For example type the goal grandfather X Y And hit lt Enter gt The inference engine answers X person Pam female Y person Bill male 1 solution As is shown in figure 6 2 79 lolx Reconsulted from lt 4D ata hdy Documents Wisual Prolog 7 projects IntroVisualProlag files pro grandfathers Y X peroni Pam female Y person Johr male 1 Solution ela Figure 6 2 Dialog with the family Notice that in this case variable X contains everything we know about the person Pam her name and her gender It is also possible to approach the name and gender separately Enter in the Dialog this goal grandfather person ChildName ChildGender person GrandName GrandGender Take care of the uppercase
267. in integer Number Number stdIO read Number lt gt 0 stdIO write Factorial by predicate stdIO nl factorial facpred Number Factorial stdlO write Factorial of Number is Factorial stdlO nl stdlO write hit lt Enter gt 165 _ stdIO readchar _ stdIO readchar stdIO write Factorial by function stdlO nl FuncFactorial factorial facfunc Number stdlO write Factorial of Number is FuncFactorial stdlO nl run run This last piece of code is quite horrible so you d better copy and paste it from the pdf file of the book Now build and run the program The results of both the predicate and the function are the same but they tackle the problem in different ways It is up to you to decide which one you like best 10 2 Lists A list is a sequence of elements Lists and list processing is a powerful technique in Prolog In this section we explain what lists are and how to declare them and then give several examples that show how to do list processing in your own applications We also define several well known Prolog predicates and take a look at list processing from both a recursive and a procedural standpoint In the end we look at compound lists that is a list with different types of elements What Is a List In Prolog a list is an object that contains an arbitrary number of other objects Lists correspond roughly to arrays in other languages but unlike
268. inally we must remove the smallest element from the list I use the predicate remove 3 with declaration remove integer integer integer procedure i 1 0 And clauses remove Element Element Tail Tail remove Elem1 Elem2 Tail1 Elem2 Tail2 remove Elem1 Taill Tail2 remove _ 0 If the element_to_be_removed is the first one then the resulting list equals the Tail If not we must remove it from the Tail Finally we must adjust the run 0 predicate Add the clauses to produce the select sorting sort selectsort UnsortedList SelSortedList stdIO write The sorted list select sort is SelsortedList stdlO nl The third sorting technique I want to show is called bubble sort It is performed by taking the following steps 185 1 Take the first two elements of an unsorted list 2 Compare them If the first element is smaller than the second do nothing If the second element is smaller than the first one change their places 3 Take the right right means here not the left one element and compare it to its neighbor on the right side 4 If the element is smaller than its neighbor do nothing If neighbor is smaller than element change their places 5 Go to step 3 until you compared every element with its neighbor 6 Goto step 1 and repeat the process until there are no more changes of places Then the list is sorted In this way a big element will move from left to right l
269. indow you should never do it on your own always use the Code Expert otherwise things may become corrupt Especially you should take care when you lt delete gt code that was inserted by the IDE E g when you want to delete the code for onFileNew you open the Task Window win in the Dialog and Window Expert as you did in the previous section look for id_ file new gt onFileNew highlight it and then click on lt delete gt What happens is that the IDE deletes the line of code for the listener but it does NOT delete the program code that you changed The reason is that you probably have changed that code Therefore it is difficult to decide what exactly should be deleted A second reason is that by deleting the IDE could destroy code that you want to keep to use it later or elsewhere That s why you must delete that code yourself by hand Things may go wrong in this case There are two possibilities 1 You delete the listener but not your own code When you build the project in this case you get a Warning that there are unused predicates in your program The warning points at your own code that is not used anymore as there is no listener to activate it It is a warning because the remaining code doesn t harm the program it is simply not used 2 You delete your own code but not the listener When you build the project in this case you get an Error that there is an unused identifier say variable in your program The listener poin
270. ing is that the world consists of objects An object can be anything literally any thing The only condition is that you must be able to identify an object that is you must be able to distinguish an object from its environment So a drop of rainwater on your window is an object a drop of water in the sea is not an object But you will understand that this is hardly a serious restriction 2 The second basic idea is that an object has certain characteristics Characteristics are called attributes An object can have many attributes Think of a human being as an object and how many attributes you can imagine Length Weight IQ you name it An attribute is a characteristic but it is also a point of measurement You can link a value to an attribute In this way you create facts A fact is an Object Attribute Value tuple that is true in a certain place at a certain time 3 The third basic idea is that objects can do something they perform activities also called tasks So every object has next to its attributes some activities that it can perform In OOP we call these activities operations or methods 4 The fourth basic idea is that objects can be brought together in groups Of course you can do that in many ways but in OOP the most important way is to bring together those objects that share the same attributes and the same operations The set of objects that have the same operations and the same attributes is called the class of those object
271. is a list expression That means that the list comprehension call is in fact an expression and as such it must be bound to a variable Now take a closer look at Exp Gen In this expression Gen is typically a nondeterm term That means when you call Gen it can fail and backtrack You may think of Gen as a functor that is called and then generates values as solutions Compare it to the item functor above Exp is evaluated for each solution of Gen and the resulting Exp s are collected in a list The Exp corresponding to the first solution of Gen is the first element in the list etc This list is the result of the list comprehension term On return the list is bound to ResultList Exp must be of mode procedure or erroneous Both Exp and Gen are cut scopes You can paraphrase the list comprehension as Give me the list of Exp such that Gen Now let s go to an example Say there is a list L that contains numbers If we want to retrieve the even numbers in this list we could call EvenNumberList X isMember X L X div2 0 In plain English this says put in EvenNumberlist the list of X s that are in list L and that are even It is comparable to findall 3 but in this case we have added an extra condition Another example of the use if list comprehension is OddNumberList X 1 isMember X L X div 2 0 Here the collected expression is more complex In English one could say put in OddNumberlist the list of X
272. is is not the way you will use them later on but for the moment it is a convenient way Please start a new project and give it a name e g Ch03p01 27 The simplest way to tell the user something is to write a note When you do the user sees a small window with a short text in it x like the one you see in figure 3 1 A This is a message To generate a message like that one we first have to create an event In this case and in the rest of this chapter the event will be a mouse click on a menu option in the Taskmenu So first we will create a new option in the Taskmenu then we will add the standard code and finally we will change the standard code to generate the Figure 3 1 a note message This pattern will be repeated throughout this chapter ME C ee rs Constant Prefix id_file Item Constant id_tile Figure 3 2 the icon toolbar in the Task Menu Editor To create a new menu option go to the folder Task Window in the Project Tree select TaskMenu mnu and open it in the Menu Editor by double clicking on it Just below the title bar you see seven icons representing options in this editor see figure 3 2 When you put the cursor on one of them you will get the name From left to right they are New first Item Inserts a new menu item before the first item In the menu in your program it appears left of the former first item or above it New item inserts a new menu option under the selected
273. is text based etc Building The project is built i e compiled and linked by the IDE Compiling means that the Prolog code is translated into machine code so the processor can digest it Linking means that the various parts of the program are knotted together Browsing The IDE and the compiler collect information about the project which is utilized in various ways for quick localization of entities etc e Development During the development and maintenance of the project the IDE is used to add and remove source files and GUI entities to the project and to edit these e Debugging The IDE is also used for debugging the project The debugger is used to follow program execution and exploit the program state when the program runs The debugger helps to find and repair mistakes that prevent your program from proper execution In the sequel we will look more detailed at each of these things Now and then especially in the beginning of this chapter I shall repeat things that have been said before to keep the text complete and readable In this chapter I shall use a project to illustrate things and to give examples You should create that project and keep it at hand no better way to learn than by doing Creation First we will create a project by selecting Project New in the menu In response to this you are presented to a dialog window that contains various properties of the project It should be familiar to you by now For this example
274. ist of Container Name property values For example if you see GroupBox1_ctl N none GroupBox1_ctl GroupBox_ctl this means that the control is placed inside two group boxes with names GroupBox_ctl and GroupBox1_ctl If you do not need to relate the control to any container then you should select none 253 X Y Width Height These properties determine the control coordinates and size X The X coordinate of the upper left corner of the control in Dialog Base Units Y The Y coordinate of the upper left corner of the control in Dialog Base Units Width The Width of the control in Dialog Base Units Height The Height of the control in Dialog Base Units Left Top Right and Bottom Anchors These properties determine how the control coordinates and may be size will be handled when the dialog or form containing this control is resized The general rules are described in the Using Anchors for Positioning GUI Controls while Dialog Resizing in section A2 Left Anchor Possible values are True and False True defines that the Left boundary of the control is always positioned at the specified distance equal to the X property from the Left boundary of the dialog form containing this control The distance is specified in the Dialog Base Units Top Anchor Possible values are True and False True defines that the Top of the control is always positioned at the specified distance equal to the Y property fro
275. istbox the selected items are returned as a list This list is bound to Name _T This last expression is a typical Prolog expression It represents a list but at the same time cuts the list in two parts The first element is bound to the variable Name the rest of the list is bound to the variable _T The underscore is a warning for the compiler that we will not use this rest Because in this case we allow the user only a choice of one element it suffices to bind the first and only element of the list to Name person getSomeIDs Name IDList We use Name in the calling of the predicate getSomeIDs This predicate that we yet have to make will be placed in the class person and we will turn to the code later For now we assume that we give Name as input and get back IDList as a list of IDs 104 idBox_ctl clearAll id_ctl setText name_ctl setText Because a new name was selected we clear the idBox and the edit fields for ID and Name idBox_ctl addList IDList We now add the list of IDs to the idBox_ctl onListBoxSelectionChanged Source Don t think too much about this line For the moment it is not too important Above I introduced the predicate getSomelDs It resides in the class person and it finds all the IDs that go with a given Name It is straight forward getSomeIDs Name IDList findall ID person ID Name IDList I put it in person pro right after getA
276. istcom implement listcom clauses sumlist 0 0 sumlist H T Sum N sumlist T S1 N1 Sum H 81 N 1 N1 person Sherlock Holmes 22B Baker Street 42 person Pete Spiers Apt 22 21st Street 36 person Mary Darrow Suite 2 Omega Home 51 end implement listcom in main pro 180 run console init L Age listcom person _ Age listcom sumlist L Sum N Ave Sum N stdio write Average Ave _X stdIO readchar The list comprehension clause in this program creates a list L which is a collection of all the ages obtained from the predicate person If you wanted to collect a list of all the people who are 42 years old you could give the following subgoal List Who my person Who _ 42 The following code will create the list of positive items List X X list getMember_nd 2 8 3 6 X gt 0 where getMember_nd is a predicate from the class list to retrieve the members of a list Finally I want you to meet another construction Remember that we used to print a list of numbers with a predicate that looked like write_a_list List member Element List write Element nlO fail Write_a_list _ This writes every Element from the List or better said it writes each Element that is generated as a solution by the nondeterministic predicate member To generate a complete list we make the predicate fail That is not really elegant
277. it is placed like and looks like any other subgoal Now there is a cut between subgoal b and c This cut succeds as a subgoal so the engine passes it The side effect is that when the inference engine passes the cut any subgoal in front of it here a and b will no longer be considered as a potential backtrackpoint This is a way of telling Visual Prolog that you are satisfied with the first solution it finds to the subgoals a and b Although Visual Prolog is able to find multiple solutions to the call to c through backtracking it is not allowed to backtrack across the cut to find an alternate solution to the calls a or b Also it is not allowed to backtrack to another clause that defines the predicate r1 So all in all you lose the backtracking of the the subgoals in this clause AND you lose the backtracking of alternate clauses of the same predicate rl As a concrete example consider the next clauses person Pete haircutter 20 66 person John piano tuner 50 person Amna piano tuner 40 person Carl piano tuner 44 69 There are four persons in a database Each person has a name a profession and asks a certain amount of money for a service Assume that you are looking for a piano tuner but you don t want to pay more than 44 Euro s for tuning a piano Then you could have the enference engine search this database with the goal run person Name piano_tuner Am
278. jects and one by one takes the head off the list asks the object for its name and number and then proceeds with the tail This continues till the tail is empty The predicate showAllObjects 1 is a private class predicate so I declare it in the file person pro class predicates showAllObjects person CreatedObjects clauses showAllObjects Head Tail stdlO write Name Head getName with number Head getNumber stdIO nl showAllObjects Tail showAllObjects Finally we have to attach the code to the taskmenu item So go to the standard code that was generated by the IDE and change it into predicates onshowall window menultemListener clauses onshowall Source MenuTag person showAllPersons The program now will show a list of the objects in the Messages Window A list is nice but in a database you often only retrieve data of a single object So let s now design code for the next taskmenu item ShowSingle This option should show a single object I decided to first show a list of the names to select an object and then to use the name to find the object and the other data in this case the number Let s enter the code for this I changed the standard code that was generated by the IDE for the Taskmenu option ShowSingle into predicates onshowsingle window menultemListener clauses onshowsingle _ Source MenuTag person getAllNames Nameslist b_true vpiCommonDialog
279. ke father and parent between the entities or objects If we instead first focus on the entities then the result will become different Our main entities are persons Persons have a name in this simple context we will still assume that the name identifies the person in a real scale program this would not be true Persons also have a gender Persons have many other properties but none of them have any interest in our context Therefore we define a person predicate like this person Bill male person John male person Pam female The first argument of the person predicate is the name and the second is the gender If you want to see a predicate as a relation you can think of the person predicate as a relation between a name and a gender Or as a relation within an entity instead of between entities A predicate like this is called a functor More about functors in section 6 3 Instead of using mother and father as facts I will choose to have parent as facts and mother and father as rules parent Bill John parent Pam Bill father Person Father parent Person Father person Father male mother Person Mother parent Person Mother person Mother female Notice that when father is a derived relation like this it is impossible to state female fathers So this theory also has a built in consistency on this point which did not exist in the other formulation You see that it is possible
280. keep the bound variables in a list When you need to know about an object simply look in the list of variables and trace the object that you need and then ask your question about its data In this section I shall use another method I simply put the created objects in a list and when I need a certain object I ll search this list till I find the wanted object By the way as I m going to use a list you may want to read the first section of chapter 9 on lists first Let s assume that we want a database of people and that we want to know the name and a number of everyone The name can be chosen freely the number will be computed by the program To show you how this works I created a program target GUI with the name ch12classDB It looks a little like the program ch12class but I decided to make it another project and not to change ch1 2class In the project I created a package person with in it a class person Here I am going to store the data The data are stored in an internal facts database in the class person Please create the project and the package person and the class person Take care that the class person does create objects As we are going to make a database we have to implement the basic functionality for a database That is we must be able to create objects show one or mor objects and delete objects So I decided to add four options to the TaskMenu e CreatePerson to create an object ShowAll to show the
281. l parent Child Parent mother Child Parent Mind the semicolon father Child Parent And then consider this goal father AA BB parent BB CC This goal states that we want to find the names of three persons AA BB and CC such that BB is the father of AA and CC is a parent of BB All three identifiers start with upper case so at the beginning they are variables and they are not bound to a value As mentioned before the inference engine always solves the goal from left to right so first it calls the father predicate When executing the father predicate it goes along the clauses in the program from top to bottom The first predicate is the mother predicate It is skipped because the engine looks for a father predicate The next predicate in the program is the first father predicate The predicate name matches what the engine is looking for so it will be evaluated But first the engine creates a backtrack point to the second father predicate clause and then it uses the first clause for evaluation Using the first clause we arrive at a solution when AA is made equal to Bill and BB is made equal to John As AA en BB are variables that have not yet a value nothing stands in the way to do so So the engine binds AA Bill and BB John It now has a solution for the first sub goal so it turns to the second sub goal parent BB CC But BB has just got a value So we now effectively have the goal parent John
282. lare class facts starts with the keyword 148 class facts The section class facts is syntactically equal to facts section So a class facts database can contain factVariables and factFunctors Et cetera Facts can only be declared in a class implementation the file with the name lt className gt pro and subsequently they can only be referenced from within this implementation file So the scope of facts is the implementation in which they are declared But the lifetime of object facts is the lifetime of the object to which they belong Likewise the lifetime of class facts are from program start to program termination 9 7 Section predicates The section predicates declares a set of object or class predicates in the current scope The section begins with the word predicates that is followed by one or more predicate declarations A file can contain more than one section predicates In its simplest form a predicate declaration consists of a predicate name followed by a colon and a list of arguments between round brackets and it ends with a period lt predicateName gt lt argumentlist gt The lt predicateName gt is a lowerCaseldentifier The lt argumentlist gt states the types of the arguments Types can be standard types or declared domains The argumenttypes are separated by commas An argument may be followed by a variable name in that case there is no comma between type and variable name An example
283. lass person When you click lt Create gt the IDE opens two edit windows One is called person cl the other one is called person pro For the moment you can close the file person cl we will attend that one later on Now go to person pro This is the file where we will place what we need for the database First we have to declare that there will be a database In VIP we use the declaration word facts for this In person pro add a clause for class facts in the code The parts for constants and clauses are already there constants _ className person person classVersion 91 class facts person string ID string Name integer Weight integer Length integer IQ clauses classInfo className classVersion With this declaration you state that there is a database within this class that consists of facts Each fact is a functor with five arguments two of them are strings and three are integers Now save the file and close the editor As this is probably clear to you I shall not mention to save the file in the remainder of this chapter When you try to execute the program the database is empty In fact the compiler warns you that there is an unused predicate person To be able to use the database we must add functionality to the program It seems to be wise first to fill the database with a few records I chose to create a menu option in the task menu to hold the functionalit
284. lauses onDatabaseConsultdatabase Source MenuTag person consultDatabase 110 As you can see I keep the predicates for manipulating the data in the facts database in the class person So now go to the file person pro and add the code for the two new predicates Here it is saveDatabase file existFile personDB txt file delete personDB txt file save personDB txt personDB stdIO write The database has been saved to file personDB txt stdIO nl saveDatabase file save personDB txt personDB stdlO write The database has been saved to file personDB txt stdIO nl consultDatabase file consult personDB txt personDB stdIO write Records have been added from file personDB txt stdIO nl In both predicates the central clause is to call the standard predicate from the class file and to give it the arguments it needs The first argument is the external filename that will be used to write the data to or to read from the second argument is the name of the internal facts database in the program In both cases we also write a message to the Message Window The clauses for the consultDatabase 0 predicate are fairly straightforward the use of saveDatabase 0 is a little bit more complicated That is because save 2 adds the records to the contents of the file when it already exists When you take no precautions this will result in the same record appearing more then once when you
285. le parent person Pam female person Jane female o parent person Jane female person Joe male grandfather Person Grandfather parent Father ParentOfPerson father ParentOfPerson GrandFather father Person person Name male 81 parent Person person Name male The result should look like in figure 6 3 X Data My Documents isual Prolog 7 projects Intro isualPralog it 3 23 Insert Indent parent person Bil1l male person John male parent person Pam female person Bill male parent ipersoni Pam female person Jane female parent personi Jane female person Joe male grandfather Person Grandfatherj parent Father ParentOfPerson father ParentOfPerson GrandFather father Person person Name male j parent Person person Name male jj Figure 6 3 Another family theory Now after you reset the inference engine and reconsult the above code into PIE and give the following goal grandFather person Pam female W the result will be grandFather person Pam female W W person John male W person Joe male 2 Solutions We get the names of both grandfathers Feel free to fool around with this program add some more clauses and ask different goals You need to get well acquainted with functors as the use of functors in Prolog is important 6 5 Functors and Predicates Functors an
286. le an object was created that can take care of everything concerning that particular stream Now let s take a look at the copying The main code is copyStream InputFile OutputFile repeat String InputFile readLine stdlO write String stdlO nl OutputFile write String n InputFile endOfStream First you start a predicate repeat It is used for repetition in Prolog and I shall leave it out of the explanation Then we bind String to a line that is read from Inputfile Again you see that like in real good object oriented tradition we ask the object Inputfile to read a line from the stream he is controlling and give it to String Then we write to the standard IO so you can see in the Messages 210 Window on the screen what s going on and finally the string is written to the Outputfile In fact we ask the object Outputfile to write the string to the stream he is controlling In the example above I used a separate class for the copy predicates That is not necessary If you want you can put everything in main pro Here is the code I made a new project target console and changed added the code in main pro into class predicates copystream inputStream InputFile outputStream OutputFile procedure i 1 clauses copystream InputFile OutputFile repeat String InputFile readLine stdIO write String stdIO nl OutputFile write String n InputFile endOfStream copystream _
287. le to stop the program in a separate clause There are other possibilities One could also write count Number Number lt 11 stdIO write Number stdlO nl NextNumber Number 1 count NextNumber count _ stdIO write That s all folks Recursion comes in very handy but at the same time you should be very careful Imagine what happens when you change the order of the two clauses count Number stdIO write Number stdIO nl NextNumber Number 1 count NextNumber count 11 stdIO write That s all folks In this case the clause count 11 is never reached so the program never stops Also the place of the write statement is quite critical See what happens when we change the program into this one 162 count 11 stdIO write That s all folks count Number NextNumber Number 1 count NextNumber stdIO write Number stdIO nl The numbers are shown in reversed order That is because now the program first goes recursively from 1 to 10 and then returning from the calls starts to write the numbers So now it first writes the final message then the number that was generated last that is 10 then the 9 and so on Note that when the program calls the recursive predicate it retains the variables and their values with every call Each recursive invocation of a clause has it s own set of variables Variables are local to the clause where they are used In the
288. letters and the full stop and hit lt Enter gt The inference engine answers CHILDNAME Pam CHILDGENDER female GRANDNAME John GRANDGENDER male 1 Solution This gives you a detailed answer More about this in the next section 6 4 Using functors If you take a look at the grandfather predicate you will notice that it has a subtle bug A person normally has two grandfathers one from the mother s side and one from the father s side but the grandfather predicate as defined earlier will only yield the grandfather on the father s side The grandfather predicate should be re written as follows grandFather Person TheGrandFather parent Person ParentOfPerson father ParentOfPerson TheGrandFather In this predicate the logic states that a father of any parent could be the grandfather of the person in consideration For this predicate to work we need to define a predicate called father using the person functor This predicate would troll through a database of facts explaining the parents defined in the system This is a more elegant method for finding out the fathers instead of presenting them as facts as shown previously because later on we can extend this concept to find out mothers in a similar fashion This can be done in either of the following ways 1st version Let us assume that a variable Person is bound to a Name and a Gender and that now we call the predicate father Person Father parent Person Father
289. level then you must add change or delete a fact by calling a class predicate In the latter case you must declare the fact to be a class fact See the end of this section Facts sections can be declared only in class implementations that is in the file lt className gt pro Fact data bases can be given a name If the fact database is named an additional compound domain is implicitly defined This domain has the same name as the fact database and has functors corresponding to the facts in the fact section A facts declaration starts with the word facts followed by the one or more facts declarations When you want to give the facts database a name the declaration looks like facts name_of_the database The name is an lowerCaseldentifier The word facts and the name are separated by a dash The facts database is now called a named facts database Each declaration after the word facts is ended with a period The declaration of a fact Variable differs slightly from the declaration of a factFunctor The simplest fact declaration is the declaration of a factVariable It states the name of the fact its type and an initial value lt factVariableName gt lt factType gt lt initial Value gt The lt factVariableName gt is a lowerCaseldentifier The lt factType gt can be any declared domain The lt initialvalue gt must of course be in the declared domain and when it is an expression must evaluate to a va
290. ll for pred3 This behavior is technically explained thus The scope visibility of pred3 is within the same scope of pred2 Hence there is no need to clarify that pred3 is from the same class as pred2 The compiler will automatically look for the definition of pred3 within the scope area as defined for class2 The scope area of a particular class definition is restricted to the class that is implemented in a particular file i e code that is written within the implement end implement keywords The predicates defined therein can call each other without the class qualifier and the double colon token preceding it The scope area of a class can be extended by using the keyword open This keyword informs the compiler to bring in names of predicates constants or domains that are defined in other files If a scope area is extended then we need not write the class qualifier with the double colon this is a part of the clauses in class2 open class 1 pred3 pred2 pred1 Note class1 qualifier is not needed anymore as the scope area is extended using the open keyword pred3 end of the part from class2 The section open does not look like a section it looks more like a line that starts with the keyword open followed by a comma separated list of names of classes An example open another_class yet_another_class It ends not with a period but with a hard return 156 Ope
291. llNames This predicate teaches you another use of findall 3 Here we call findall 3 and tell it to find values for ID that is the first argument and put the values found in a list called IDList But at the same time we tell findall that it should only take into account the records with the value Name as second argument Remember that in Name we store the name selected in the listbox So findall restricts the search to the records with the specified Name Finally we have to declare getSomeIDs in person cl This is the declaration getSomelDs string Name string IDList procedure i 0 Put it in person cl right after the declaration of getAllNames When you now build and execute the program it shows the ID s when you select a name That is to say only when you first filled the database But still the fields with name and ID of the record to be deleted are not shown This should be done after you select an ID Because this looks much like what happens when you choose a name I give the code without explanation Go to deletePerson dlg in the Project Tree and open it in the Forms editor Select idBox_ctl click once on it and open the events list in the properties window Select the event SelectionChangedListener en give it the value onIDBoxSelectionChanged Add the standard code double click and open the editor In the editor change the standard code into predicates onIdBoxSelectio
292. ls along their tops Center the selected controls horizontally Align the selected controls along their bottoms Spacing the selected controls evenly between the leftmost and the rightmost controls Toggle the Grid see below Spacing the selected controls evenly between the topmost and the bottom most controls Make the selected controls the same size as a model control 246 Make Same Make the selected controls the same width as a model Horizontal Size control Make Same Vertical Make the selected controls the same height as a model Size control Size To Contents Resize the selected controls to optimally display its titles Control Properties Table When you open a GUI dialog or a Form GUI window in the IDE Designer then next to the opened dialog form the Control Properties table appears If the Control Properties table is closed then double click a control the Control Properties table appears Also the Control Attributes command from the IDE Designer speed menu that pops up when you right click the Dialog can be used to open the Control Properties table You will find a comprehensive description of the Control properties in section A1 3 To change properties of a GUI package control first select this control by clicking in it in the dialog form The current set of the control properties appears in the Control Properties table Now you can edit the displayed GUI control properties l all xi ok Rapiesan
293. lse If it is True then the edit control displays all typed in characters as asterisks The default value is False displays typed characters as is You can use the editControl getPassword 0 gt predicate to get whether the control has this flag IsPassword controlName getPassword IsPassword true and the editControl setPassword 1 predicate isPassword true controlName setPassword IsPassword to set this flag programmatically ReadOnly Possible values are True and False If it is True then the edit control is in the read only mode The default value is False editable You can use the editControl getReadOnly 0 gt predicate to get whether the control has this flag IsReadOnly controlName getReadOnly IsReadOnly true and the editControl setReadOnly l predicate IsReadOnly true controlName setReadOnly IsReadOnly to set this flag programmatically Rows Possible values are positive numbers For list button and list edit controls it specifies the maximal number of rows which can be displayed in the opened list box sub control The default value is 3 You can use the listControl getMaxDropDownRows 0 gt predicate to get this number MaxDropDownRowNumber controlName getMaxDropDownRows and the listControl setMaxDropDownRows 1 predicate 261 MaxDropDownRowNumber 5 controlName setMaxDropDownRows MaxDropDownRowNumber to set this number programmatically Sort Possible valu
294. lue at compile time The lt initialValue gt may be omitted but then the factVariable must be initialized in the constructor that is the fact must be initialized at the moment when the object that it belongs to is created The fact Variable cannot exist without a value To change the value of a factVariable you must use the assignment sign So when you declare the factVariable 147 facts myFact integer 0 then the factVariable myFact initially gets the value zero To change the value you must use a statement like myFact 3 The declaration of a factFunctor consists of the name of the factFunctor followed by a colon the arguments of the functor between round brackets and optionally the mode of the factFunctor It looks like this lt factFunctorName gt lt argumentlist gt lt factFunctorMode gt The lt factFunctorName gt is a lowerCaseldentifier The lt argumentlist gt contains the types of the arguments and an optional variable name for each argument The types are separated by comma s When you add variable names there must NOT be a comma between the type and the name An example facts person string integer defines a factFunctor with two arguments The declaration person string Name integer Length defines the same factFunctor The compiler neglects the names of the variables The lt factFunctorMode gt is optional It can only be used with a factFunctor and it indicates the number of fa
295. ly onto Delta 2 dialog base units So the Size of the control changesw proportional to the change in size of the Form The other Control properties are specific for different kinds of controls They are described in the next section Cut Copy and Paste Undo and Redo You can Cut or Copy a group of selected controls onto the Windows clipboard and Paste controls from the clipboard back into a dialog The Undo and Redo commands serve to delete or restore the last editing operations of your dialog Grid When the menu or the pop up menu entry Resource Grid is activated or when the EH button from the Layout toolbar is pressed the following dialog appears Grid Specification Grid Spacing Dialog Base Unit hs Horisontal E Vertical 2 M Snap to Grid Display Grid ms xi Cancel Help The Dialog to Specify the Grid Properties With a grid in place arranging the controls inside a dialog is easier Also it is possible to tell the IDE Designer by the Snap to Grid that it should place controls at the grid intersections to give a satisfactory result 249 LM xi GroupBo Push Button C Radio Button 1 Radio Button 2 Radio Button 3 Edit Cancel Help Using a grid in the Dialog Window Form Editor Test Mode The IDE Designer has the Test Mode so it is possible to see how the dialog with the created controls looks like and behaves To illustrate the Test Mode we will assign some d
296. m the Top boundary of the dialog form containing this control The distance is specified in the Dialog Base Units Right Anchor Possible values are True and False True defines that the Right boundary of the control is always positioned at the specified distance from the Right boundary of the dialog form containing this control The distance is specified in the Dialog Base Units The distance value can be seen near the arrow which appears when the control is selected Bottom Anchor Possible values are True and False True defines that the Bottom of the control is always positioned at the specified distance from the Bottom boundary of the dialog form containing this control The distance is specified in the Dialog Base Units The distance value can be seen near the arrow which appears when the control is selected Enabled Possible values are True and False They define whether the control will be created initially enabled True or disabled False The default value is True enabled You can use the window getEnabled 0 gt predicate to get whether the control is enabled IsEnabled checkButtonName getEnabled IsEnabled true and the window setEnabled 1 predicate IsEnabled true checkButtonName setEnabled IsEnabled to set this flag programmatically Visible 254 Possible values are True and False They define whether the control will be created initially visible True or invisible False The default value is True vi
297. m the collection in class vpiCommonDialogs Between brackets is the string that should be displayed on the screen Close the editor save the changes and build and execute the project In the Task Menu the new menu option appears click it and next click on WriteNotel and the message should appear Click on lt OK gt to close the message 30 When the message window appears you will notice the word note in the title bar If necessary you can add a different title Let s add another item in the submenu under Messages and call it WriteNote2 Add the code for this item and change the clauses into clauses onMessagesWritenote2 _ Source MenuTag vpiCommonDialogs note This is a title This is a message Save the changes build the project and execute it When you choose the option WriteNote2 you see that the first argument has become the title and the second one is the message body So note 1 lets you send a short message to the user in a window with a preset title note 2 allows you also to edit the title of the window When you look back at the code you may think that it is cumbersome to type the name of the class vpiCommonDialogs every time you want to use a procedure from that class It is and luckily there is a quicker way to make the procedures in a class available Go to the Project Tree go to the folder Task Window and double click on Task Window pro This is the file where the IDE keeps the Prolog code for t
298. machine code by an interpreter and executed In VIP there is a compiler in stead of an interpreter A compiler compiles the whole program into machine code before execution and at runtime the processor executes the machine code So a compiler can check the code while it is compiling before you run the program Because of this feature in Visual Prolog the overall efficiency of a programmer improves The programmer does not have to wait till the program is actually executed to detect a bug In fact those of you who are experienced in programming would understand what a life saver this can be for often the particular sequence of events that is causing a runtime exception to happen at run time may be so elusive that the bug may actually turn up after several years or it may manifest itself in some crucially critical or other embarrassing situations All this automatically implies that the compiler has to be given explicit instructions regarding the predicates and other things that exist in the code For this instructions there are two distinct concepts In the declaration you describe in general what e g a predicate will look like and what kind of arguments it will use in the definition or implementation you enter the specific clauses for that predicate Please take care not to mix up these two notions They are used throughout this chapter and the VIP documentation Sometimes you can combine the two in one statement as you can with domains and cons
299. me Le the character x will be displayed underlined thus visually indicating that the x is an accelerator key If you need to display the ampersand amp symbol in the Text of a control then use two of them amp amp Anchoring a Control GUI controls have additional Anchor properties which help to reposition and may be resize GUI controls when the containing GUI dialog or form is resized Each GUI control has four anchor properties Left Anchor Top Anchor Right Anchor and Bottom Anchor You find them in the properties Table Each anchor determines that the specified Left Top Right or Bottom control boundary should be always positioned on the stated distance from the nearest correspondent border of the dialog frame Each of anchors can have the value True or False When the anchor value is True then this anchor is active that is when the dialog is resized the anchor is used to position the control For example when the Left Anchor is active has the value True then the left control boundary should be positioned on the stated distance from the nearest left border of the dialog frame The simplest way to see which anchors are active for a control is to select the control Then some red arrows between control boundaries and correspondent boundaries of the Dialog can bee seen Each arrow specifies that the correspondent anchor is active For example the arrow from the control top to the dialog top identifies that the Top An
300. me with it You learned to work with lists and how to assert and retract facts in a database Quite a job for one chapter Congratulations By the way in VIP there is a library of predicates to manipulate a database Take a look in the VIP Wikipedia at http wiki visual prolog com and locate the article on Visual Prolog External Database System For a short introduction take a look at chapter 20 in Prolog for Tyros by Eduardo Costa 112 Chapter 8 Object oriented programming classes and objects Visual Prolog is an object oriented programming language When you are going to program in Prolog you must know about the principles behind object oriented programming The first thing to know is that object oriented programming is shortened to OOP The other things to know are explained in this chapter Of course I cannot explain everything that is to know about OOP in a single chapter in a introductory text But the basics that you need are here I think 8 1 An OO view at the world Everywhere in and outside science people use models to describe the world around them Models are very diverse there seems to be hardly any restriction to how people describe the world In that view OOP is just another way of modeling But it has turned out to be a very powerful way of modeling Euclid used 5 axioms to create geometry Let me explain to you the fundamentals of OOP in 5 basic ideas 1 The first basic idea behind OO modeling and programm
301. ming and how it is implemented in Visual Prolog In this chapter you will find stuff about class declaration class interface and class implementation 4 Different from other Prolog dialects Visual Prolog is a strongly typed language In chapter 9 we take a look at the declarative side of VIP 5 The last few chapters are on special topics that are important in Prolog like recursion and lists Also we take a look at well known data structures as stacks queues and trees Finally there is a chapter on how to decently handle input and output By then you are ready to explore Prolog and VIP by yourself There are a few very good books available there are several articles on various subjects on the PDC site Besides as this book is for the very beginners by then you are no longer a beginner So it is time to close the book I wish you a lot of fun in reading and trying this book Please let me know when you encounter trouble I like to improve this book as much as I can You can reach me at t w de boer rug nl Please indicate in the subject line of your message Visual Prolog Acknowledgments I have to thank Eduardo Costa Sabu Francis and Thomas Linder Puls for their generosity to allow me to freely use texts they have written I extensively used and maybe abused their writings I tried to be precise and I indicate in every chapter where the text is originally from When I missed something please let me know I hope it will be clear
302. mn displays type of the error and a certain number of the message The type can be Error e or Warning w When it is an Error the compilation continues as far as possible but no linking or execution will occur When it is a Warning compilation and linking continue and if wanted execution takes place The color of Errors is red and the color of Warnings is magenta For example 193 e609 means an Error with number 609 The message w507 means a warning with number 507 You can find detailed descriptions of Errors and Warnings in the Visual Prolog Help File When the cursor is positioned on some line inside the Errors Warnings Window and you press the F1 key then the Visual Prolog Help is activated and opens the topic containing the detailed description of the error reported in the selected line The Description column contains description of the message The Filename column contains the name of the file with relative path where an error or warning has occurred When you double click the line in the Errors window which contains a compiler error or a warning message then the IDE opens the referenced source file and puts the cursor on the reported error warning position The lower edge of the Error Window contains a full description of the error full path of the file with position in text full name of message including number and description If for some reason F1 cannot be used then you can find the topic with detailed err
303. mpare it with the same predicate that was defined in chapter 5 After all the logic of that predicate has not changed So you would not find any changes to the variables used inside that predicate The biggest advantage of using a functor is that you are free to change the internal arguments of the functor without changing much of the predicates that uses such a functor as you keep revising the code That means if you decide in a later version of the code you are writing to have one more argument for the functor person you still need not change anything in the grandfather predicate That is to say as long as you bind the functor as a whole If you want to retrieve a specific value from within the functor then you could be obliged to change the calling code 6 6 Functors as arguments In the previous section the person functor was having two arguments the Name and the Gender Both happened to be simple domains in this case both were of domain string The values that are used like Bill and male are in general called constants as they do not change in the course of the program However there is nothing that prevents us from putting a functor itself as an argument of another functor Suppose you wanted to define a functor for a couple i e a husband and a wife Here is an example of how you would use such a functor A couple consists of a male and a female at least let us assume that So a couple is a kind of package
304. mples writeSomeTerms Intfile inputstream_file openfile intSource txt Stringfile inputstream_file openfile stringSource txt FunctorFile inputstream_file openfile functorSource txt readExamples readSomeTerms Intfile Stringfile FunctorFile readExamples writeSomeTerms readExamples readSomeTerms Intfile Stringfile FunctorFile readExamples writeSomeTerms _ stdIO readchar end implement main I made three files in Unicode File intsource txt contained two integers 123 456 File stringsource txt contained three lines with a string here is another string And again another one File functorsource txt contained two values for the functorfact person AnotherThomas 203 person Anna Take care to put each person on a new line and NOT to use a period at the end of the line In main three objects are created that are connected to these three files Then we call the class readexamples and make it write the values of three fact variables The class readexamples looks like this class readexamples open core predicates readSomeTerms inputStream Ints inputStream Strings inputstream Functors procedure 1 1 1 writeSomeTerms procedure end class readexamples And the implementation is domains person person string Name class facts intfact integer 1 stringFact string This is a string functorFact person person Thomas clause
305. mpty you can combine Listl and List2 to form List3 by making the head of List1 the head of List3 In the following code the variable H is used as the Head of both List1 and List3 The tail of List3 is Tail3 which is composed of the rest of List namely Taill and all of List2 In Prolog append H Tail1 List2 H Tail3 append Taill List2 Tail3 The append predicate operates as follows While Listl is not empty the recursive rule transfers one element at a time to List3 When Listl is empty the first clause ensures that List2 hooks onto the back of List3 One Predicate Can Have Different Uses Looking at append from a declarative point of view you have defined a relation between three lists This relation also holds if List and List3 are known but List2 isn t However it also holds true if only List3 is known For example to find which two lists could be appended to form a known list you could use a goal of the form append L1 L2 1 2 4 With this goal Visual Prolog will find these solutions L1 L2 1 2 4 L1 1 L2 2 4 L1 1 2 L2 4 L1 1 2 4 L2 4 Solutions You can also use append to find which list you could append to 3 4 to form the list 1 2 3 4 Try giving the goal append L1 3 4 1 2 3 4 The inference engine finds the solution L1 1 2 This append predicate has defined a relation between an input set and an output set in such a way that the relation applies both ways Given that
306. n To add i AboutDialog pro code to the File New option go B Taskwindow cl back to the project tree and look B Takwindow i E Ls P Taskwindow pack for the file TaskWindow win In T B taskWindow ph this file you manage and control TaskWindow pro the Prolog code that is linked to a 7 the various parts of the Task EY TaskMenu E Ms Window The code is eventually EA Projectlcor Attributes placed in the file E fort frm sai TaskWindow pro You could go E main cl Code td i directly to TaskWindow pro and main pack EAS ES main nh zi Figure 2 10 Open the Code Expert enter code But for now it is better not to do so Do not place the code directly It is better to use the part of the IDE that is called the Code Expert The Code Expert keeps track of the places where code is inserted When the program grows this Code Expert will guide you Trust him Click on TaskWindow win in the Project Tree with the right button of the mouse That opens a floating menu In this menu choose the option Code Expert figure 2 10 The IDE now opens the file TaskWindow win in the Dialog and Window Expert and you see a list of the many options that you have in the Task Window of your program Here you can specify many things You see the Dialog and Window Expert in figure 2 11 To add code to the menu option File New you first have to find it It is hidden in the folder Menu Open it and you see the subfolder TaskM
307. n book X Y write X Y writePossession guitar X write X Compound domains have no subtype relations to any other domain If a domain is defined as being equal to a compound domain then these two domains are synonym types rather than subtypes Meaning that they are just two different names for the same type Compound domains can have a recursive definition They can also be mutually indirectly recursive Example I domains tl ff gg integer t1 tl is a compound domain with two alternatives The first alternative is the null ary functor ff while the second alternative is the two ary functor gg which takes an integer and a term of the domain tl itself as arguments So the domain tl is recursively defined The following expressions are terms of the domain t1 ff 22877 ffQ ge 33 gg 44 gg 55 ff0 Example II domains t1 ff0 gg t2 t2 hh tl t1 145 tl is a compound domain with two alternatives The first alternative is the null ary functor ff while the second alternative is the unary functor gg which takes a term of the domain t2 as argument t2 is a compound domain with one alternative the functor hh which takes two tl terms as argument So the domains tl and t2 are mutually recursive The following are terms in the domain t1 ffO gglhh ffO 0 gg hh gg hh ffO ff ffO ggglhh ffO ghhCfO ff0 gg hh gg hh ffO ffO ge hh ffO ff0 In chapter 11 we will see more compound domain
308. n qualifications are used to make references to class level entities more convenient The open section brings the names of one scope into another scope so that these can be referenced without qualification Open has no effect on the names of object members as these can only be accessed by means of an object anyway But names of class members domains functors and constants can be accessed without qualification When names are brought into a scope in this way it may happen that some names becomes ambiguous Open sections have only effect in the scope in which they occur Especially this means that an open section in a class declaration has no effect on the class implementation 9 11 Class predicates class facts and where to declare them When a class with name lt className gt can create objects the code for it is divided in VIP over three files They are The file lt className gt cl is called the class declaration the file lt className gt 1 is called the interface e the file lt className gt pro is called the implementation These files were described in chapter 8 To repeat in short the class interface contains the declarations that are public can be called from within other files and that are connected to the objects that are generated by the class The class interface can be seen as a description of what predicates facts the outside world sees of the objects of a class So you can think of an interface as a
309. n the second clause Conclusion In this chapter you learned about simple ways of communication with the user You changed the Task Menu and used pop up windows In the mean time I hope you became familiar with the Code Expert and grew accustomed to using it There are a lot of other possibilities for communicating with the user we will meet them in later chapters In chapter 7 e g we will customize forms and other dialogs By now you are able to do the following without much trouble e Create a new project Create new items in the Task Menu Activate items in the Task Menu Create A Form or a dialog and put some controls on it e Enter Code with the help of the Code Expert In the next chapter we will take a closer look at the several parts of the Integrated Development Environment IDE and explore a little more what you can do with them 37 38 Chapter 4 A closer look at the IDE The Integrated Development Environment IDE is a very nice and helpful tool to create Prolog programs In this chapter we will make a tour around the IDE to give an overview 4 1 The IDE in general The IDE is used to create develop and maintain your Visual Prolog projects Briefly speaking you will use the IDE for the following tasks in a project life cycle Creation The project is created in the IDE at creation time you choose important properties of your project such as whether the project is an executable or a DLL whether it uses GUI or
310. nChanged listControl selectionChangedListener clauses onIdBoxSelectionChanged Source ID _T Source getSelectedltems person getPerson ID Name _ _ id_ctl setText ID name _ctl setText Name onIdboxSelectionChanged _ Source 105 When the selection in idBox is changed we get the chosen ID and then ask the module person to return the data for the person with this ID The module person returns the wanted value for Name and we use it to fill the Edit Field To make this work we need to add the predicate getPerson to person pro Please go there and enter after the code for getSomeIDs the following code getPerson ID Name Weight Length IQ person ID Name Weight Length IQ getPerson _ ID 0 0 0 0 The second clause is probably never used but the compiler likes it to be there Finally enter the declaration for getPerson ID Name Weight Length IQ in person cl getPerson string ID string Name integer Weight integer Length integer IQ procedure 1 0 0 0 0 Now we are almost done We only have to insert the code to delete the record when the user has selected a record and then hits lt OK gt To insert the code open deletePerson dlg in the editor select the lt OK gt button and open the Events list in the properties window Select ClickResponder and give it the value onOkClick Add the standard code in the editor go to the code and c
311. nDatabaseFilldatabase Source MenuTag person filldatabase This code tells that when you click on menu option FillDatabase the program should call the predicate fillDatabase that resides in the class person The names of the class and the predicate 93 are connected by a double colon It indicates that person is a class name and fillDatabase is a predicate name Dialog and Window Expert TaskWindow Show Al Handled Unhandled J Window 23 Menu E By TaskMenu id file J id_edit E id_database y id_database_filldatabase gt onD atabaseFilldat 2 03 id_help Scrollbar ntrol Co Key Mouse Miscellaneous fid_database_filldstabase JonD atabaseFilldatabase Set Figure 7 4 adding code for fillDatabase When you now run the program and click on the menu item Database FillDatabase then the code for fillDatabase in the class module person will be executed and the four records will be added to the database Go ahead and try it Close the files that are open and execute the program If you get some questions if you want to include one or more files answer yes When you click the option Database FillDatabase the records are added to the empty facts database and a message is shown in the Message Window By the way in this chapter 1 11 do all the talking via the Message Window That s because it t
312. nal or Identification 7 2 The database in VIP Let s create a new project for the minimal database I called this project chap07db but of course you are free to give it another name Choose GUI as the target Create and build the project and then it is time to add the part where the database will reside As it will be a database of people I decided to create a module to contain the database To keep the structure of the project clear I also decided to put this module in a separate package that I called person So first create the package person In the Project Tree click on the root and then create the package click File New in the Create Project Item Dialog choose Package in the leftmost window and enter the name of the package person See figure 7 1 S al Peon B Taskwindow Name person a o Parenl Directory main p nnn asa i main pro B resourcal dentifiarc i Used Packages Public FroDir pFe Private 23 ProDirl ofc exception Figure 7 1 Create Package person Click lt Create gt and the package appears in the Project Tree Now build the project again somehow I think it is better to build the project very often but that may be too often and now we must create the module where the database will reside VIP is an object oriented language I will elaborate on that in the next chapter but that means that in this chapter you will encounter things that may
313. nd from this moment on you can refer to the stream by way of the variable The Error stream should be set previously by the predicate setErrorStream 1 stdio nl_error 0 nl error procedure Description The predicate writes new line symbols to the current error stream stdio setErrorStream 1 setErrorStream outputStream Stream procedure i 201 Description This predicate sets Stream as current error stream stdio writeQuoted_error writeQuoted error procedure Description The predicates writes specified variables to the current output stream It can be called with an arbitrary nonzero number of input arguments For all string char variables the prefixed and post fixed double single quotes will be written into the stream too stdio write_error write error procedure Description The predicate writes specified variables to the current error stream stdio writef_error writef error string Format formatString procedure i Description The predicate produces formatted output to the current error stream See description of format string in outputStream interface Finally there are some predicates for special purposes that you should know They are stdio consult 1 consult factDB FactsSectionName procedure 1 Description The predicate consult 1 reads facts from the current input stream and asserts them in the internal facts section with the name Fa
314. nd the other well known Windows menu options This menu will also be referred to as Main Menu The Task Menu Bar It contains icons for the most used options in the Task Menu The Project Overview It lists the names of the projects that have been processed in the IDE When you start Visual Prolog for the first time there probably will be no recent projects e The Message Window This window is typical for Visual Prolog and the IDE Here you will find the messages from the IDE and later on you are going to write short messages in the Message Window yourself A lot more can be said about the IDE but this is enough for the moment Now it is time to do something with the IDE You are going to create a simple program As programming is a project we will use the words program and programming project interchangeably 1 2 Creating a project in VIP Let us create an empty project to which you will add functionality later on Step 1 Create a new project There are two ways to create a project When the Project Overview is still open you can click lt New Project gt in the project overview The other way when the Project Overview is closed is to click the option Project New from the Task Menu as shown in figure 1 2 isual Prolog 7 1 Commercial Edition Licensed to Thomas W de Boer File Edit View Insert Project Build Debug Goto Tools Web Window Help Figure 1 2 Task menu Project New When you click the option the
315. ne It says findAlI ID person ID _ _ _ IDList This standard predicate finds all the occurrences records that are in the database gathers the value of a specified field argument of a functor and puts the values found in a list Phew this needs a lot of explaining A list in Prolog is a set of values A list specifies the elements between square brackets and separates the elements with comma s The values can be of any kind but here I will only use strings and integers Examples of lists are 1 2 3 4 5 6 John Thomas Anna Lidy The number of elements is not restricted In fact you can also have a list with no elements It is called the empty list and looks like this One restriction however the elements in a list must be of the same type When I call the predicate findall findAlI ID person ID _ _ _ IDList 98 then the first argument ID states that I am looking for the variable ID the second argument person ID states that this variable can be found on the first position in the functor person and the third argument states that I expect findall 3 to bind the list of the values found to the variable IDList So after the call findAlI ID person ID _ _ _ _ IDList I know that all the values of ID in the database are stored in the list IDList Then in the next statement I use this list to find the maximum value of ID in the database You should re
316. ne of the four by one of the names e mesbox_iconinformation shows the information icon If you want you can also use the number 0 as an argument mesbox_iconquestion shows a question mark Alternative is the number 1 e mesbox_iconerror shows the error icon number 2 mesbox_iconexclamation shows an exclamation mark number 3 These four names are called constants They are predefined within Visual Prolog that is they represent a value that is constant within the Visual Prolog environment Instead of the names you can also use the mentioned integers in fact these integers are the values that the constants represent but your program will be better understandable when you use the names You should place the icon name after the Title string and the Message string as the third argument The buttons At the fourth place you enter an indicator of the button s that should appear in the message window The available names and their equivalent numbers are e mesbox_buttonsok 0 shows only an lt OK gt button so this looks like the note message e mesbox_buttonsokcancel 1 shows two buttons lt OK gt and lt Cancel gt mesbox_buttonsyesno 2 shows two buttons labeled lt Yes gt and lt No gt e mesbox_buttonsyesnocancel 3 shows three buttons labeled lt Yes gt lt No gt lt Cancel gt e mesbox_buttonsretrycancel 4 shows two buttons labeled lt Retry gt lt Cancel gt e mesbox_buttonsabortretryignore
317. ner clauses onDatabaseListdatabase _ Source MenuTag person listDatabase 95 This should do the trick Build and execute the program When you click in the program on Database Listdatabase you will see the four records appear in the Message Window Don t forget first to fill the database otherwise you will see nothing 7 3 Manipulating the data add a record So far so good But the fun of a database is to manipulate the data Adding records deleting records and changing the content of records In this section we will add the necessary code to do that We ll start with the functionality to add another record To enter a record we need a form In VIP there are two possibilities that are almost the same You can create a Dialog or you can create a Form In this project I shall use Dialogs but you are free to use Forms there is hardly any difference To be honest I even don t know the difference In this Dialog we gather the data of the new person for the new record and when the user clicks lt OK gt we shall enter the record in the database To keep the structure of the project clear I shall put the Dialogs in another package that I shall call forms just for fun So please got to the Project Tree click once on the root to highlight it and then add another package called forms Now highlight this new package and add a Dialog Click File NewInExistingPackage choose Dialog in the leftmost window and create a Di
318. new domain lt realPrecisionDescription gt and lt realRangeDescription gt The lt realPrecisionDescription gt declares precision of the domain measured in number of decimal digits that is the number of digits right of the decimal point You specify the lt realPrecisionDescription gt by declaring the keyword digits followed by the integral number of digits that should be used in the decimal part If lt realPrecisionDescription gt is omitted then it will become the same as for the parent domain If there is no parent domain then it will be the natural precision for the processor or given by the compiler in Visual Prolog v 6 the compiler limit is 15 digits The precision has upper and lower limits given by the compiler if the precision that you specify is larger than that limit then numbers will only obtain the processor compiler specified precision anyway 143 The lt realRangeDescription gt is the same as for integral value ranges two number between square brackets and separated by two dots As this is a range for real numbers the minimum and maximum are real numbers You can use expressions for these number but they must evaluate to a floating point value at compile time That is the compiler must be able to calculate the real domain precision and limits at compiling time The real range description declares minimal and maximal limits for the real domain If a limit is omitted then it will be the same as for the parent doma
319. nfo 2 and run 0 respectively Think of these predicates as procedures It also contains two constants className and classVersion and there is also something called a goal When you double click on an item the IDE opens an editor and puts the cursor at the place where the code for that item is In the left pane you see that each filename has an extension The extensions indicate the type of file To understand these types you must know about object oriented programming I will explain it in chapter 8 but for now a short explanation should suffice In object oriented programming OOP for short the central idea is that programs consist of parts that are called objects Objects are created in a so called class A class is a general description of a certain type of objects But a class is also a collection of activities that the objects can perform When a program is run the objects are created and perform activities These activities produce the result that the programmer intended As an example think of a database of students This program could consist of a class main that is used to start the program by creating a taskwindow with a main menu In this menu there will be options to create a student record or to search for a record To create a student record there could be a class called student that is used to enter and store the data of a student When you click the menu option to create a new student record the class
320. ng like this you end up with a tree structure The elements in red are at the end of a branch They are the leaves and together they form the sorted list 4 2 5 3 1 2 6 4 2 3 1 2 4 4 5 6 1 2 2 3 4 5 6 FA A 1 1 2 1 3 18 When you now walk from left to right along the leaves of the tree the red numbers neglecting the empty lists you will see that the elements are sorted We will apply this principle in the predicate quicksort As usual I shall presume that this predicate takes an unsorted list and produces a sorted list So the declaration is familiar quickSort integer integer procedure 1 0 From the description you can see that the predicate will be recursive So the first clause is not too difficult it is the usual sorting of an empty list quicksort Things get a bit more complex when the list is not empty Then we have to take the first element and divide the rest of the list into two new lists one with the smaller elements and the equals and one with the bigger elements Here we go quicksort SplitElem Tail SortedList split SplitElem Tail LowerList UpperList 187 first split the tail into two lists using SplitElem as a splitting element quicksort LowerList SortedLowerList then quicksort the lower list into a sorted list quicksort UpperList SortedUpperList then quicksort the upper list into a sorted list append
321. non graphic screen Think of it as the MsDos Command Prompt Window under Windows This Windows console is a plain text window for console applications within the system of Windows Application Programming Interface APJ It is called the Win32 console A Win32 console has a screen buffer and an input buffer These buffers are used as an intermediary between the keyboard and the computer and between the computer and the screen Think of it in this way When you enter characters on the keyboard they are put into the input buffer and when you hit lt Enter gt the computer reads the input buffer When there is some output the computer writes it to the buffer and when an End Of Line character is written to the buffer then the contents of the buffer are written to the screen as a single line Win32 consoles are typically used for applications that do not need to display images In VIP you create console computer programs when you set console as a target for your VIP project SThe general stuff in this chapter is based on Wikipedia and other theoretical sources much of the specific stuff is from the VIP help files and there are a few things from private correspondence with Thomas Linder Puls Thank you Thomas 191 In VIP there is a special class of procedures to read from and write to the console Think of a VIP program as a process Then you can imagine that when you work with a few programs at the same time there are many processes going on at
322. nsists of many tables but in this section we will build a minimal database It will consist of one table In the table we put data about people To prevent you from a lot of typing I will define the table as having five columns that contain respectively PersonID Name Weight Length and IQ of a person just like in table 7 1 The number of rows is not too important Let s take the data of the four persons from table 7 1 To represent a table in Prolog I shall use a functor The functor will have the structure person id name weight length iq I do take care not to use uppercase to prevent that Prolog will interpret it as a variable There is a convenient way to store data in a Prolog program It is with the help of facts The facts section is a special part of a Prolog program There you can store facts what wil not be a suprise But what is a fact The simplest form of a fact is the tuple of an Object an Attribute and a Value an OAV tuple E g I am object Thomas I have an attribute Length and an observation will tell you that with that attribute there goes a value of 190 I hope you understand that this value means 1 90 meter When you take a look at table 7 1 you see that it is loaded with facts In fact every entry in the table is a fact on its own To represent facts we could write fact Thomas Length 190 fact John Length 185 fact Anna IQ 130 Et cetera But you will remember the funct
323. nt that data represented as functors can be used like any regular Prolog variable and we can even perform all operations including recursion on such data We also learnt that we must spend time modeling our subset of the real world for which we are developing the software and get a feel for the data We should simultaneously experiment with the predicates that we may wish to use with the data that is being modeled so that they can be refined further 86 87 Chapter 7 Using Forms or Dialogs and Controls a minimal database In this chapter we will take a closer look at the form and the so called controls in it As an introduction I develop a simple database with some functionality and a simple user interface that consists of a few forms To this forms we will add the desired functionality That will make you understand how the controls in the form are used to trigger your program In appendix A1 you will find a comprehensive description dialogs and forms The text in that section comes from the Visual Prolog Help File In the first sections I shall repeat things that were already said before If you find that boring skip it 7 1 A minimal database A database consists of a structured set of data and procedures to work with them In the past several structures have been proposed In practice the relational database structure is the most applied one A relational database consists of flat tables that are somehow connected The structure of
324. nt copystream I shall comment on most of the clauses the rest is clear I hope The program starts with calling in main the predicate copy 2 copy InFile OutFile When we call it Infile is bound to source txt and Outfile is bound to destination txt These are files on disk Then we proceed with the clause of copy 2 First we open both files InputFile inputStream_file openFile8 InFile OutputFile outputStream_file create OutFile Source txt is opened as an inputstream_file and the mode is ANSI Source txt is bound to the variable Inputfile From now on when we want to refer to source txt we can simply use the variable Inputfile In this way you can copy many files by simply changing the names in the call You could also ask the user for names For the output stream a new file is created in Unicode mode It gets the name destination txt and it is bound to the variable Outputfile Now we can start copying Now we call copyStream InputFile OutputFile This predicae takes care of the copying And when the copying is done we close both the input file and the output file InputFile close OutputFile close Take good notice of the difference between InputFile inputStream_file openFile8 InFile and InputFile close In the first clause you call a predicate from the class Inputstream_file In the second clause you call a predicate from the object Inputfile Clearly when you opened the fi
325. ntegral value within the limits 0 and 300 For the new domain name we only state that is is a string Now turn to the functor person The functor was named person and it had three arguments for Name Length and Weight In chapter 7 we declared the functor person as a fact facts person string integer integer But now with the new domains at hand we can make the declaration more meaningful facts person name length weight By using domains you reach two goals the code of your program becomes more understandable and the compiler can do more precise checks In the declaration in the previous chapter the second and third argument were both of type integer and could easily be confused But with this declaration the compiler knows that the second argument must be of type length and that is something different than type height When you declare a domain you can use the name to create another domain E g you could use the domain weight to declare a new domain lightWeight domains lightWeight weight 0 50 to indicate light weight people And so on 141 Now let s take a look at the formal syntax For a start you may see a domain as a value range for a variable but in this section we will see more kinds of domains A domains section starts with the word domains that is followed by one or more domain declarations A domain declaration is at the same time a domain definition A domain definition de
326. nterface are always object predicates and predicates declared in a class declaration are always class predicates already So a class can privately i e inside the implementation file declare and define more entities than those mentioned in the declaration and or the interface files Especially an implementation can declare fact databases that can be used to carry class and object state An implementation is a mixed scope in the sense that it both contains the implementation of the class and of the objects produced by 158 the class The class part of a class is shared among all objects of the class as opposed to the object part which is individual for each object Both the class part and the object part can contain facts and predicates whereas domains functors and constants always belong to the class part i e they do not belong to individual objects By default all predicate and fact members declared in the implementation of a class are object members To declare class members the section keyword i e predicates and facts must be prefixed with the keyword class All members declared in such sections are class members Class members can reference the class part of a class but not the object part Object members on the other hand can access both the class part and the object part of the class An interface declares how the outside world sees the objects of that type But take care When you declare constants and domains in the class
327. nth 2 gt nth core positive Index Elem List gt Elem Item procedure Returns the nth element in List list remove 2 gt remove Elem List Elem Value gt Elem ListWithout Value Removes the first occurence of Value from List list removeAll 2 gt removeAll Elem List Elem Value gt Elem ListWithNoV alues Removes the all occurences of Value from List list removeDuplicates 1 gt removeDuplicates Elem List gt Elem ListWithNoDuplicates Removes the all duplicates from List Last occurences are retained this causes the order in the resulting list list reverse 1 gt reverse Elem List gt Elem Reverse Reverse contains the elements of List in reversed order 266 list setNth 4 setNth core positive Index Elem List Elem Item Elem NewList procedure 1 1 1 0 Replaces the nth element in List with Item Returns the NewList with the n th element changed to Item list sort 1 gt sort Elem List gt Elem SortedList Sorts the List list sort 2 gt sort Elem List sortOrder Order gt Elem SortedList SortedList is the sorted version of List Order specifies the sort order it can be ascending and descending Default is ascending list sortBy 2 gt sortBy comparator Elem Comparator Elem List gt Elem SortedList SortedList is the sorted version of List Comparator is an us
328. nto class person person open core predicates classInfo core classInfo constructors new string Name predicates getCreatedCount gt unsigned Count end class person Why do we declare this predicate in the class declaration person cl Well think of this Each object that is generated knows everything about itself But it doesn t know anything about other objects Especially in this case it doesn t know if it is the latest object generated or even if there are other objects at all But the class knows Because the class generates the objects it can count them and retain the number You may think of a class as an umbrella covering the objects or maybe as an mother duck covering little ducks So when you want to know the number of objects created you have to ask the class not any object So this is a class predicate and it must be declared in the class declaration This is a strict rule with no exceptions Public predicates are declared in both the interface and in the class declaration The class predicates performed by the class are declared in the class declaration The object predicates performed by any of the objects are declared in the interface declaration It is not allowed to declare object predicates in a class declaration and it is not allowed to declare class predicates in an interface 122 You have declared getCreatedCount as a public class predicate Next it will need a specification in
329. o a module Normally Visual Prolog defines each class in its own separate files During the program execution it often so happens that the program may need to invoke a predicate that is actually defined in another class file Similarly data constants or domains defined in a class may need to be accessed from a different file Visual Prolog allows such code and data references across classes using a concept called scope access This can be understood by an example Suppose there is a predicate called pred1 defined in a class called class which is written in the file class1 using the IDE and we need to call that predicate in the clause body of some predicate pred2 in a different file say class2 then this is how predl would be called within the clause body of pred2 this is a part of the clauses in class2 155 pred3 pred2 class1 pred1 pred1 is not known in this file It is defined in some other file Hence a class qualifier class1 is needed pred3 end of the part from class2 In the above example you can see that the clause body of pred2 calls two predicates predl and pred3 As predl is defined in another file which defines class1 the word class with the token two colons precedes the word pred1 This is called a class qualifier But the predicate pred3 is defined within the same file as pred2 itself Hence there is no need to indicate class2 prior to the predicate ca
330. o the root node than q then p is an ancestor of q and q is a descendant of p So Cathy is an ancestor of every other node in the tree and Charles is a descendant of Michael The size of a node is the number of descendants it has including itself In degree of a node is the number of links arriving at that node from ancestors The root is the only node in the tree with in degree 0 e Out degree of a node is the number of links leaving that node to descendants In a binary tree the out degree can be 0 1 or 2 12 6 Trees as a Data Type Recursive types were popularized by Niklaus Wirth in his book Algorithms Data Structures Programs Wirth derived Pascal from ALGOL60 and published his work in the early 70 s of the last century He didn t implement recursive types in Pascal but he did discuss what it would be like to have them Visual Prolog allows truly recursive type definitions For example you can define a tree as follows domains treetype tree string treetype treetype This declaration says that a variable of type treetype will be written as the functor tree whose first argument is a string that could represent the name of the node and whose second and third argument are of treetype We can interpret the second argument as the left subtree and the third argument as the right subtree In this way the treetype consists of things we call them nodes that have a name and 19 from Wikipedia 226
331. odule and classes are declared in a cl file The file AboutDialog i is an interface more about that in chapter 8 Finally the code for the About Dialog is in file AboutDialog pro This file consists of two parts One part where you can freely enter delete and change code the second part is maintained by the IDE It is generated every time you build the project after you changed AboutDialog dlg This pattern is repeated throughout the IDE Now take a look at the files with the name TaskWindow By now you can guess that the file TaskWindow cl contains the declaration the file TaskWindow pro contains the Prolog code the file TaskWindow win is the file to be edited In this case when you double click on this file name you get a dialog in which you can change the properties of the window You do not really edit it as you do with e g the About Dialog This is because the TaskWindow is build up from separate items like the taskmenu the About Dialog the Project Icon et cetera 43 The file TaskMenu mnu contains the main menu of your program You met it before The Project Icon is a little picture that appears in the top left corner of your program Extensions in file names are important in the IDE They indicate what type of file a file is and they help the IDE to open the correct editor when yo double click a name There are several different extensions e dlg file contains a dialo
332. of a male and a female and their names A clause for the functor couple could look like couple person John male Anna female In your program code there could be a predicate myPredicate that could do something with a couple myPredicate ACouple ACouple couple person Husband male person Wife female In this example you will notice that the functor couple is defined by using two functors both of the compound domain person each of which is a mixture of variables and constants This can be done to reflect the logic of the data being represented The logic used here is that a husband is always a male and wife is always a female and a couple consists of a husband and a wife All of which is consistent with a common interpretation of what one means by a couple But of course it describes only a part of the fascinating reality around us Though in PIE you cannot predefine the kind of grammar functors can actually have in Visual Prolog you can make such definitions The advantage of defining a functor in such a manner is that 83 later when you create actual data using this functor the Prolog engine will always check if the data being created are consistent with the grammar that data was supposed to adhere to This comes to another characteristic of functors the couple functor has been defined with two arguments and the position of these arguments reflects their logical association It w
333. of it Delete Oops it deletes the toolbar Be careful e Properties Displays several properties of the toolbar In this case the Code Expert is not available as it is only used with the TaskWindow 4 3 Creating a new item in the Project Tree The Project Tree shows the directories of your project and the items that are stored in them To add a new item to the Project Tree choose the option File New from the task menu But wait Remember that every item is stored in a file so in fact you are creating a new file In an object oriented environment like Visual Prolog you will create many files So a wise woman will see to it that the Project Tree will be a logical hierarchy of folders and files and that files that belong to a group will be The following text is partly from chapter 4 of Prolog for Tyros by Eduardo Costa 44 placed in the same folder Such a group is called a package you can think of a package as a folder in Ms Windows From time tot time you will need to create folders to store parts of your project In this section we shall create a package in the project ch04p01 and put a form in it The package will get the name pack01 and the form will get the name query Step 1 Create the package Add a new package to the project tree ch04p01 pack01 Take the following steps In the Project Tree click on the root directory ch04p01 to highlight it In the Task Menu click on File New in existing package the Create Proje
334. oject Tree and highlight ch01p01 by clicking on it With ch01p01 highlighted you are ready to create a form and it will be placed in the root directory of ch01p01 To create a form choose the option File New in Existing Package from the task menu as in figure 2 1 ch01p01 prj6 isual Prolog 7 1 Commercial Edition Licensed to Thomas W de Boer File Edit View Insert Project Build Debug Goto Resource Tools Web Window Help New in New Package CtrieN New in Existing Package Ctrl Alt N Add Ctrl Shift 4 Open Ctri O Sloss ch01p01 prj6 X data My Documents Visual Pray oj x Saye FZ Save as TI th LI 1 E QOS See Q 1 x Print Gtrl P Exit main pro rasourcel dentifiers i Figure 2 1 File New in Existing Package The IDE opens a window that offers the possibility to create several project items see figure 2 2 The name of the window is Create Project Item that is what you want to do and it is a form with two panes The left pane shows the items that you can create Take a moment to review the possible items gt This chapter is a slightly rewritten version the second chapter of Prolog For Tyros by Eduardo Costa 14 You can create a Package a Class A Dialog A Form just to mention a few In the left pane you choose the type of item that you want to create The right pane offers some options for the item Creale Project Ilem xl Name fami C New
335. ole getconsoleoutputstream stdIO setoutputstream NewOutput stdlO write Thats all When you put these lines at the end of the clauses for fill 0 then the last write statement will appear on the screen again What s left to do is the code for main pro Here it is clauses run console init 197 database fill _ stdlO readchar succeed place your own code here In the remainder of this section I give an overview of the most important predicates from stdIO Most important means that these are for a beginner the most useful ones At least I think they are I group them in four categories predicates for input for output for error and for databases For every predicate I give the name the declaration and a short description What follows is more or less an edited copy of parts of the helpfile First let s take a look at the predicates to read from the input stream stdio getInputStream 0 gt getInputStream gt inputStream Stream procedure Description The predicate returns current input stream That means that it binds a variable to the stream and from this moment on you can refer to the stream by way of the variable The variable is of type Stream you cannot print it The help file says that it should be set previously by the predicate setInputStream 1 but when I tried it worked also when I didn t set it myself You may use this predicate to temporary change the input stream and
336. olog code of your program resides in file main pro There are two ways to go to the code in main pro The first one is simply to double click on the name in the Project Tree An editor is opened with the code in it Try to find the piece of code that says clauses run Task Window task Window new Task Window show Clauses are pieces of code that are executed This clause says something like if you run this program first make a new TaskWindow and then show it to the user This is what happens when you execute the program ch04p01 The other way to reach a particular piece of code is to single click on the name in the Project Tree The right pane then shows the parts that are present in the file you clicked on as shown in figure 4 2 ch04p01 prj6 X data My Documents Visual Prolog 5 chO4p E es main E ProDir EU predicates 3 Task Window Oi a classimfo 2 E main cl RB run 0 main pack EL constants E main ph className class ersion resourceldentifiers 22 goal Figure 4 2 The Project Tree and the contents of main pro 41 393 66 In the right pane you see words like predicates constants and goal Don t care too much about what these words mean The contents of the file main pro are presented as a tree structure This tree shows that the file main pro contains a class called main which contains two predicates called classi
337. omains newDomain integer bitsize 8 0 10 If lt sizeDescription gt is omitted then the compiler will assume it to be the same as the parent domain If there is no parent domain it will become the natural size for the processor If a lt rangeDescription gt is omitted the range of the parent domain is used If there is no parent domain then the range will depend on the lt SizeDescription gt E g an unsigned num ber that is stored in 8 bits cannot be larger than 255 decimal Of course you must not specify a range where the minimum exceeds the maximum Also minimal and maximal limits should satisfy the limits implied by the specified lt SizeDescription gt If you want to you can use arithmetic expressions for lt sizeDescripton gt and lt rangeDescripton gt but they must result in a number at compile time An example constants three 3 domains subint integer 0 three Real Domains Real domains are used to represent numbers with fractional parts i e floating point numbers Real domains can be used to represent very large and very small numbers The built in domain real has the natural precision for the processor architecture or the precision given by the compiler Just like it is done with integers you declare a real domain by stating domains lt realDomainName gt real lt realPrecisionDescription gt lt realRangeDescription gt After the word real you can specify two special properties of the
338. omitted for the flow pattern anyflow then it is assumed to be a procedure Flows can also be applied to functors and lists but I ll leave that out 9 8 Section clauses A section clauses begins with the keyword clauses and consists not surprisingly of a set of clauses A file can have more than one section clauses The clauses section is meant to define or specify implementations of predicates or initial values of facts A single clause section can have clauses for several predicates and facts On the other hand all clauses for one predicate or fact that is clauses or facts with the same name and arity must be grouped together in one clauses section and without intervening clauses of other predicates or facts Clauses are used to define predicates A single predicate is defined by one or more clauses Each clause is executed in turn until there are no more clauses left to execute If no clause succeeds the predicate fails If a clause succeeds and there are more relevant clauses in a predicate left the program control can later backtrack to the clauses of this predicate to search for other solutions This is called setting a backtrackpoint Thus in principle a predicate can fail succeed and even succeed multiple times A clause consists of at least a head and optionally a body The head of a clause consists of a predicate name followed by the right number of arguments between round brackets e g person John
339. on Father and then solve father Father GrandFather Or even like this When grandFather Person GrandFather is called first call father Person Father and then call father Father GrandFather With this procedural reading you can see that predicates correspond to procedures or subroutines or functions in other languages There is a distinction between procedures subroutines and functions A function returns only one value and a procedure returns one value for every output variable But in Prolog a predicate can return several solutions to a single call compare it with a goal that gives more than one solution or even fail To fail means that is doesn t give a solution at all It is one of the funny sides of Prolog that failing can be very useful These things will be discussed in details in the next sections 5 5 3 Failing A predicate call or invocation might not have any solution in the program for example calling parent Hans X has no solution when there are no parent facts or rules that apply to Hans We say that in this case the call to the predicate fails The goal as it is a kind of predicate can also fail If the goal fails then there is simply no solution to the goal in the theory There are two ways for a predicate to fail The first way is when there is no matching fact for itself or one of its premisses The second way is when the word fail is encountered in the premisses of a predicate At that point the predi
340. oooo 58 Dialog WindoW o oo ooooo ooo 79 A O es 143 disable ias oda Dr 19 disabled aia 19 DEE eet chek rta 10 E E 76 domains sess wearin oe Ge ek 140 142 GOMAINS A ee eR es a 167 double QuOtes oooooooooooo 138 Sa ean LN ROS Aa AE ESTE 193 Edita ita ora id aa 50 Edit Field IcoM o oo oo o 46 CLEMENS aida 166 elIPSISs a eh as 135 CMAN A PT 19 SNA ble dirs aid ale aida 19 encapsulation ooooooooooooo 114 end implement 139 140 endOfStream o oo ooooooo ooo ooo 212 endOfStream 0 0 000 000 ee 203 Engine Reconsult 58 79 Engine Reset o o oooooooooooo o 79 O a as T7 enumeration values 144 CITONCOUS 2 eee 151 EOL ponia oda titi ee tae 193 error S AM it datada Gok 193 error STEAM ici id iia ada 192 Error T copes cate eds Soh a 32 CUT OT Zi do os os 32 error waming position 194 Errors Window 000002 193 AE A a a iaa 73 Events rerea ea 24 50 O ADT hens 10 270 Existing Package oooo oooooo 46 exponential part 0 138 SE eee eee eee 42 TACT soe Seen oe a lo Ee Y 54 89 147 fact database ooooooooooo o 147 fACHHUNCHON 5 ogc ce Mee ee a 147 factorial uo aS oes Haven SS 163 PACTS dada seed asses 91 119 127 140 147 PACTS oir of Sayeed Sn aes oe 157 facts database o oooooooooo o 126 facts declarations 000
341. oooo o o o 128 151 CONSU an Aon Matern Ea 212 consular le ia 202 Control vais ad cis 16 Controls toolbar 46 96 101 CICALC ii a ETIE 214 Create a form 0 0 0 0 momo 14 Create a new project 0 00000 9 create ObjectS ooooomomomoo o 119 create Project Item 14 Create Project Item dialog 45 46 Create Project Item Dialog 90 91 A EEE HA ag Atel ee hed A derek 214 Creates Objects oooooooooo 91 Creationin na siete nice Shower ats Bacio eed 39 CUlTent 204l ed in es Mien 63 Un Sacer eta anise be ee ee aca 68 CUE AA IN Me Seat ook EA 151 CUU SCOPE Yok sects Waa rece ote Soe 179 182 data Es ao data Ea E 88 data Mat orto ads aa 88 data structure 0 000 ce eee 221 data structures 0 0 0 0 cece eee 215 dataset is 88 decimal dOt o oo ooooooooo 138 decimal part 0 0 cece eee o 138 declaration oooooooooooooo 137 declaration o oooooooooooooo 135 Declaring Lists 00 5 167 deduction ooooooooooomoo ooo 55 definition o 115 135 137 Det a a eo en 21 depth first search o oooo o 234 depth first traversal 234 description sss iai eas aE E A eee eee 86 DAM id ea 148 150 deterministic predicate 72 Dial See a eee ee 96 Dialog and Window Expert 48 Dialog Window o oo oooo
342. open the class list in your program you can simply state List append Front Tail What follows here is a short description of the most important predicates in the class list In the helpfile you can find the other available predicates list append 2 gt append Elem Front Elem Tail gt Elem List List is the result of appending Tail to Front list appendList 1 gt appendList Elem ListList gt Elem List AppendList 1 takes for an input a list of lists The result List is the result of appending all the lists in ListList list getMember_nd 1 gt getMember_nd Elem List gt Elem Value nondeterm Returns the members Value of the list List list isMember 2 isMember Elem Value Elem List determ Succeeds if the element Value is member of List list length 1 gt 265 length Elem List gt core positive Length Returns the length of the List as a natural number gt 0 What else could you expect list maximum 1 gt maximum Elem List gt Elem Item Returns the element with maximal value in List list minimum 1 gt minimum Elem List gt Elem Item Return the minimum element in List list nDuplicates 2 gt nDuplicates Elem Element core positive N gt Elem List Takes for an input a single Element and a positive number N and then creates a list that contains N duplicates of Element list
343. operties window shows the UserListaner properties of the About Dialog Mark the ContextMenuResponder two tabs at the bottom of the properties ClaseR esponder El window They are labeled Properties HScr ollListener and Events Click on the tab Events MouseDblListener Th tent of the wind h MouseDownListener e content of the win ows changes Ma Moras Instead of the properties you now see a Mousel pListener list of the events that can be handled by WScrollListener the dialog It looks like in figure 4 8 CharResponder EraseB ackgroundResponder In this list you see the events that can KeyPownh esponder KeyUpResponder happen when the About Dialog is open Painthiesponder Now locate the event with the name CloseResponder This event is connected to the lt Close gt button that you find in the top right corner of a program Properties Events Figure 4 8 the events in the properties window of About Dialog 50 Whenever you click that button the program recognizes that the event CloseWindow should happen and the CloseResponder responds by closing the window But it is also possible to add more activities to this event Let s add a note that simply says what is happening To add code select CloseResponder The value field gets an arrow Click on it and the possible values for CloseResponder appear In this case the only possibility is onClose Choose it and double click on it Now the I
344. or description if you open the Index tab in the Visual Prolog Help and specify the reported error number like this e507 There is a class with predicates to write directly to the Messages window The classs is called vpiMessages and you will find details in the VIP help file It is tempting to write to the Messages Window and maybe the Error Window directly But again I must strongly advice you not to do so unless you have very good reasons It is better to use the Standard Input Output class stdIO Just like it was the case with the console you are not sure if the Messages Window is there to accept your writings The user may have closed the messages window In that case your writings are lost When you use stdlO in stead these predicates will always write to the current output channel 11 3 Streams In the old times there were not many devices to read from or to write to That has changed considerably You read from and write to USB sticks hard disks CD s DVD s ftp sockets graphical interfaces your computer receives radio TV telephone FAX and video signals It s a mess Imagine that you as a programmer had to take care of all these different signals in different buffers and that you had to code and decode every signal in its own way and maybe in its own language A hopeless job That s why nowadays we think of input and output as coming and going through channels that are called streams Every input an
345. or from chapter 6 In stead of all the separate facts we can state every fact about one object with a functor In this case it could be the functor person with the appropriate arguments In PIE we could simply use the functor in Visual Prolog we have to tell VIP that we are going to use this functor That is done by declaring that we are going to store facts in the form of a functor At the appropriate place we declare facts person string ID string Name integer Weight integer Length integer IQ With this declaration we tell that there will be a functor person with five arguments The first will be of type string and will be referred to as ID e The second will be of type string and will be referred to as Name e The third will be of type integer and will be referred to as Weight The fourth will be of type integer and will be referred to as Length e The fifth will be of type integer and will be referred to as IQ The names ID Name et cetera are for the human reader The compiler skips them After this declaration we can use in our program clauses like person 1 John 85 185 110 person 2 Thomas 75 190 150 It may seem strange to make the ID a string while it is clearly a number but there are technical reasons to do this The main reason is that the ID looks like a number but it is a code so the type 89 string is more appropriate A statistician would say that the scale of measurement is Nomi
346. oted_erTot o oooooooo o o 202 WEN rs iii 73 275
347. otype in which already three controls are put by the IDE Figure 2 5 Form Edit Window 16 The fourth window titled Properties a i x contains the properties of the selected Prop 215 x item on the form When there is no item F orm formt gt selected the properties of the form are r J Ue orm shown When you don t see the s 50 properties of the form click anywhere in Y 40 the form but not on the buttons and the Width 240 properties will look like they are in figure Height 120 i 2 6 The properties tell you the title that Font Meee Semt is shown in the title bar the coordinates men li ah aie Assigned Toolbar lt unset gt X and Y where the form will be placed Default Button lt unset on the screen the Width and Height of TitleB ar True the form and many more characteristics CloseBox True of this form Also you can set properties MaximizeB ox True by changing the value True into False Minimizebox True f Enabled True E g for not showing the title bar a Visible True closebox et cetera Feel free to ClipSiblings True experiment a little with controls and ClipChildren True properties you cannot do much harm HonzScrollB ar False Please note that the name in the Title Verts crollBar False field is th that in the Titl Minimized False jeld is the name a appears in the Title E order Size Border Bar of the form It is not the name of the form that is form1 Feel free to change the title of the form
348. ou will use the standard windows interfacing That s OK The Target Type is the kind of program the IDE should produce Exe is fine as it means that the IDE wil produce a stand alone Windows Program The other option is DLL If that means nothing to you forget about it for now The Base Directory is the folder where you want your programs to reside The base directory is not the definite folder where this program ch01p01 will reside that folder is specified in the next field The Base Directory is the folder where your program folders are located In the Sub Directory you give the name of the folder where this program ch01p01 should be stored This folder will be created if necessary in the Base Directory When you typed the name of the project the IDE assumed that the name of this sub directory would be the same But you can change this if you want For now we suggest that you keep the names of project and sub directory the same By the way you understand by now that we use the words folder and directory interchangeably There are more project settings You see them when you click the other Tabs lt Directories gt lt Build Options gt et cetera but in this chapter we only need to fill in the fields in the lt General gt tab When you are satisfied with the project settings press the button lt OK gt and you will get the Project Tree The Project Tree is an overview of the files of your programming
349. ou are an ancestor of a Person if you are a parent of Person The second clause states that you are also an ancestor if you are an ancestor of a parent of Person If you are not already familiar with recursion you might find it tricky in several senses Recursion is however fundamental to Prolog programming You will use it again and again so eventually you will find it completely natural Let us use the two ancestor clauses in a program with other family clauses parent Bill John parent Pam Bill ancestor Person Ancestor parent Person Ancestor ancestor Person Ancestor parent Person P1 ancestor P1 Ancestor and try to execute an ancestor goal ancestor Pam AA We identify ourselves with the inference engine and create a backtrack point to the second ancestor clause and then we use the first clause finding the new goal parent Pam AA This succeeds with the solution AA Bill Then we try to find another solution by using our backtrack point to the second ancestor clause This gives the new goal parent Pam P1 ancestor P1 AA 72 Again Bill is the parent of Pam so we find P1 Bill and then we have to evaluate the goal ancestor Bill AA To solve this goal we first create a backtrack point to the second ancestor clause and then we use the first one This gives the following goal parent Bill AA This goal gives the solution AA John So now we
350. ount Amount lt 45 write Name In this case the inference engine would come up with two names Anna and Carl that meet the requirements But that is too much Given your requirements the solution Anna suffices To make the engine stop after the first solution we introduce the cut run person Name piano_tuner Amount Amount lt 45 write Name I place the cut after the subgoal that fulfills the last requirement needed In this case when Amount lt 45 succeeds you know for sure that the solution is feasible and that there is no need for backtrtacking to another person What happens is that the engine skips Pete as he is no paino_tuner Then it finds John but the subgoal Amount lt 45 fails so the engine backtracks to person Anna Now the subgoal Amount lt 45 succeeds the engine passes the cut and loses every backtrack point that it has in this clause and in the predicate run So it writes the name Anna and stops 2 Prevent Backtracking to the Next Clause The cut can be used as a way to tell Visual Prolog that it has chosen the correct clause for a particular predicate And that there is no need to go to another clause for this predicate For example consider the following code r 1 a b c r 2 d r 3 r _ write This is a catchall clause Using the cut makes the predicate r deterministic That means that of the predicate r only one clause will be used It
351. pears at the specified location in the container To change properties of the GUI control you one should select it Then the list of GUI control properties appears in the GUI control Properties table There are the following icons in the Controls Toolbar Icon Insert a new Description Push Button After selection of this control the cursor becomes After selection of this control the cursor becomes ta Check Box Radio Button After selection of this control the cursor becomes O After selection of this control the cursor becomes Static Text Control ex l a i B After selection of this control the cursor becomes di z z m Edit Control After selection of this control the cursor becomes EE z List Box After selection of this control the cursor becomes E fa List Button de i After selection of this control the cursor becomes E List Edit p Horizontal Scroll Bar After selection of this control the cursor becomes 3 After selection of this control the cursor becomes Ee TE Iie Vertical Scroll Bar 13 244 After selection of this control the cursor becomes Group Box i After selection of this control the cursor becomes Icon Control 3 After selection of this control the cursor becomes Custom Control Selecting and Deselecting Controls in the Dialog Designer Click inside a control to select it A frame appears around the select
352. persons are also siblings if they have the same father Le if you have rules like this sibling Person Sibling mother Person Mother mother Sibling Mother sibling Person Sibling father Person Father father Sibling Father The first rule reads Sibling is the sibling of Person if Mother is the mother of Person and Mother is the mother of Sibling The second rule reads Sibling is the sibling of Person if Father is the father of Person and Father is the father of Sibling The reason that you receive siblings twice is that most siblings both have the same father and mother and therefore they fulfill both requirements above And therefore the inference engine finds a solution twice We shall not deal with this problem now currently we will just accept that some rules give too many results A fullBloodedSibling predicate does not have the same problem because it will require that both the father and the mother are the same fullBloodedSibling Person Sibling mother Person Mother mother Sibling Mother 61 father Person Father father Sibling Father You see that the four premisses that were spread over two rules are now in one rule connected with and 5 4 Prolog is a programming language From the description so far you might think that Prolog is an expert system rather than a programming language You ask a question and Prolog gives the answer And indeed Visual Prolog can be used as an expe
353. pert When the changes have been made close the editor save the changes and build execute the project You will see the messages appear as before You will have noticed that you can use the procedure note with one and with two arguments This may seem confusing but to Prolog it is clear the procedure note l is different from note 2 because the number of arguments differ The number of arguments is clear from the declaration or from the calling lines Another way to send the user a message is by way of the procedure error that is also contained in vpiCommonDialogs The use is identical to the use of note There are two variants of error Error 1 takes as an argument the message text error 2 allows you to also add a title for the message window I suggest that you add two more options to the menu WriteError and WriteError2 and that you add with the help of the Code Expert the following lines of code at the appropriate places predicates onMessages Writeerrorl window menultemListener clauses onMessages Writeerrorl _ Source MenuTag error This is an error message predicates onMessages Writeerror2 window menulteml istener clauses onMessages Writeerror2 Source MenuTag error The title The error message Save the changes and execute the project and you will see the error message boxes pop up Notice that there is a small difference with note 1 and note 2 The message window generated b
354. ple above When you enter the data of a new student your OO information system literally creates a new object that represents the new student Every class in a OO program has a method called new That method is called the constructor of the class it creates new instances In the example the new object will contain the data 114 of the new student To store the data in the attributes of an object each object has methods of which the name begins with set When a student object has attributes Name Address Place then there are methods called setName setAddress and setPlace When you want to retrieve those data you will ask that object after its Name or Address For retrieval there are methods that begin with get So in this case student objects will also have a method getName getAddress and getPlace An object can contain many attributes and many get and set methods These methods are sometimes called the getters and setters of a class Communication between objects is performed by calling methods In the example above the user interface calls a method in the report generator that creates the report The report generator calls a method in object student to get the wanted data Objects from other classes that interact with a specific object have to know what methods are available Methods that are available to other objects that are callable from within other objects are called public Now to make the classes as independent as po
355. ple person Bill male person Pam female or myPredicate C2 C2 couple person Pam female person Bill male In the calls you bind the gender of Person to PersonGender and the gender of Spouse to SpousGender In the remainder of myPredicate you will have to find out who is who It should be pointed out that in the PIE and many other Prolog Engines there is no way to indicate whether a functor will receive simple domain or compound domain arguments by looking at the variables that define the functor This stems from the fact that in PIE compound domains are directly used without being declared anywhere other than in the mind of the programmer For example if you wanted to use a compound domain as person Name Gender then PIE would as easily accept a logical variable such as regardingAPerson Somebody Somebody person Pam person Pam person Pam 84 person Pam female ae which actually does not make any logical sense in the current context Luckily Visual Prolog does a much better job of differentiating between simple domains and compound domains so when you get to the next chapters you would not have any such problems 6 7 Recursion Using Functors When data are described using functors they can be treated like any other piece of data For example you can write a predicate that can be made to recursively search through data which use compound domains using functo
356. postOrderTraverse Right stdlO write Element Enjoy the program You may want to experiment with the program E g in the program as it is a new element is placed in the left tree when it is smaller then or equal to the node element See what happens when you change that Change treeinsert Newltem tree Element Left Right tree Element Newleft Right Newltem lt Element treeinsert Newltem Left NewLeft Into treeinsert Newltem tree Element LEft Right tree Element Newleft Right Newltem lt Element the change is in this line treeinsert Newltem Left NewLeft Especially note what happens when you create the tree twice Then the insertions and traversals change 237 Appendix A1 Everything about Dialogs and Forms This appendix gives a complete overview for editing Dialogs and Forms The first section is on creating a Dialog and editing its attributes the next section is about designing the Dialog and the controls the third section focuses on the Control Properties The text is mainly a copy of parts of the VIP Help File In the text the words Dialog Form Window refer to the same thing a window where the user can click and edit controls Sometimes this window is called a container as it contains the controls In the Project Tree you can recognize files by their extension A Dialog has extension dlg a Form has extension frm There are little differences between Forms and Dialo
357. predicate count Number stdlO write Number stdIO nl NextNumber Number 1 count NextNumber you see that the recursive call is the last statement That implies that the program does not have to return here when the called predicate has been handled This is called tail recursion and it is important to look for when you write a program Tail recursion means that when the program is executed the compiler can replace recursion with iteration That reduces the needed space for the stack Recursion leads very easily to a huge number of calls and it saves space when these calls don t have to remember a return address I like to end this section with a classic example the factorial The formula n Say n factorial stands for n 1 2 3 4 n A program to calculate the factorial of a number could contain these clauses fac 1 1 fac Number Factorial NumberMinus1 Number 1 fac NumberMinus1 FactorialMinus1 Factorial Number FactorialMinus1 When the number is 1 the factorial is also 1 When the number is greater than one the factorial of the lesser numbers are calculated and finally the factorial of Number is calculated Let s write a program for this Create a new project or use the project from the previous section I created a new program and called it ch10factorial with strategy console In the project I created a 163 new module class that does not create objects factorial In fac
358. predicate that we need in person pro is changePerson Here it is changePerson ID Name Weight Length IQ retract person ID _ _ _ assertz person ID Name Weight Length IQ stdlO write Person Name with ID ID has been changed stdIO nl It simply removes the old record and inserts the a new one with the content that comes from the edit fields in the dialog Because I use here assertz the new record is placed at the end of the database The declaration for this predicate should be entered in person cl Please insert Finally enter a new item in the task menu I called it Change Person and put it between Add Person and Delete Person Then in the Code Expert insert the standard code and then change it into predicates 109 onDatabaseChangePerson window menultemListener clauses onDatabaseChangePerson Source MenuTag NewDialog changePerson new Source NewDialog show Don t forget to delete the underscore in front of Source 7 6 Saving and consulting the database So far so good You can add records change their content and delete them But when you exit the program the database is lost This is the fate of every internal database that is kept in facts in a program The reason to use an internal database is that it is faster than a database on disk In practice it is of course unacceptable that you lose data So let s add two more predicates
359. project It is set up like the directories in Windows Explorer in fact it shows shows a part of the directory structure on your hard disk Figure 1 4 The Project Tree The Project Tree is also called the Project Directory It shows the set of folders and modules in which the IDE keeps track of your program When you created the new project ch01p01 a directory was created somewhere on your hard disk that is resembled in the Project Tree The root name of the Project Tree is ch01p01 It is the name of the project that you entered in the project settings It is not the name of the sub directory that you indicated in the project settings In fact you don t see the name of the directory in which the program is stored on your hard disk The IDE only copes with the name of the project In this case it is ch01p01 In the root directory of the Project Tree are two folders named ProDir Lib and TaskWindow They were created by the IDE you can skip these for the moment we will come back to them later In the project directory there is also a file named main pack This file contains the basic information about your project You should be aware that there is not yet something like a program To create the program of this project we use the generating power of the IDE Based on what you declared about the project the IDE generates the Prolog code that is necessary Although you only declared the name of the project and maybe t
360. q Name Amount is pushed at the end of the queue stdIO nl Now run the program and try to manipulate the queue Just for fun I added two more predicates One to duplicate the first element in the queue and one to swap the first two elements in the queue Here are the clauses for the predicates that I placed in classQ pro duplicate Name Amount q Name Amount asserta q Name Amount duplicate 0 0 swap Namel Amountl Name2 Amount2 retract q Namel Amount1 remove the first one in the queue assert qout Namel Amount1 put the first one in a temporary place retract q Name2 Amount2 try get the second one in the queue asserta q Namel Amount put them back in the queue in reversed order asserta q Name2 Amount2 retract qout Namel Amount remove the temporary fact swap 0 0 0 0 retract qout Namel Amount1 remove the temporary item asserta q Namel Amount1 restore it s position in the queue The declarations in classQ cl are duplicate string Name real Amount procedure 0 0 swap string Namel real Amount1 string Name2 real Amount2 determ 0 0 0 0 To use the predicates add two more options to the taskmenu I called them Duplicate and Swap Here is how to change the code that the Code Expert generates in taskmenu pro predicates onQDuplicate window menultemListener clauses onQDuplicate _ Source MenuTag classQ duplicate Name
361. r clicks the lt Create gt button the constructor predicate new of class person is called This predicate should reside in person pro and it looks like this clauses new Name name Name createdCount createdCount 1 stdlO write Hello World I am a new object person My name is getName 128 number getCreatedCount stdIO write My number is number addToObjectsList This stdIO nl The first line gives the Name provided by the user to the attribute name Then the counter createdCount is raised by one and a message is shown In the next line the attribute number is set to the value of the raised createdCount In this way every object will have an unique number After the number is written there is a call to the predicate addToObjectsList This This predicates is a private class predicate It can only be called from within the class so it must be declared within person pro Here are the declaration and the clauses predicates addToObjectsList person NewPerson procedure i clauses addToObjectsList NewPerson createdObjects NewPerson createdObjects When we call this predicate we use the argument This This is a special variable This can be used in predicates It refers to the object that is in control at that moment So when we ask an object for its name This refers to the object that is asked to give its name In this
362. r foreach loop generates lists from the list_of lists each L is a list The inner foreach loop iterates X through the elements of L and writes the elements There are a number of things to notice e There is no comma before the keywords do and end foreach e The body of a foreach term is a cut scope so a cut met in the body will not influence the iteration A foreach term always succeeds or raises an exception and no extra variables are bound after the evaluation of a foreach term Therefore a foreach term can only be used for its side effects I realize that there may be some abracadabra in these last sentences Sorry for that Just keep in mind that foreach is a simple but powerful construction to proces the members of a list 10 5 Sorting In the previous section we said that the order of the elements in a list matters That brings us to the subject of sorting If you are not interested in sorting then you can skip this section There is a 182 standard predicate available for sorting But sorting a list is interesting because it gives you further insight into lists and recursion That s why I included this section in this book There are many ways to sort a list In this section I shall show you four ways Three of them insertSort selectSort and bubbleSort are just for fun because they are not very efficient The fourth way quickSort is a very famous one that you have to know I think Another restriction is that I s
363. ram can be more complicated and in the next sections it will but when you understand the basics till now you can handle also greater complexity 5 5 5 Preventing Backtracking the Cut Backtracking leads to what is called the relentless search for solutions by the inference engine The engine tries to find every possible solution Sometimes that is OK but sometimes it s not Think of the case that you one solution will satisfy your goal E g when you are looking for a piano tuner you look into the yellow pages and probably you are satisfied with the first name that you see No need to search the complete database for every piano tuner in town In this case you want the program to stop after it finds the first solution Visual Prolog contains the cut which is used to do this It prevents backtracking and thus prevents the search of more solutions The cut is written as an exclamation mark The effect of the cut is This section leans heavily on the language tutorial that came with Visual Prolog 4 0 68 simple once you pass the cut it is impossible to backtrack to predicates that precede the cut The cut is like burning your resources behind you Whenever you pass it there is no way back to an alternative You place the cut in your program in the same way you place a subgoal in the body of a rule When processing comes across the cut the call to cut immediately succeeds and the next subgoal if there is one is called
364. ram to perform some other operation on the elements rather than printing them 229 Depth first search is strikingly similar to the way Prolog searches a knowledge base arranging the clauses into a tree and pursuing each branch until a query fails If you wanted to you could describe the tree by means of a set of Prolog clauses such as father_of Cathy Michael mother_of Cathy Melody father_of Michael Charles mother_of Michael Hazel So you have two different ways to describe the family with a tree structure or with separate clauses The last one is preferable if the only purpose of the tree is to express relationships between individuals But this kind of description makes it impossible to treat the whole tree as a single complex data structure and as you ll see complex data structures are very useful because they simplify difficult computational tasks 12 8 Creating a Tree One way to create a tree is to write down a nested structure of functors and arguments as we did in the preceding example But this is not a good way It is too easy to make mistakes and you cannot change the tree dynamically So ordinarily we create trees by computation We start by creating a tree that consists only of the root node and then successively add the next nodes When you have to add a node in a tree you first have to find the place where to add the node So you go down the branches to the leave where the new node is to be inse
365. rating code that is placed in several files Take a look at figure 1 6 The IDE has added among others the file ch01p01 pro This is the file where you will find the kernel Prolog code of your program The other files also contain Prolog code To see what is inside simply doubleclick on the name but please don t change the code ch01p01 prj6 X dataMy Documents Visual a JE Task window main cl main pack maith pro resourceldentifiers i Figure 1 6 The Project Tree after the project is built Now that you have built the program you are ready to execute it To execute it choose Build Execute from the Task Menu and a window like the one in figure 1 7 will pop up on the screen TT dol File Edit Window Help ix Figure 1 7 Execution of program ch01p01 12 You see that the IDE created a normal Windows looking program with the usual menu options File Edit Window and Help When you click one of these nothing happens That is because you did not specify what should happen Only a few options really work in this program The Minimize and Maximize buttons in the upper right corner and the menu option File Exit These options work because they are standard options that are the same in every Windows application For the IDE it was not hard to guess what code you wanted here One last remark concerns the Messages window It is typically for Visual Prolog You don t have to bother about it yet By the
366. rd The engine looks in the program at the facts with the predicate father and decides for Yes or No But what when you enter as a goal not a fact but the conclusions of a rule like grandFather X Z In that case there is no fact to match with With a goal like this one the inference engine uses the rule to replace the compound goal grandFather with the two sub goals in the premisses father X Y and father Y Z Now it has two goals to solve but these sub goals are not compound and can be found in the program facts and so the engine can solve the goal There are several possibilities and you should know how the engine handles these e When you use object names in the goal the engine transfers the names from the compound goal to the sub goals E g when the goal is grandFather Sue Jim then the sub goals become father Sue Y father Y Jim 62 e When you use variables in the goal these are also transferred from the compound goal to the sub goals E g GrandFather Sue Grandfather becomes father Sue Y father Y Grandfather In the next section we see how the inference engine uses matching to solve the goal The strategy is to breakdown the goal in simple sub goals that we hope to find in the facts in our theory 5 5 2 Solving the goal When you as a human being try to find a solution to a goal like father X Y you can do it in many ways For example
367. re is a little problem here The value in an Edit Field is by definition a string As the variable Name is also a string the value produced by gettext can be directly bound to Name But for the variables Weight Length and IQ I need integers So before I can bind the values to the variables I first have to convert them with toterm 1 Now we are ready Please build and execute the project and then enter one or more new persons But before you do that fill the database with fillDatabase If you don t you will get an error Remember that when you add a new record the existing records are used to compute an ID for the new record But when the database is empty the function maximum gets nervous because it cannot find any record So please first fill the database then add a record and check the results with listDatabase Whenever you find this bug a nuisance here s how to change the code for addPerson to prevent the problem addPerson Name Weight Length IQ findAlI ID person ID _ _ _ _ IDList IDList lt gt NewID toString toTerm maximum IDList 1 assertz person NewID Name Weight Length IQ stdlO write Person Name has been added with ID NewID stdIO nl addPerson Name Weight Length IQ assertz person 1 Name Weight Length IQ stdIO write Person Name has been added with ID 1 stdIO nl Don t forget the exclamation mark Now the program detects it
368. ream It can be called with an arbitrary nonzero number of arguments For all string char variables the prefixed and post fixed double single quotes will be written into the stream too stdio writef 1 writef string Format lt formatString gt procedure i Description The predicate write produces output that is formatted automatically With this predicate you produce output that is strictly formatted E g when you want to produce neatly formatted columns The trick is that you precede the list of variables to be output by a string of format specifications As this is an overview of predicates I postpone the details of the formatting till section 11 6 And here are the predicates to write to the Error Stream They look a lot like the predicates for the Output Stream So I give them without much further comment stdio flush_error 0 flush error procedure Description This predicate forces the contents of the internal buffer of the current error stream to be written to resource The flush is useful when output is directed to a serial port and it becomes necessary to send the data to the port before the buffer is full It is also used to ensure that other process will get the latest data from the stream stdio getErrorStream 0 gt getErrorStream gt outputStream Stream procedure Description This predicate returns the current error stream That means that it binds a variable to the stream a
369. res In the previous chapter you have seen several structures to store or represent data facts functors databases and lists In this chapter I want to show you how to represent three other structures that are widely used in programming They are the stack the queue and the tree I shall present ways to implement these structures in VIP and then tell you that partly you can find them in standard classes in VIP But by then you will understand what they are and how to use the standard classes when available 12 1 Data Structures A data structure is a way of storing data in a computer program so that it can be used efficiently In the design of many types of programs the choice of data structures is a primary design consideration as experience in building large systems has shown that the difficulty of implementation and the quality and performance of the final program depends heavily on choosing the best data structure After the data structures are chosen the algorithms to be used often become relatively obvious Sometimes things work in the opposite direction data structures are chosen because certain key tasks have algorithms that work best with particular data structures In either case the choice of appropriate data structures is crucial A programming language should offer you several options to implement the data structure that you need A well designed data structure allows you to have a variety of operations be performed using as f
370. rs Let us get back briefly to the ancestor predicate which we have used in chapter 5 In order to determine if somebody is an ancestor of someone else we used a recursive definition for the predicate i e a definition that is defined in terms of itself like this ancestor Person Ancestor parent Person Ancestor ancestor Person Ancestor parent Person P1 ancestor P1 Ancestor This declaration states that a parent is an ancestor and that an ancestor to a parent is also an ancestor If you examine the variables Person and Ancestor in the above definition they can very well stand for either simple domains or compound domains Let us define the data thus parent person Bill male person John male parent person Pam female person Bill male If we now ask PIE to solve the following goal P person pam female ancestor P Who this is what we ll get P person Pam female WHO person Bill male P person Pam female WHO person John male The PIE engine has recursively examined the parent facts to reveal the two possible solutions to the answer By the way in the above query you also see that we have bound a variable P to the person compound domain when specifying the goal to PIE This was done to make the code more readable but it also demonstrates again the fact that we can bind a piece of data specified as a compound domain into any Prolog variable 6 8 Strategies for Using Functors Soft
371. rt system but it is designed to be a programming language To make it a usable programming language We need two important ingredients to turn Horn Clause Logic into a programming language Rigid search order and or program control When a program is executed it should behave predictable That is it must not react in a random way Imagine a computer program that behaves on Monday differently than it behaves on Tuesday For that we need among others a strict way in which the program handles the goal and the rules E g it should search the rules in a rigid order Side effects Horn clauses are great for answering questions and detecting whether a proposition is true But a computer program needs to communicate with the user and its environment For this we need something that is called side effects in Prolog An example of a side effect is writing to the screen as we did in the previous chapter We will deal with these subjects in the next two sections 5 5 Program Control Prolog is a programming language so Prolog knows a strict search and program control In this section we will look into detail into the way Prolog tries to solve the goal and we will look at two important notions fail and backtracking 5 5 1 Finding a match I said before that the inference engine tries to find a match for the goal that you enter When you enter a goal that is simply a fact like father Sue Jim then the search is straightforwa
372. rted and there the insertion takes place The program to insert a new node acts in the same way In each step an empty subtree is replaced by a nonempty one through Prolog s process of unification To create a tree we need a predicate Let s call it createTree To create a one cell tree from an ordinary data item is trivial create_tree N tree N empty empty This says If we want to insert a node N in an empty tree then the result is a node with two empty subtrees and we write tree N empty empty that is a one cell tree containing node N Building a tree structure is almost as easy The following procedure takes three arguments Argument one and two are for input the new node and the existing tree en argument three is for output the new tree Each argument is a tree The procedure inserts the first tree as the left subtree of the second tree giving the third tree as the result insert left X tree A _ B tree A X B 1 10 Notice that this rule has no body there are no explicit steps in executing it All the computer has to do is match the arguments with each other in the proper positions and the work is done Suppose for example you want to insert tree Michael empty empty as the left subtree of tree Cathy empty empty To do this just execute the clause insert_left tree Michael empty empty tree Cathy empty empty NewTree 230 and NewTree is immediately bound to tree Cathy tree Mi
373. s 0 0 0 c cece eee eee 194 CAT Heist air ap eeu ade ayaa heats 76 Character aa ii aoe eerste ees ahs 138 character literal 138 characteristics o ooooooooooo 113 CLASS en ols AI 27 42 O T NS 90 91 113 class declarati0M 122 AE aaae a E EA 123 147 Class fatto 5 eka ess ets da 123 124 class factS 124 140 149 157 class facts oooooooooo o 91 127 class Member o o ooooooooo ooo 158 class predicate o oo o 122 129 class predicates 121 140 157 class predicates oooooooooooo o 124 class Stata ater a eon ia 124 Classes a td li 121 clas idas aia 153 Clauses ee eee eee o 55 140 153 Clickresponder 00 00 0000 ee 99 269 ClickResponder oo oooooooooo o 106 COSA a aa Os eae 212 CloseResponder o ooooooomoo ooo 50 Code Expert eieo eia nea 20 48 93 COMMON ankea naa p aea eee 136 compile time errors 4 135 COMP aa rat Re wales 134 COMP iru ds 135 compound domain ooooo ooooo o 78 compound domain 205 82 compound domains 77 144 compound goal 0 00 ee ee eee 62 CONCLUSION its ata 54 conditions e aese ho Me We 153 console ui a A 191 console computer programs 191 constants 0000000 139 146 constructor 00000000 117 127 ConstructOr 2 eee 114 constructors o o oo
374. s 5 The fifth idea is that objects are related to each other There are several kinds of relations but for now I skip this What is the use of all this Well basically it is a way to cope with the complexity of programming computers When you use a program you want it to do some job but at the same time you want it to interact with other programs you want it to have help files maybe you want it to check your spelling to give you access to internet and so on At the same time you want to be free in choosing what the program should do at a certain moment You don t like programs that allow you only one option at a time To create such a program is quite a job OOP is a way to create programs that are built from say modules Each module takes care of a part of the job and therefore is less complex than the total program 113 Let me give an example Imagine that you have a database system of students teachers and courses When you program this information system in say Ms Access you create the necessary tables and the user interface Then you write the code that is necessary to operate the system When the user clicks an option in the interface the program recognizes the option gets the data from the tables and performs the task that the user indicated Every option in the menu is handled by the same program In an OOP environment we would use a class of students For every living student the class would generate a different object tha
375. s An example is domains person person string integer integer Although the declaration is correct it is not very clear An alternative is domains person person string Name integer Length integer Weight In this case the compiler ignores the variables Name Length and Weight But your program is better understandable This can become more complex Let s assume that we want to make a database of persons and their belongings Suppose that a person can have a book with title and author or a guitar with a brand name Now we can declare domains personName string author string 144 title string brandName string possession book author title guitar brandName notice the semicolon a logical or class facts person personName possession Facts will be treated in the next section Now the declaration says that the functor person contains two arguments a personName and a possession A personName is from the domain string or for short is a string a possession can be from the domain book or from guitar The semicolon is read as a logical or In your program you can initialize the facts with clauses like person John book Eduardo Costa Prolog for Tyros person John guitar Fender And you can use statements like person Name Possession The two variables are bound here writePossession Possesion And then have the clauses for writePossesion writePossessio
376. s classInfo className classVersion clauses writeSomeTerms stdlO write intfact intfact nstringfact stringfact nfunctorFact functorfact stdlO nl readSomeTerms Intfile StringFile FunctorFile intfact Intfile read stringfact StringFile read functorFact FunctorFile read end implement readexamples I initialize the facts with a value When readsometerms is called new values are read from the three files Although the three facts are of different type the program determines what to read in each case from the type declarations Sometimes it is not possible to give such neat type declarations like the ones in this example In that case you can force the typing of a variable with the help of a special predicate hasDomain 2 It takes two arguments a type argument and a variable name The use is like this 204 predicates readint InputStream S gt integer Value clauses readint S Value hasDomain integer Var Value S read By the use of hasDomain 2 you explicitly state that the variable Value has type integer so the predicate read 0 will try to read a integer from inputstream S In stead of integer you can of course use other types predefined or declared 11 6 The predicate writef en the format string The predicate writef produces formatted output to the output stream To format the output you provide a so called format string The general synta
377. s listSelect Select a name NamesList 0 Name Index person showSinglePerson Name onshowsingle _ Source MenuTag Then I declared the new predicates in person cl getAllNames string Nameslist procedure 0 showSinglePerson string Name procedure 1 The predicate getAllNames 1 produces a list of the names of the objects This is the code clauses getAllNames NamesList getNames createdObjects NamesList 130 class predicates getNames person CreatedObjects string NamesList procedure 1 0 clauses getNames ObjectsList NamesList Head Tail ObjectsList Name Head getName getNames Tail TailNamesList NamesList Name TailNamesList getNames The predicate getAllNames 1 uses a private class predicate getNames 2 to produce the list of names Just like it was done before the list is given as an argument and getNames 2 one by one takes the head off the list and asks the object for its name The gotten names are collected in a list and upon termination of the predicate returned to the calling predicate in Taskmenu pro The list of names is then offered to the user with the help of the standard predicate listSelect from the class vpiCommonDialogs that was introduced to you in chapter 3 The user chooses a name form the list and then the predicate showSinglePerson 1 in person pro is called It shows the data of the person wanted Here is the
378. s a variable It means that in this place you can use any object that is of type person Of course the rule will not hold for any combination of persons but that is another matter Notice that in this rule the premisses are facts It is also possible to use conclusions of other rules as premisses 54 If you know about logic then you will recognize the pattern that is known as deduction The difference is that in logic we are used to write something like IF p AND q THENr while in prolog we would write ri p q When the premisses are true then the conclusion is also true This is more or less the case in Prolog The general form of a rule is conclusion premiss1 premiss2 premissN Let s return to the grandfather rule As it is formulated it is difficult to understand E g from the way it is written you cannot see directly which Person is supposed to be the grandfather It is better to use understandable names for the variables instead of Personl Person2 and Person3 In Horn Clause Logic you can do that just as you like it You are free to choose almost any variable name as you like so you can e g rephrase the rule to grandFather GrandChild GrandFather father GrandChild Father father Father GrandFather Realize that this is only convenient for human readers of your code Computers don t realize that a variable Father should have a child Your computer would be just as happy with a rule ex Y X Z
379. s and returns the name of the object as a string The variable Name is used later on The other procedure setName needs one argument of type string and sets the name of the object This is all you as an outsider need to know of the object You ask for the name and you get it What is going on inside the object is none of your business as long as you get the name Of course you should realize that there are a lot of conventions behind this all GetName and setName are familiar procedures in OO programming When you create other procedures be sure to make clear to the outside world what they do By the way you have declared the procedures as predicates This is normal in Prolog a procedure is a kind of predicate In the next chapter I shall elaborate on declarations Also you should realize that you only have declared the predicates What they do and how they do it say the Prolog code for it is another matter Now close the file person i and take a look at person cl It is the class declaration of the class person In it is the following code class person person open core predicates classInfo core classInfo short Class information predicate Adetail This predicate represents information predicate of this class vend end class person Don t bother about the lines that start with a sign This sign marks a comment A comment is some insertion in a program that explains what is happening at that place It is
380. s like trees List Domains List domains represent sequences of values of a certain domain Normally we think of a list as a series of elements between square brackets and separated by comma s The elements must be of the same type An example 1 2 3 2 1 is a list of integers You declare a list by naming the domain of the elements followed by an asterisk domains realList real 9 5 Section constants A constant is a variable that has the same value throughout your program One uses a constant to make the program more understandable It makes more sense to use in the code the variable V ATpercentage than the value 0 20 Besides when you use a constant and the value changes taxes do change then you only have to change the constant declaration This will prevent mistakes A constants section defines one or more constants in the current scope It starts with the keyword constants For every constant you create a line that states the name of a constant followed by a colon the type of the constant an sign and its value The line ends with a dot So in general the declaration of a constant looks like constants lt constantName gt lt constantType gt lt constantV alue gt The lt constantName gt is a lowerCaseldentifier The lt constantValue gt should be conform the lt constantType gt If it is an expression then the compiler must be able to evaluate it at compile time Examples constants vat real 0 19
381. s that are in list L and that are even but before putting a number in the list increment it with 1 So you get a list of odd numbers Not the odd numbers in the list If you want you can get the same list with 179 OddNumberList Y isMember X L X div 2 0 Y X 1 In English The list of Y s such that there exists an X which is member of L and which is even and Y equals X 1 List comprehension is a generalization of findall 3 Consider the schematic findall term findall V Gen L This is equivalent to writing L V Gen The main differences between findall and the list comprehension is that findall is a predicate that binds the result to a variable whereas the list comprehension is an expression Now let s use list comprehension in a program To repeat when we say ListParam VarName mypredicate then it means e The first argument VarName specifies which argument in the specified predicate is to be collected into a list The second argument mypredicate indicates the predicate from which the values will be collected The output ListParam is a variable that holds the list of values collected through backtracking The next program uses list comprehension to print the average age of a group of people class listcom domains name string address string age Integer predicates person name address age multi 0 0 0 sumlist age age integer procedure 1 0 0 end class l
382. s that were added before have to be removed before the new element can be accessed Queues provide services in computer science transport and operations research where various entities such as data objects persons or events are stored and held to be processed later In these contexts the queue performs the function of a buffer The defining attribute of a queue data structure is the fact that it allows access to only the front and back of the structure Furthermore elements can only be removed from the front and can only be added to the back In this way an appropriate metaphor often used to represent queues is the idea of a checkout line Other examples of queues are people traveling up an escalator machine parts on an assembly line or cars in line at a gas station The recurring theme will be clear queues are essentially waiting lines In each of the cases the customer or object at the front of the line was the first one to enter while at the end of the queue is the last element that entered Every time a customer finishes paying for his bought items or a person steps off the escalator or the machine part is removed from the assembly line etc that object leaves the queue from the front This represents the queue pop function Every time another object or customer enters the line to wait they join the line at the end and this represents the push function The queue size function would return the length of the line and th
383. s to be used in traditional Prolog we can use it without first forewarning the Prolog engine about our intention to use it We simply use a functor the moment we feel the need for it However in Visual Prolog we first need to declare the existence of a predicate to the compiler before writing down the code for the clause body of it Similarly before using any functors they need to be declared and their presence is to be informed to the compiler The reason a forewarning is required in Visual Prolog is essentially to ensure that runtime exceptions errors bugs are converted into The text in this chapter is partly from the tutorials by Sabu Francis partly from the VIP helpfile and partly from the Visual Prolog Language Tutorial that came by Visual Prolog Version 4 0 It describes only a part of the declarations that are possible for beginners 134 compile time errors as far as possible By runtime exceptions we mean the errors that turn up when you run the program For example if you intend to use an integer as the argument of a functor and instead of the integer you erroneously use a real number things will go wrong when you run the program We say that it is a runtime error and the program will fail there This is the case in many other programming languages that have a so called interpreter In these languages the program is stored as a text and only at runtime the necessary program code is translated statement by statement into
384. save more then once So in the first clause of saveDatabase 0 we look if the file already exists This is done by the standard predicate existFile 1 If the named file exists this predicate succeeds otherwise it fails If the file exists we use delete 1 to delete the existing file and then save 2 creates the file a new and adds the records If the file doesn t exist the predicate existFile 1 fails and the engine goes to the second clause There it simply writes the data to a newly created file personDB txt Finally add two declaration lines to person cl Here they are saveDatabase procedure consultDatabase procedure Now the program is capable of save and retrieving data from an external file on disk Take care that this implementation of saving and retrieving is error prone You must first save data to disk before you can consult the file otherwise the program will crash Also the name of the file on disk is preset the data are always written to personDB txt You will find this file in the same folder as the program ch07db exe It is the folder with name Exe in the project folder ch07db Another nuisance is that when you consult the data on the external file they are added to the existing data in the internal file So consulting two times in a row results in two times the number of records If you want to avoid this you must add a clause that deletes all records in the internal facts datab
385. se new options in a separate menu item According to Windows tradition the options File and Edit should stay on the left side of the menu bar and Help should stay on the right side So our new options should be placed between Edit and Help To achieve that highlight amp Edit and click lt New Item gt on the toolbar The IDE opens a small edit window where you can enter the name of the new menu option Let us call the option Messages The editor should start to look like the one in figure 3 3 If you want you can add an ampersand amp somewhere in the name If you do so e g you enter amp Messages then Windows will allow the user to use the letter after the ampersand as a shortcut Compare amp File or amp Edit You can close the edit field with lt Enter gt or with a mouse click anywhere Below Messages we want to create a submenu Highlight Messages and click lt New Subitem gt on the toolbar Below the option Messages the editor opens an edit box where you can enter the name of the new option Let s call this option WriteNotel See figure 3 3 If you want you can check the new menu by clicking the option T on the toolbar The Taskmenu changes into your new menu Click on the Project Window to return to the original Taskmenu Close the menu editor by clicking on the x in the upper right corner and don t forget to save the changes you made We now have a new menu option so it is time to generate som
386. se are needed at compile and link time In the Project Tree this folder is shown as a subdirectory 40 of your project but in fact it may reside somewhere else on your hard disk That s why we call this a logical node Below ProDir comes another directory with the name TaskWindow which is a real subdirectory of the project directory This directory contains all the code needed to produce the Task Window of your program its menu and toolbar s and the about dialog The Task Window is the main window of your program that is shown at startup Because there are some traditional rules as how this window will look under MsWindows the IDE knows what is expected and can create parts of the code without your specifications Finally you see a number of files with the names main and resourceidentifiers i I will neglect the file resourceidentifiers 1 for now The other files are the files that will contain your program Of course you will probably add more and more files but these files are a start You should realize that your program is spread over several files This is a consequence of the fact that Visual Prolog is an object oriented language Each file contains a part of your program I shall elaborate on these files in chapter 8 for now a short introduction suffices Your program is declared defined in file main cl The extension cl stands for class For now you can think of a class as a module The Pr
387. se it in your program The code that you need is in a special so called class Why it is called a class is not important at this moment I will come to that in a later chapter For now you can think of a class as a set of procedures that have been written and can be used whenever you need them Procedures are little programs that can be used from within your program I deal with procedures in the next section The class we will be using in this chapter is called vpiCommonDialogs It contains over 30 procedures for communication with the user If you want you can take a look in the helpfile Go to the index and look in the index for vpiCommonDialogs 3 1 About procedures When you start programming you will realize after some time that the code that you are writing must have been written a thousand times before by other people Well maybe not exactly the same code but surely many times before people wrote code to open a form to enter some input to say Hello to the world It would be nice when you could use these pieces of already written code in your program Reusing existing code is an efficient way of writing programs There are several ways to implement reusing code One way is to have snippets of code at hand and copy and paste them in your program when necessary Another way is to include in your program a library of pieces of code that perform the needed tasks In this chapter we will use the second way 26 You ad
388. sed the named predicate to start with However this is not the case with Visual Prolog In Visual Prolog programs are compiled and the compiler has the responsibility to produce efficiently executing code for the program you write It is important to notice that compiling the program and executing the compiled program are two different things First the program is compiled and then later the compiled program is executed Hence the compiler needs to know beforehand the exact predicate from which the code execution will start so that later on when the program is called to perform it can do so from the correct starting point As you may have guessed the compiled program can run independently without the Visual Prolog compiler or the IDE itself 154 In order to obtain a compiled program there is a special section that starts with the keyword goal This section contains a special predicate run that is used without arguments That predicate is the one from which the entire program execution will start So the goal section is the entry to a program It starts with the word goal followed by a clause body An example goal mainExe run main run I made a program called procfunc that writes to console The goal says that the engine should look for the predicate run in the class main This class was created by the IDE when I created the new program project The clause for run in main looks like t
389. should show It is possible to show all the events that is the default but it is also possible to choose one of the options Handled and Unhandled When you check Handled only the events that have code attached are shown When you check Unhandled only the events that have not yet code attached are shown When a window contains many events it is sometimes handy to choose Handled or Unhandled For now you can simply keep the default All In chapter 2 I told you to be careful The Code Expert is a handy tool but it should not make you careless I repeat here a section from chapter 2 because I think it is important as an illustration of what I mean When you lt delete gt code that was inserted by the Dialog and Window Expert please do 49 good housekeeping E g when you want to delete the code for onFileNew you open the Task Window win in the Dialog and Window Expert as you did in the previous section look for id_file_new gt onFileNew highlight it and then click lt Delete gt What happens is that the IDE deletes the line of code for the listener but it does NOT delete the program code that you changed The reason is that you probably have changed that code Therefore it is difficult to decide what exactly should be deleted A second reason is that by deleting the IDE could destroy code that you want to keep to use it later or elsewhere That s why you must delete that code yourself by hand Things may go wrong in this c
390. sible You can use the window getVisible 0 gt predicate to get whether the control is visible IsVisible checkButtonName getVisible IsVisible true and the window setVisible 1 predicate IsVisible true checkButtonName setVisible IsVisible to set this flag programmatically TabStop Possible values are True and False They define whether this control belongs to a subset of controls through which the user can move using the TAB key The default value is True the control belongs to a group of controls between which the user can navigate with TAB You can use the control getTabStop 0 gt predicate to get whether the control has this flag IsTabStop checkButtonName getTabStop IsTabStop true and the control setTabStop 1 predicate IsTabStop true checkButtonName setTabStop IsTabStop to set this flag programmatically If IsTabStop is true then the focus will be able to jump to this control while using the Tab for navigation Pressing the TAB key moves the focus to the next control which has the TabStop True A1 3 2 Specific Properties of Different GUI Control Types The following properties are specific to different types of controls 3State Possible values are True and False They define whether this check box can have 3 states unchecked checked undetermined You can use the checkButton getStyle 2 predicate to get whether this state is set to a control checkButtonName getStyle _ IsAuto Is3State
391. simple way to avoid PIE s own output is to make sure that the goal has no solutions Consider the following goal ancestor Pam AA write Ancestor of Pam AA nl fail Fail is a predefined call that always fails i e it has no solutions The first three predicate calls have exactly the same effect as above an ancestor is found if such one exists of course and then it is written But then we call fail this will of course fail Therefore we must pursuit a backtrack point if we have any When pursuing this backtrack point we will find another ancestor if such one exists and write that and then we will fail again And so forth So we will find and write all ancestors and eventually there will be no more backtrack points and then the complete goal will fail Imagine we find every solution but the goal fails There are a few important points to notice here e The goal itself did not have a single solution but nevertheless all the solutions we wanted were given as side effects Side effects in failing computations are not undone These points are two sides of the same thing But they represent different level of optimism The first optimistically states some possibilities that you can use while the second is more pessimistic and states that you should be aware about using side effects because they are not undone even if the current goal does not lead to any solution Anybody who learns Prolog will sooner or later exper
392. sing the IDE Programs under Windows consist for a great part of code that is used for communication with the outside world and the user is an important part of it So it is useful to know about how to communicate with the user Besides it is an easy way to get to grips with VIP and the IDE In this section we will show the user some messages and ask for input Writing a computer program is a lot of fun and often quite frustrating But it is not a goal in itself There is a user out there you will have to talk and listen to that user In this chapter I introduce some simple ways to do that You will learn to create messages and create ways for the user to enter input At the same time you will become more and more at home in the IDE and you will see patterns in programming As I like to go slowly because I think you are a beginning programmer I will be repeating things telling the same story again and again If I go too slowly for you you can skip parts of this chapter When you use a Windows program you are familiar with several popup windows that tell you things you need to know In this section I introduce standard ways to talk to the user like notes error messages and message boxes They are available in VIP so you can easily use them Available in VIP means that there is a part in the IDE where you can find the code In your program you will have to tell the IDE that when it builds the program it has to get that code and u
393. sively removing the head of the list and usually doing something with it until the list is an empty list 189 The standard list predicates can be found in the list class Visual Prolog provides a built in construction list comprehension which takes a goal as one of its arguments and collects all of the solutions to that goal into a single list It has the syntax Result Argument myPredicate Argument Because Visual Prolog requires that all elements in a list belong to the same domain you use functors to create a list that stores different types of elements Sorting a list can be done in various ways Not every way is an efficient way 190 Chapter 11 Reading writing streams and files To be useful a computer program has to communicate with its user So a computer needs to know where it can read input from the user and where it can send output to the user During the development of computers and computing the communication with the user has developed as well In this chapter we take a look at the way your program communicates with the user To understand the way input and output is handled in VIP you need to know a bit more about the background and history of computer input and output So I decided to make this chapter more than only a summing up of the predicates that are there for input and output But take care As I am a newbee in object oriented programming myself I had to gather everything from several sources and then
394. son Bill male And we specify Pam as person Pam female To indicate that Bill is the father of Pam we now have to write father person Pam female person Bill male The pattern is the same as when we wrote father Pam Bill in chapter 5 the only difference is that now the arguments are from a compound domain The family program becomes father person Bill male person John male father person Pam female person Bill male 78 99 99 father person Sue female person Jim male grandfather Person Grandfather father Father Grandfather father Person Father Enter these lines in the program editor of PIE the result will look like in figure 6 1 x Data My Documents Yisual Prolog 7 projects Intro isualer 4 1 Insert Indent father person Bill male person John rmeale father peraon Pan fenale person Hill male father peraon Sue fenale person Jir mele j grandfather Person Grandfather father Father Grandfather father Person Father Figure 6 1 A program with compound domains Please note that in the PIE Prolog Inference Engine that we are using we can directly use a compound domain without any prior intimation to the Prolog engine for example we need NOT declare a predicate or a fact called person for our code to work In fact you can declare a compound doma
395. sponder The ClickResponder responds to the 51 event that you click the button Choose the event onButtonOkClick it is the only option and go to the code The code that you see is something like predicates onButtonOkClick button clickResponder clauses onButtonOkClick _ Source button defaultAction Change the clauses part into clauses onButtonOkClick _ Source button defaultAction vpiCommonDialogs note OK I ll close the AboutDialog Now close and save the code and other parts and build and execute the program Click on Help and choose About A more or less standard About Dialog opens When you close it you will see the note In this case you see clearly that closing the window by way of the lt close gt button is a different event than closing the Dialog by clicking the lt OK gt button In practice you probably will never add program code to the About Dialog but I thought it would be a funny example that shows that you can indeed change every item of a program To add code to other parts the procedure is the same Open the item Dialog Form Toolbar whatever in an editor select the item push button radio button whatever where you want to insert code and open the events list in the properties window Select the event double click on it and in the code editor that opens change the inserted standard code Sounds easy isn t it It is not too difficult But what code to insert
396. ssible you don t want every method to be public Because when you change a method you could be forced to go to every other class that calls this method to change the call So we prefer to keep methods out of sight as much as possible This is called loosely coupling and it is an important principle in OOP So there are methods that are not callable from outside an object These methods that are only callable from within the object itself are called private methods Summarizing an OO program consists among others of classes Classes create objects and objects perform activities because they get messages procedure calls from the user interface or from other objects Visual Prolog is an OO programming language That means that when you program you will have to think in classes and objects Every operation is performed by a method in an object When you use PIE this is not the case PIE resembles Prolog from before the OO era That is why PIE is a good place to learn to understand the inference engine in Prolog because you are freed from the OO and GUI hullabaloo But when you are going to write a program in VIP that will be used by other people you will have to learn and apply OO thinking and GUI practices 8 3 Classes and objects in Visual Prolog To see how classes objects and messages go in VIP we shall create a project to generate objects of a class person You will create new objects and delete objects Please create a new project I call
397. ssible values are Insensitive Upper Lower The default value is Insensitive Insensitive Defines that the edit control leaves the typed in text as is This is the default value Upper Defines that the edit control converts all typed in characters into the upper case You can use the editControl getUpperCase 0 gt predicate to get whether the control has this flag IsUpperCase controlName getUpperCase IsUpperCase true and the editControl setUpperCase 1 predicate IsUpperCase true controlName setUpperCase IsUpperCase to set this flag programmatically Lower Defines that the edit control converts all typed in characters into the lower case You can use the editControl getLowerCase 0 gt predicate to get whether the control has this flag IsLowerCase controlName getLowerCase IsLowerCase true and the editControl setLowerCase 1 predicate IsLowerCase true controlName setLowerCase IsLowerCase to set this flag programmatically Class Specify a name of a Windows class for the custom control your application code must register the custom control by this name Default Possible values are True and False They define whether the push button control is the default push button When lt Enter gt is pressed by the user the action of the default button is activated You can use the dialog tryGetDefaultButton 0 gt predicate to get the object of the default button in the dialog Similarly you can use
398. st For example 167 the head of a b c is the element a the tail of a b c is the list b c What happens when you get down to a one element list The answer is that the head of c is the element c the tail of c is the list The empty list still a list but empty If you take the first element from the tail of a list enough times you will eventually get down to an empty list The empty list cannot be broken into head and tail This means that conceptually lists have a tree structure just like other compound objects The tree structure of a b c d is list a list b list Cc list d An explanation of this structure is that it says that the list a b c d consists of the element a and the list b c d The list b c d consists of the element b and the list c d Et cetera Further a one element list such as a is not the same as the element that it contains because a is really the compound data structure shown here list a List Processing Prolog provides a way to make a head and a tail of a list explicit Instead of separating elements with commas you can separate the head and tail with a vertical bar For instance a b c is equivalent to a b c In this way you can split up the list in the head and the tail This process can be continued a b c is equivalent to a b c which is equivalent to a b c You can even
399. stack without removing it from the stack In VIP the list is a convenient way to create a stack The procedures for pop peek and push are relatively simple To show how to use a stack in VIP I created a project called ch12Stack with target GUI In this project I created a class that does not create objects with the name stackprocs to contain the procedures For the stack I created a facts variable with the name stacklist It s declaration in stackprocs pro is class facts 217 stacklist stringlist ddd ccc bbb aaa The stacklist was initialized with ddd ccc bbb aaa so I didn t have to push some elements first But normally your program will start with an empty stack In stackprocs cl I declared the domain stringlist domains stringlist string In stackprocs cl I declared the predicates for pop push et cetera as procedures but in this case I guess it is equally well to declare the predicates peek and pop as functions predicates push string Element procedure i peek string Element determ 0 pop string Element determ 0 The procedure push puts an element at the first position of stacklist and returns the new stacklist push Element stacklist Element stacklist Put the new element in front of the stack Peek takes a look at the first element and returns it without changing the stack list peek Element Element stacklist take a
400. sts and puts the elements together into one list thereby retaining the order of the elements Because I call the predicate with append SortedLowerList SplitElem SortedUpperList SortedList You should realize that the splitting element is already in place So now I only have to take care of the elements of the lower list Here is the declaration append integer integer integer procedure i 1 0 These are the clauses append List List append Head Tail List2 Head List3 append Tail List2 List3 To run the program I adjusted the run 0 predicate run console init UnsortedList 4 2 5 3 1 2 6 4 188 stdIO write The unsorted list is UnsortedList stdlO nl sort insertsort UnsortedList InSortedList stdlO write The sorted list insert sort is InsortedList stdlO nl sort selectsort UnsortedList SelSortedList stdIO write The sorted list select sort is SelsortedList stdlO nl sort bubblesort UnsortedList BubSortedList stdlO write The sorted list bubblesort is BubSortedList stdlO nl sort quicksort UnsortedList QuickSortedList stdIO write The sorted list quicksort is QuickSortedList _X stdIO readchar succeed This should do the job If you think it is a pity that you don t see how the sorting is going on feel free to insert write statements at the appropriate places As you can see in the code the predicate
401. syntax of such domain declarations and they cater to all possible kinds of domains that will be used later on in the code As this book is a basic one we shall only partly describe the domains declarations that are possible Facts The keyword facts designates a section which declares the facts that would be used later on in the code of the program A fact can be a single variable but it can also be a functor that holds many values Each fact is declared with its name and in case of a functor the arguments that are used for the respective facts along with the domains that those arguments belong to Predicates The section that starts with the keyword predicates contains the declarations of the predicates that will later be defined in the clauses section of the code Just like the facts declaration the names that would be used for these predicates along with the arguments and the domains to which the arguments belong to are indicated in this section Clauses Of all the sections that are present in a Visual Prolog code the section that starts with the keyword clauses is the one that closely mimics a traditional Prolog program It contains the actual definitions of the previously declared predicates And you will find that the predicates used here will follow the syntax as declared in the predicates section Class facts A fact denotes a kind of attribute Most facts will be attributes of an object In that case the fact will
402. t second and last are answered with a simple yes or no because you are asking if a fact is true For other goals like the third we seek a solution like X Bill Some goals may even have many solutions For example father X Y has three solutions there are three facts that give a solution X Bill Y John X Pam Y Bill X Sue Y Jim A Prolog program consists of a theory and a goal When the program starts it tries to find a solution to the goal in the theory This search for solutions is performed by a part of Prolog that is called the inference engine In the search for solutions it uses the rules and the facts from the theory more or less in the same way as a human being would do Of course the search for solutions by an inference engine is very formalized but the analogy is useful when you want to understand how Prolog and the built in inference engine work In the next section we elaborate on that 5 2 PIE Prolog Inference Engine Now we will try the example created above in PIE the Prolog Inference Engine PIE comes as an example with Visual Prolog Before we can use the program you should install and build the PIE example When you haven t yet installed the examples please do so now Select Install Examples in the Windows start menu Start Visual Prolog Install Examples Start Visual Prolog You find project PIE in the folder Examples that is installed Open the project PIE in t
403. t T stdio write H stdio nl If you are not sure go ahead and change the program Counting List Elements Now consider how you might find out how many elements there are in a list We call the number of elements the length of the list How can you find the length of a list The easiest way is to count the elements That gives a simple algorithm 1 The length of is 0 2 When a list is not empty we can represent it with Head Tail The length of this list is 1 plus the length of Tail Can you implement this In Prolog it is very easy It takes just two clauses predicates length of integer List integer Length procedure i o end class implement listlength clauses length_of 0 length of _ Tail Length length _of Tail TailLength Length TailLength 1 end implement run console init listLength length of 1 2 3 Length I called the class listLength stdio write the length of the list is Length _ StdlO readchar Take a look at the second clause first Crucially _ Tail will match any nonempty list binding Tail to the tail of the list The value of the head is unimportant as long as it exists 1t can be counted as one element 171 So the goal listlength length_of 1 2 3 Length will match the second clause with Tail 2 3 The next step is to compute the length of Tail When this is done never mind how TailLength will get the value 2 an
404. t click the Dialog in the IDE Designer You see the popup menu in figure A1 3 Unda Redo Gut Copy Paste Select All Controls gt Layout b Grid Test Mode Tabstops Visit Order Code Expert Dialog Attributes Gontrol Attributes Figure A1 3 the Popup Menu Controls Toolbar When the IDE Designer is opened the Controls toolbar is displayed next to the IDE Designer window You see the Controls Toolbar in figure 7 Figure A1 4 The Controls Toolbar By clicking icons in the Controls toolbar you select a type of a control to be placed into the edited Dialog When you click a control type icon the current cursor is replaced with the cursor correspondent to the control type to be inserted Move the cursor to some position in the Design Area the container client area and click the left mouse button The control with the default size will be inserted at the specified position Instead of dropping a control into a container with a click it is possible to drag out the rectangle When you have selected a Control drop it in the Design Area but keep the mouse button pressed As long as you keep the button pressed you can change the size of the Control When you release the 243 button the Control is placed with the indicated size In this way you can specify not only the control position but also the control size in one action After a GUI control is placed with the mouse into a GUI style container it immediately ap
405. t contains the data the attributes and the values of that particular student Also there would be a class of teachers and a class of courses that generate the particular objects for teachers and courses Now think of the classes as modules in the information system Add two more modules a user interface and a report generator When a user wants to have a list of students she tells the user interface what she wants The user interface asks the report generator to produce the report The report generator starts to create the output it creates headers et cetera and then asks each student object to provide the data of that student The student objects provide the student data the report generator finishes the report and sends it to the user interface This is more or less how an OO program works The advantage of this way of programming is that you split up the job in simple activities that are performed by different objects and that are relatively independent The user interface only has to take care of what the user enters and to send the necessary message to the other parts of the program The student objects only have to take care of the data of one student and to provide these when asked This architecture may seem to be more complex than necessary but there are advantages Imagine that some day the user interface needs a change Then you only have to change that module Or imagine that extra attributes are added to the student class Then it is enoug
406. tControl setWantReturn 1 predicate IsWantReturn true controlName setWantReturn IsWantReturn to set this flag programmatically Wrap 263 Possible values are True and False They define whether the text in the control will be wrapped to several lines The default value is True wrap You can use the textControl getWrap 0 gt predicate to get whether the control has this flag IsNoWrap textControlName getWrap IsNoWrap true and the textControl setWrap 1 predicate IsNoWrap false textControlName setWrap IsNoWrap to set this flag programmatically 264 Appendix A2 List manipulating predicates In the class list VIP offers a lot of predicates for manipulating lists All these predicates act as functions It means that when you call them you must bind the result to a variable E g the predicate append 2 is used to append a list to another list The declaration of this predicate is append Elem Front Elem Tail gt Elem List From this declaration you can see that it is a polymorphic predicate That means that it takes any input as long as it is a list So you can use this predicate for integer lists string lists et cetera The predicate mode is omitted so it is procedure The flow is omitted so all the arguments are input To call this predicate use List list append Front Tail The variable List will contain the result that is a list in which Tail is appended to Front When you
407. tants Sometime you have to declare and define in different statements as you do with predicates This chapter is about what to declare how to declare it and where to put the declarations in your program It is not complete I will restrict the myself to the declarations that are used most and that are important to a beginning programmer 9 2 Basic notions and overview of keywords In programming a computer you are using a kind of language In this book the language is Prolog To be able to talk about that language we need a number of basic concepts This section is about these basic notions Identifier An identifier is a name It could be the name of a predicate of a class of a variable of anything An identifier must conform to some rules There are four kinds of identifiers in VIP They are called lowercaseldentifier uppercaseldentifier anonymouslIdentifier e ellipsis A lowercaseldentifier is a sequence of letters digits and underscores that starts with a lowercase letter An uppercaseldentifier is a sequence of letters digits and underscores that starts either with a capital letter or with an underscore The anonymousldentifier is the underscore _ You use the 135 anonymousldentifier in places where a predicate needs an argument but you don t use this argument E g when a functor describing a person like in chapter 7 contains Name and Age and you only want to know the Name In that case you can call th
408. ten as 12 4 3 The expression is evaluated from the left to right using a stack We call a number an operand and the signs for addition and multiplication are called operations The action rules for evaluation are when you encounter an operand push it onto the stack e when you encounter an operation pop two operands apply the operation to them and push the result onto the stack e finally when the input line is empty pop the top element and show it The processing of 1 2 4 3 goes like this In this example the Stack is displayed after Operation has taken place Input Operation Stack 1 Push operand 1 2 Push operand 2 1 Pop two elements apply operand and push result 3 4 Push operand 4 3 220 e Pop two elements and multiply 12 Note this is the number twelve 3 Push operand 3 12 Pop two elements and add 15 At the end of the calculation the final result 15 lies on the top of the stack 12 4 The Queue A queue pronounced kju is a sequence of elements in which the elements are kept in order and the principal or only operations on the sequence are the addition of entities to the rear terminal position and removal of entities from the front terminal position This makes the queue a First In First Out FIFO data structure In a FIFO data structure the first element added to the queue will be the first one to be removed This is equivalent to the requirement that whenever an element is added all element
409. ter Definition A definition is the specification of the program code in your program E g the definition of the predicate square could be square Input Answer Answer Input Input This clause defines specifies the declared predicate To separate the declaraton and the definition of a predicate we precede the declaration with the word predicates ans we precede the definition with the word clauses The declaration must precede the definition So for the predicate square we get predicates square integer Input gt integer Answer procedure i clauses square Input Answer Answer Input Input The definition is executed at run time At compile time the compiler uses the declaration to check the use of the predicate in the definition In this case the compiler could find out that somewhere in your program you erroneously use a string as input when you call the predicate square Punctuation marks Punctuation marks in Visual Prolog have syntactic and semantic meaning to the compiler most of them do not specify by themselves an operation that yields a value They are used to separate clauses to end a declaration et cetera Some punctuation marks either alone or in combinations can also be Visual Prolog operators In VIP these punctuation marks are accepted by the compiler eae ee eee lt Mls Sete The meaning and use will become clear in the remainder of this chapter Operators Opera
410. ter of taking care of events by defining and coding the right event handlers Please take some time to consider what you did and saw In the previous section you added code to File New here you added code to MouseDown The place and names are different but the pattern is the same You open a prototype TaskMenu mnu or form1 frm e You select an event File New or MouseDown e You lt Add gt the code for the event e You go to the inserted code to change it according to what you want to happen Let s draw some conclusions There is a pattern in the way we are programming When a program should show some action you first think of an appropriate event Then you think of the user action that will invoke this event It could be a click on a button or a click on a menu option or a click anywhere You create and enable the necessary item e g a button in a form or e g an option in a menu or in whatever is appropriate Next you use the IDE to insert a listener or a so called responder that is another kind of listener and some standard code and finally you change and or insert the necessary code Until now the code looks probably like some magical words to you but the pattern should be clear 25 Chapter 3 Simple user interfacing In this chapter I will elaborate a little bit more on user interfacing There are several ways to communicate with the user and you will need to know about it At the same time it will give you experience in u
411. the end implement but then it must be exactly the same as the name that is given after implement Now that I introduced the most used keywords the next sections will give a detailed description of each keyword its meaning and how to use it 9 4 Section domains Let me introduce the domains section in an informal way Every variable in Prolog is of a certain type The type of a variable indicates what values are acceptable for a variable E g an variable of type integer can only have an integral value that is a number like 123 or 456 In Prolog there are a few standard types The ones to know were already presented in the previous section They are unsigned integer real and string When you use only these standard types in your program you can omit the section domains The standard types are used to create your own types An example In the previous chapter we used a functor with the length and weight of persons Length and weight cannot be negative So we could want to indicate this to the compiler To do it we create a domain of positive integers Let s assume that length cannot be more than 300 what is quite tall I think and weight cannot be more than 250 and that is quite heavy Now we declare three domains one for length one for weight and one for name It will look like domains length integer 0 300 weight integer 0 250 name string In this declaration we state that length is an i
412. the implementation of the class and it will also need a fact for storing the count This fact must be a class fact because it does not belong to each object but it belongs to the class as a whole It is shared among all the objects Facts are declared in the implementation file person pro In the implementation of a class you can declare and define private object items facts and predicates as well as private class items To declare class items you prefix the corresponding declaration section with the keyword class All in all our class person can now be implemented like this implement person open core class facts createdCount unsigned 0 clauses getCreatedCount createdCount facts name string constants className person person classVersion clauses classInfo className classVersion clauses new Name name Name createdCount createdCount 1 stdIO write Hello world I m a new object My name is name stdIO nl stdIO write I am object number createdCount stdIO nl clauses getName name clauses setName Name name Name end implement person I have added a class fact createdCount which is initialized to zero I hope you will take the precise syntax for granted I will explain more in the next chapter I have also added a clause for the predicate 123 getCreatedCount which returns the current value of createdCount Finally I have added the code
413. the objects that are created When an object cannot be referenced anymore the garbage collector removes it from memory So when we removed the reference to the object from the list it was removed from memory by the garbage collector 133 Chapter 9 Declarations in Visual Prolog In Visual Prolog the program that you write is compiled before it is executed Compilation is the translation of your Prolog code into machine code that is executed by the processor in your computer Compilation is quite a job It is done by the so called compiler In order that the compiler can do its work properly it is necessary that you provide it with all the information it needs This chapter is about that In your program you provide the code Part of the code is not about the things to do but is about what the code is Is it a predicate is it a clause is it a fact For every piece of code in your program you have to provide the compiler with information about it This is done in the so called declarations in the program Or to say it in another way your program will consist of several sections Each section is preceded with a keyword that informs the compiler what it can expect in that section This chapter is about these keywords their meaning and the contents of the various sections in a program But first we have to go back to basics What are the building bricks of your program After that we will see what keywords there are and what their meaning is an
414. the same time Each of these processes has a connection to a console and at the same time each process has no more then one console For input and output the console supports three streams an input stream where it reads characters an output stream where it writes characters and an error stream where it writes special messages in case of errors More on the concept of streams in a later section for now think of it as a channel through which the computer gets or writes a stream of characters The input stream is by default connected with the keyboard the output and error streams are by default connected to the screen But they can be redirected When you want to write directly to the console use the predicates in the class console But I must strongly advise you not to do so unless you have very good reasons and know what you are doing Writing to the console means among others that you must be sure that there is a console In VIP and the IDE this is clear from setting the target to console But what happens when you export your program to another computer In that case the program may not work as intended Instead of writing to the console you ve better use the class for Standard Input and Output called stdIO that we used before The procedures in this class will always write to the right output device In a console environment it uses the Win32 console in a GUI environment it uses the Message Window in the Task Window
415. the speed menu right click to make it popup of the IDE Designer can also be used to open the Control Properties table A1 3 1 Common Properties of Almost All GUI Controls There are some basic control properties which are used by almost all GUI controls These are Name The Name input box is where you can specify some correct Visual Prolog name which will be used as the control name identifier The default Name is generated from the control type for example pushButton checkBox listButton etc and the ctl suffix If several controls of the same type are created then the control type is suffixed with the number for example pushButton1_ctl You can edit this default name or simply type in your own version Representation 252 This property can have one of the following two values Fact Variable Variable Fact Variable When Fact Variable is specified then the IDE generates the fact variable which will store the object of the control This fact variable is used to reference to this control in the code This fact variable name is determined by the Name property For example ok_ctl Notice that in this case the fact variable name is automatically converted to be started with the lowercase character Variable When Variable is specified then the IDE unifies the object of the control with an ordinary variable This variable will be used to reference to this control in the code This variable name is determined by the Name property
416. then return to the previous one stdio read 0 gt read gt _ Term procedure Description The predicates reads a specified term from the input stream The type of Term can be any legal Prolog term That means that with this predicate you can read integers reals chars strings facts whatever The program decides what is to be expected and reads the appropriate term type E g when you have declared a variable to be of type integer and you read it somewhere in your program then the program knows what it must expect When you are not sure or when a variable has not been declared then you can use the predicate hasdomain 2 to exactly define the term s domain Because this may sound tricky I devote the next section to this subject stdio readBytes 1 gt readBytes byteCount Size gt binary Binary procedure 1 Description When the input stream has mode binary then your program should read bytes This predicate reads a number of bytes from the input stream and stores them into the variable Binary The number of bytes to read is given in the variable Size when you call the predicate If this predicate meets the end of the input stream before the number Size of bytes is read then it returns in Binary the number of bytes actually read 198 stdio readChar 0 gt readChar gt char Char procedure Description The predicate reads and returns a single character from the current input stream To my surprise
417. ther list add 1 to the head and make it the head of the result and then add 1 to each element of the tail and make that the tail of the result Load the program and run the goal with the specified goal add1 1 2 3 4 NewList The goal will return NewList 2 3 4 5 Tail Recursion Again Is add1 tail recursive If you re accustomed to using Lisp or Pascal you might think it isn t because you think of it as performing the following operations e Split the list into Head and Tail Add 1 to Head giving NewHead e Recursively add 1 to all the elements of Tail giving NewTail e Combine NewHead and NewTail giving the resulting list This isn t tail recursive because the recursive call is not the last step But and this is important that is not how Prolog does it In Visual Prolog add1 is tail recursive because its steps are really the following e Bind the head and tail of the original list to Head and Tail Bind the head and tail of the result to NewHead and NewTail NewHead and NewTail do not have values yet e Add 1 to Head giving NewHead Recursively add 1 to all the elements of Tail giving NewTail When this is done NewHead and NewTail are already the head and tail of the result there is no separate operation of combining them So the recursive call really is the last step More on Modifying Lists Of course you don t actually need to put in a replacement for every element Here s a program that sc
418. top to bottom For each clause the head is unified with the arguments from the call that is the input arguments are bound to the values that are provided by the calling predicate If this unification succeeds then the body of the clause if such one exists is executed The clause succeeds if the match of the head succeeds and the body succeeds Otherwise it fails When a predicate is a function then you must declare right after the heading the sign and the variable that is bound to the value that must be returned by the function E g when you declare square integer gt integer Square procedure i to declare a function that squares an integer then the clauses section should contain the necessary clauses e g square Number Square Square Number Number The expression Square tells the compiler that upon termination of the clause for the predicate square 1 the value of the variable Square must be returned to the calling predicate To call a function you must have a variable ready to be bound to the returning value E g NewSquare square 3 This binds Newsquare to the value 9 9 9 Section goal The goal section defines the main entry point into a Visual Prolog program In traditional Prolog you provide the inference engine with the predicate 1t should start with That can be any predicate Remember the family program you used in PIE It was up to you to enter the wanted goal and the program u
419. torial cl I declared the predicate fac predicates classInfo core classInfo Ashort Class information predicate Adetail This predicate represents information predicate of this class vend fac integer Number integer Factorial procedure 1 0 end class factorial Then in file factorial pro I entered the clauses clauses classInfo className classVersion fac 1 1 fac Number Factorial NumberMinus1 Number 1 fac NumberMinus1 FactorialMinus1 Factorial Number FactorialMinus1 end implement factorial Then I built the project and finally in the file main pro I changed the run predicate into clauses run console init stdIO write Type a number 0 to end gt hasdomain integer Number Number stdIO read Number lt gt 0 factorial fac Number Factorial stdlO write Factorial of Number is Factorial stdlO nl _ stdlO readchar run run When you run this program it will ask you for a number and then give the factorial of that number It is not a foolproof program When you input a negative number or a number that is too large then things will go wrong For the arguments of the predicate fac we use integers Integers are restricted in their maximum value The permitted range is from 2147483648 to 2147483647 When you choose a too large number on my machine the Number 13 generates already too large factorials the f
420. tors specify an evaluation to be performed on involved operands As an example take a look at 1 2 The numbers 1 and 2 are called the operands the plus in words is the operator The arithmetic operator specifies that the number 1 and 2 should be added In VIP these operators are known divmodquotrem lt gt lt gt gt lt lt gt All operators are binary operators that means that they need two operands But and also exist as unary operators That means that to the compiler 3 is meaningful Although these operators are quite familiar there are slight differences between their use in Prolog and other languages 137 6699 e The operator is not an assignment operator it is a comparison operator Essentially this operator compares the two operands Its use depends on whether the operands are bound or not If both operands are bound then the comparison succeeds if both operands are the same otherwise it fails If one of the operands is bound and the other is not then the unbound one is bound to the value of the bound one If both are unbound then they are bound to each other If later on one gets a value the other one will automatically get the same value e The operators div and mod are reserved words The operators div and quot give as a result an integer division but both do it slightly different Please take a look in the help file to find out the difference T
421. trieve values when calling a predicate is the function A function is a special kind of predicate It returns one value simply by calling without using an explicit output argument Let me give an example Take a look at the predicate squarePred 2 that computes the square of an integer This is the predicate declaration predicates squarePred integer Input Var integer OutputSquare procedure 1 0 And this is the clause clauses squarePred InputNumber OutputSquare OutputSquare InputNumber InputNumber When this predicate is called the InputNumber is used to compute the square that is bound to OutputSquare So when you call it with squarePred 2 Square then Square will be bound to the number 4 Now take a look at this predicate predicates squareFunc integer InputVar gt integer OutputSquare procedure i clauses SquareFunc InputNumber OutputSquare OutputSquare InputNumber InputNumber This predicate takes the InputNumber to compute the square and binds OutputSquare to it It then returns the value of OutputSquare to the calling program When you call this predicate you must provide a variable to catch the returned value like this Square squareFunc 3 This will bind Square to what the functon squareFunc 1 returns In the declaration the part gt integer OutputSquare indicates that this predicates is a function that will upon termination return the value that is bound to the variable OutputSqu
422. ts to an identifier variable onFileNew that it needs to react to clicking File New But you deleted that identifier so it is not found in the program code so the listener cannot direct the program control to it That s an error you should repair it by entering your own code or by removing via the IDE the listener 2 5 A mouse event Clicking on File New is an event but events are not restricted to clicking menu options In this section you are going to add another mouse functionality to the program More specific you are going to extend your program so that it will react when you click the left mouse button anywhere in the form Clicking the mouse generates a so called mouse event When you click a mouse button in fact two events take place You push the button down and after that release it These events are known as MouseDown and MouseUp Windows is capable to detect both events as different events Here we will use MouseDown and neglect MouseUp The place where we want to make MouseDown trigger a reaction from the program is in the form form1 frm That means that we must insert the needed program code in the form 23 Go to the project tree and click with the right mouse button on form1 frm The IDE opens a floating menu From this menu you choose the option Edit The IDE opens the form editor that you already know Another way to open this editor is to double click on form1 frm We want the form to react
423. uctures isis end Sede ei eerie ee deus tae as 215 122 Aca the lit O LG ne AG Ae wind hel bard WAAL ale the eae RO 215 1233 Th Stack ge fist E eee ahaa E NN 217 IDA Tne Quedem a ana a dina tana 221 LLO ETCCS cl dais sa ta da sae via ay data cdas seen 225 12 6 Trees as a Data TyPl o o ooooooooooor ence enn e nen 226 ci a ITEE dren ae e yaaa ilai aie i e dang at iE iat rire ac p a E a a0 228 12 8 Creating a Tree Yuen A E E ra ia iia 230 12 9 Binary Search Trees la A E ema dang i a wae Wass 232 12 10 Other tree traversalS issor ge etacie sae a Ri a aa E et 234 12 11 A program for tree traversal 0 2 nenun 235 Appendix A1 Everything about Dialogs and Forms 0000 cece eee eee ae 238 Al 1 Create a Dialog or a Form 0 2 nent nnn ens 238 AK2 Edita Dialogs ccs A aida aan alto 241 A1 3 The Control Properties Table 0 0 0 ect etn t enna 252 A1 3 1 Common Properties of Almost All GUI Controls ooo ocoococo ooo 252 A1 3 2 Specific Properties of Different GUI Control Types 255 Appendix A2 List manipulating predicates 02 0 cece ccc n eens 265 Introduction This is a book on how to write computer programs in Visual Prolog Visual Prolog or VIP for short is an Object Oriented Programming Language OOPL for short that can be used to create programs that run under MsWindows This means that the programs that you write should support a GUI a Graphical User Interface
424. uld be positioned whether messages in the Messages window should be wrapped If the Messages window is closed it can be opened again by clicking the option View Messages Window in the menu e AS File ProDinpfc string string_api string_api pack compiled al File ProDinptciwpitvpi pack compiled File aboutDialog aboutDialog pack compiled File simpleWPl pack compiled File taskindow task Vindow pack compiled N Project has been built CA_MIMAS demo streeamDemo StreamDemo prj6 opened PA EAA MAA AEK gece eee saved 4 Figure 11 1 The Messages Window in the IDE In the IDE there is a separate window for displaying errors for displaying the error stream Most of the time it is hidden and of course you as a user hope it will never show The IDE displays this window when error or warning messages are generated by the compiler while compiling Prolog files or by the linker while combining object files libraries and resource files into the single target file If the Errors Window appears it looks like the one see in figure 11 2 dl Errors 1 Warnings 2 Description Filename Variable TaskWindow is not completely bound main pro Unused variable Taskindow main pro Unused variable TaskWindow main pro C Wip63 Test 3005 main pro 19 9 warning c507 Unused variable Taskindow Figure 11 2 The Errors and Warnings Window Each line in the window contains three parts Type Description and Filename The Type colu
425. umbers 138 hook bracketS oooooocoo oo ooo 134 Horn Clause LOglC 0 ooo 56 Horn Clause logiC 53 54 O hae R 49 A fom sued thet Ace e cues aches 49 E Y 8 39 identification oooooococooooooooo 88 det aS 135 o 54 153 implement oooococccco o o 139 140 implementation 4 135 implementation 118 123 In order traversing 005 234 Idex t b oe 8445 2ate gten cenas 194 inference engine 0 0 eee 57 inference engine 0000s 56 input argument 205 152 input buffer 002 2005 191 input stream oooocoocococoo 192 198 Inputstream ooe cie oeu seien ies 207 Inputstream_console 208 Inputstream_file 0 208 insert sorting 6 eee eee 183 INSTANCES So Sees 114 MLSS EL a soe wien ad a 76 INCE SEL oe aces epee awe Bacar cae ales 138 integer literal oooo o o o 138 Integral domains 142 Integrated Development Environment 8 39 Interface tas o Sh that 42 interface declaration 122 invertible clause o o 178 iteration 2 ee eee eee 163 keyboard 00 0 cece eee ee 192 keyboard s 2n cenetanrewdes rias 191 keyword iii id 136 Keywords cisco shaded aie wath beats ias 139 Last In FirstQUt o o ooooooo 217 Dayot tb e 16 271 Layout toolbar
426. unter NewCounter Counter 1 length_of T Result NewCounter end implement run console init listlength length of 1 2 3 L 0 start with Counter 0 stdio write L L _ stdIO readchar This version of the length_of predicate is more complicated and in many ways less logical than the previous one We ve presented it merely to show you that by devious means you can often find a tail recursive algorithm for a problem that seems to demand a different type of recursion Modifying the List Sometimes you want to take a list and create another list from it You do this by working through the list element by element replacing each element with a computed value For example here is a program that takes a list of numbers and adds 1 to each of them predicates add1 integer integer procedure 1 0 end class clauses add1 boundary condition add1 Head Tail NewHead NewTail separate the head from the rest of the list NewHead Head 1 add 1 to the first element add1 Tail NewTail call element with the rest of the list end implement run console init listmanager add1 1 2 3 4 NewList I called the class listmanager stdio write Newlist NewList stdIO readchar 173 To paraphrase this in natural language To add 1 to all the elements of the empty list just produce another empty list To add 1 to all the elements of any o
427. uttons are supposed to be used in the usual MsWindows tradition That means that when you keep these buttons a click on lt OK gt and lt Cancel gt will close the Dialog no matter what code you add to these buttons yourself Controls A control is a part of a Dialog that the user can use to perform some activity The best known controls are for choosing e g a file name clicking e g the lt OK gt button and for editing e g entering or changing a name Each control in a Dialog must have an identifying name that ends with ctl and that is unique within that dialog While two controls within one dialog must not have the same name it is actually good practice when controls in different dialogs have the same name when they perform the same action The table on the next page shows the types of control that are available in VIP 241 Push Button l Check Box C Radio Button Static text Edit A Push Button control serves to initiate a specialized action in an application A Check Box control lets you indicate a choice among two alternatives For example some facility may be switched ON or switched OFF A group of Radio Button controls serves to indicate one choice from among a list of alternatives A Static Text control reserves an area for the text in the dialog Although called static in fact the application can change the text during execution This is often used for prompts or field names relating to other
428. ware will be only as good as allowed by the modeled data By the term modeling we mean the establishment of a relationship between the subset of the outside real world that is being tackled with 85 the internal data structures of the software If the modeling of data is poor then it is likely that the software will also be poor or at best inefficient This is true of any software written in any programming language That was the reason that in the latter part of the previous chapter we had tried to focus on the entities and tried to correct the situation by inserting richer facts Similarly we introduced the concept of functors in this chapter to get even more clarity on the data regarding entities that are being modeled The advantage of Prolog is that it allows easy description of the real world data in a form that can be internally utilized by the code in an efficient manner Simultaneously it also makes the code very readable by fellow programmers who are working together on a project Functors can be used to create any type of compound domain to help in this modeling process You would have to carefully examine the various parts of real world data that you plan to process and convert them using functors and other data types that you will encounter in other chapters keeping in mind their usage in all critical parts of the software Some data structures that were useful in one area of the software may prove to be a hindrance in other areas
429. who know little about programming It is not for people that know nothing You should know the basics about computers and that it is possible to program them and that for a program you use a programming language But nowadays these things seem to bee widely known even among computer nitwits What when you know more Then you should look for other sources When you are programmer and know about other languages like Visual Basic or C read the book Prolog for Tyros by Eduardo Costa When you know about other languages and are curious about Prolog read the articles by Thomas Linder Puls and Sabu Francis at the Visual Prolog web site at www pdc dk When you have experience in Prolog programming look for details about Visual Prolog object orientation in the advanced tutorials at the same web site You will find the publications at the website www pdc dk gt Solutions gt Visual Prolog gt Tutorials At that site you will also find a Wiki on programming in Visual Prolog There is a lot available about Visual Prolog But for a beginner it is very hard to find his way through all the material This book tries to bring structure in the apparent chaos for the beginner I edited this book by borrowing and rewriting texts from other people with their consent and by filling in some gaps with old material that I wrote years ago It all comes together in this introductory text I have tried to keep the content as low profile as possible so ev
430. write Mother is Mother When the first claus succeeds the cut will prevent backtracking So the predicate can come back with one of fail succeed that is mode determ Finally you can also change the predicate into the mode procedure Take a look at this child Name father Name Father write Father is Father child Name mother Name Mother write Mother is Mother child _ write No known parents The last clause plays the role of a catch_all clause When the other two clauses fail the last one will succeed no matter the Name used in the call In this definition because of the cuts only one clause will succeed and because of the catch all clause at least one will succeed So mode procedure lt predicateFlow gt The option lt predicateFlow gt indicates whether an argument is an input argument or output argument When you call a predicate the input arguments must have a value On return you use the output arguments to retrieve the values that you need from the predicate The lt predicateFlow gt is given by a sequence of the characters i and o between round brackets and separated by commas The character i indicates an inputargument the character o indicates an outputargument The characters are written without the quotes They must be given in the order in which the arguments are given in the lt argumentlist gt When a predicate has five arguments and t
431. writes the number 2 then it adds 1 to make 3 then ad infinitum The predicate count is a recursive predicate It may bother you that when you call count 1 it never stops So let s add a clause to stop the recursive calling Suppose that we want to stop the counting when the number 10 is written That means that we want the predicate to stop when it reaches 11 We can do that with this clause count 11 stdIO write That s all folks 160 When we put this in front of the other clause then every time the predicate is recursively called it first will check if Number has got the value 11 If not then it continues counting If yes then it prints the message and stops Because of the cut also the backtracking is stopped Now let s put everything into a program In this chapter I shall use the strategy console for the projects because that minimizes the amount of typing The focus in this chapter is on Prolog not on the GUI So please create a new project I called it ch10count Don t forget to choose the strategy console In the project tree create a new class with the name counter Put it in the existing package and uncheck the box that says creates object as we will only use this class as a module The IDE opens the files counter cl and counter pro as usual In the file counter cl enter the declaration for the predicate count It should look like class
432. x of calling the predicate is stdIO writef lt format string gt lt list_of_variables gt The lt format string gt is a string that contains one or more lt format field gt s For every variable in the lt list_of_variables gt you must give a format field in the string The format string can become complicated because you can also add text that will be interwoven with the variables The number of actual variables in lt list_of_variables gt must match the format fields specified in lt format string gt otherwise a run time error Wrong number of arguments in the format string is generated A lt format field gt begins with the percent sign and contains up to five lt format specifier gt s A lt format specifier gt exactly indicates how the matching variable should be written in the output stream If the percent sign is followed by some unknown character not a lt format specifier gt then this character will be printed without modification and thus will appear in the output The lt format field gt syntax is lt gt lt 0 gt lt width gt lt precision gt lt type gt That means that it starts with a sign that is followed by five possible lt format specifier gt All specifiers are optional and written without lt and gt Here is a description lt gt Hyphen indicates that the field is to be left justified right justified is the default The hyphen has no effect as the lt width gt value is not set because then the wi
433. xpert like you did in section 2 3 clauses onFileNew W MenuTag S query new W S show to Task Window win CodeExpert Menu TaskMenu id file id file new here are the steps that you must follow In the Project tree e Open the folder TaskWindow of the project tree e Right click on TaskWindow win the floating menu opens e Choose the option Code Expert the Dialog and Windows Expert opens In the Dialog and Windows Expert Click on Menu 48 Click on TaskMenu Click on id_file Click on id_file_new Press the button 4dd to generate prototype code e Finally doubleclick on id_file_new gt onFileNew The editor opens the file Taskwindow pro at the place where the code was inserted In the file TaskWindow pro Add the requested code clauses onFileNew Window MenuTag New Window query new Window NewWindow show Dialog and Window Expert TaskWindow i xj Show Al Handled Unhandled Help J window Menu TaskMenu a id_file id_file_open id_file_save id_file_save_as id_file_exit gt onFileExit 3 id_edit id_help QJ Scrollbar 9 Control Key H B Mouse 1 23 Miscellaneous fid_file_new Add JonFileN ew Set Figure 4 7 Dialog and Window Expert When in figure 4 7 you take a closer look at the bar under the Title Bar you see three options for what the Dialog and Window Expert
434. y for the database So go to the TaskMenu and add a menu item Database with the sub item FillDatabase See figure 7 3 Later we will add more sub items MIE Belts Constant Prefix lid_database_fildatabase ltem Constant id_database_fildatabase y Help Constant lid help y Accelerator T Checked Shit yo T Alt Disabled amp File Ey Edit Ey Database Ey dFillDatabase Help Figure 7 3 Create menu items So now we have an menu item to fill the database but of course this only will work if we add the appropriate code somewhere in our project I decided to put all the code that has to do with the database in the class the module person So now go the file person pro and open it in an editor by double clicking the name in the Project Tree After the piece of code clauses classInfo className classVersion Enter the following 92 clauses fillDatabase assert person 1 John 85 185 110 assert person 2 Thomas 75 190 150 assert person 3 Anna 65 175 123 assert person 4 Lidy 55 160 135 stdIO write Database has been filled stdIO nl With this code for assert you tell the computer to add four records to the database Assert is one of the standard predicates that you can use to manipulate facts in a database Now the code is there but still there is a little problem T
435. y note has and T icon the error message window has a white cross in a red circle to indicate an error The procedures note and error are for short messages Next to them there is the procedure messagebox 6 This one allows you to control the layout of the message window and also to keep track of the reaction of the user To see how this procedure works add another menu item called WriteMessageBox and add the following code predicates onMessages Writemessagebox window menultemListener clauses onMessages Writemessagebox _Source MenuTag Answer messageBox TitleString Message line mesbox_iconExclamation mesbox_buttonsYesNo mesbox_defaultSecond 32 mesbox_suspendApplication note You pressed button number tostring Answer Please take care to enter everything just as it is shown This may seem to be a lot of work but it is worth the trouble The procedure messagebox has six arguments that allow you to control almost everything The first argument Titlestring contains the text for the Title Bar of the message The second argument Message line contains the text of the message The other arguments are for controlling the icon the buttons the default button and the suspending The icon There are four icons available Information Question Error and Exclamation You already know the Information icon from note and the Error icon from error Here you choose o
436. you could choose a name like Canvas Folder if it contains queries a good name is queryForms if it contains 47 document filling forms it is a portfolio and so on Never use names like Test or Folder01 unless you use the folder for tests or for number 01 A form can be edited any time by going to the Forms Editor You can reach the Forms Editor by right clicking on the file lt formName gt frm in the project tree and choosing the option Edit from the pop up menu A shorter way is to double click on on the name of the form e g Query frm Remember that the file with the extension frm is the place to change the looks of the form The file with extension pro contains the code You approach the files with code in two ways The code for the Task Window is approached by way of the Dialog and Window Expert The code for other items like forms is approached by way of the events list in the properties window More on that in section 4 5 Also you can reach the code by a double click on the file with extension pro or by a right click on that name and choose edit But I advice you to do that only when you know what you are doing 4 4 The Code Expert and the Dialog and Window Expert As its name reveals the Code Expert is used to insert code or snippets into the program code of the Task Window To reach the Code Expert you must right click on TaskWindow win Then choose the option Code Expert from the floating menu T
437. you might only consider the second fact in the theory and then you only have the solution X Pam Y Bill Or you might search the rules in a different order e g from bottom to top or from top to bottom But Prolog does not use a random search strategy instead it always uses the same strategy It starts with the question that is asked and calls it the current goal The system maintains this current goal throughout the search and tries to solve it left to right Suppose the current goal is grandFather X Y mother Y Z It says that you want to find the grandfather of X save the name in Y and then want to find the mother of Y In this case the system will always try to solve the sub goal grandFather X Y before it solves mother Y Z If the first i e left most sub goal cannot be solved then there is no solution to the overall problem and the second sub goal is not tried at all When solving a particular sub goal the facts and rules are always tried from top to bottom In short the inference engine in Prolog works from left to right and from top to bottom just like you do when you read a book or a cartoon in English When a sub goal is solved by using a rule the right hand side replaces the sub goal in the current goal Let us look at the following goal grandFather X Y mother Y Z The engine starts by solving the leftmost part that is grandFather X Y To solve it it looks in the facts and rules of

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