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Abstract - Robert Chatley

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1. 49 THE USER soonest 51 EVALUATION 57 TESTING THE ANGU AGE A 57 TESTING THE DEVELOPMENT 5 Gero 60 CONCLUSIONS m 65 FURTHER 69 4L DLE OR LO OT 71 APPENDIX A GRAMMAR FOR 2 73 APPENDIX B SOME EXAMPLE KENYA PROGRAMS AND THEIR TRANSLATIONS TO 79 APPENDIX C MORE COMPLEX EXAMPLE KENYA PROGRAMS AND THEIR TRANSLATIONS TO JJAV A eee eren eene 22 87 APPENDIX D A SURVEY OF PEOPLE S EXPERIENCES OF LEARNING iu 0 6 V Ship em 93 KENYA USER GUIDE ee eer eere e rente too ente tenente toot testo to teneo 95 Introduction and Motivation Java 1 is a popular programming language in the computer industry today Students want to learn Java but its syntax is quite complicated for teaching introductory programming courses Current methods of teaching introductory programming do not often start with object oriented design but rather concentrate on procedural programming and flow control moving on to abstract data types and classes at a later stage It is not
2. formal param list type name comma type name type name type identifier comma type name comma type name initialiser assign expression booleanliteral true true false false 75 RR KKK KK KK KKK Types ee f type basic type basic type reference type reference type basic type char char int LAE real real string string boolean boolean void void reference_type class_type class_type RR KKK KK KK KKK ke k KK KK Variable Declarations KK KKK KK KK type_param less type_param_list greater type param list type comma type comma type comma type RR KKK KK KK KK KK e ke KK A x Expressions KOK KKK KK KK expression boolexp bool expression 76 math_expression term term plus math_expression plus term minus math_expression minus term term factor factor mult term times factor div term divide factor mod term mod factor power term power factor factor function function_application number number stringliteral stringliteral readint readint readreal readreal readstring readstring fieldaccess field_access expression l_parenthese bool_expression r_parenthese number 1 intnumber fpnumber bool_expression term bool_term mathexp math_expression or bool_expression or bool_term
3. 2 println a equals 2 else println a does not equal 2 package examples public class Program public static void main String args int a 2 if 2 System out println equals 2 System out println a does not equal 2 84 Using Switch int a 2 switch a case 1 print One break case 2 print Two break default print Many break Java package examples public class Program public static void main String int a 2 switch a case 1 System out print One break case 2 System out print Two break default System out print Many break 85 args Defining and Applying a Function Kenya The definition int triple int return n 3 The application int a 2 int b call triple a Java package examples public class Program public static void main String args 1 int 2 int b triple static int triple int return 3 86 Appendix More Complex Example Kenya Programs and Their Translations to Java Star Triangle This program prints a triangle of stars on the screen Kenya int a 1 for 1 to 10 for 1 print println i Java package star public class
4. 7 THE CURRENT SITUATTION eere ree enero eene tonne tonto toten toto sento tenente tenentes toe 9 THE TURING SYSTEM 9 ACIAVA LIKE TEACHING LANGUAGE 10 PREVIOUS ATTEMPTS SOLVE THIS 1 THE JMAG SYSTEM oe pni a RARE 11 DEBER 12 SES acto bos i edi Rs eh o La DDR Lom Acor TE 13 COMPILER COMPILENRS 15 LANGUAGE 17 ISSUES WITH THE GENERATED 4 0 4 44 1 40 4 90999099 04 4 4 29 SOFTWARE DESIGN 31 fi S I 37 d sagen sas pan t pleb d E Unt sd dU dud to SEI PEE RD S UA 37 CODE GENERATION p deno Poet CRUS Dea EIER 41 TYPE CHECKING wireccscsascosconssnsticdendossivesnstevoseinsincoseasovesssvdlennesvevsanceaderensenssataved abontssstontneabensesnvensthe 47 IMPLEMENTATION OF THE KENYA
5. system again runs it in a separate process but connects to the standard input and output streams for this process This is so that output from the program can be piped into a window inside the Kenya user interface and input from the user can be fed back to the process to allow interactive programs to be written and tested OutputWindow ExecuteListener Process 5 InputBox OutputArea SwingWorker STREAM nummu The classes related to executing a compiled program 36 Translation A large part of the work in developing the Kenya system was developing a way to translate Kenya code into Java code Although this is a mechanical translation I wanted the Java code that was produced to look as human generated as possible It should be as close as possible to the code that a human programmer would write to solve the problem in Java There should perhaps be a caveat here Java is an object oriented language and the way in which good or certainly large programs are written in Java normally makes much use of this Kenya is not an object oriented language There is provision for defining classes and creating instances of these classes but there is no inheritance and therefore no type polymorphism As it happens this is the same as Visual Basic which Microsoft claims is object o
6. The parser is generated from a grammar for the language given as a text file The grammar defines the tokens and productions used in the language which need to be recognised SableCC has been used in this project because its use of the Visitor pattern gives an elegant design to the software to process the abstract syntax tree and because it produces Java code which is platform independent 16 Language Design The main concerns in the design of the language are e It should have a syntax which is easy to understand remember and write e It should not be necessary to define a main function in a program e It should not be necessary to create classes or objects to write simple programs syntax of Java should be used where it seems suitably simple so that the learner can see how to write the same programs in Java when they want to learn a new language Enough suitable keywords and structures should be included to allow the language to have sufficient functionality for it to be used to solve all of the problems which might be set as an introductory programming exercise Data Types It makes sense to share the concrete data types of Java so that no type conversion has to be done at the translation or compilation stage The basic types supplied by Java are e boolean e char e byte e short e int e long e float e double Are all of these needed It is probably not necessary to have all of these when writ
7. 10 JGuru ANTLR website http www antlr org 11 Moog T PCCTS resources and Notes for New Users http www polhode com pccts html 12 Sable Research Group SableCC 2 16 2 Homepage http www sablecc org 13 Gamma et al Design Patterns 1995 Addison Wesley 14 Jackson C and Tomlins A Java Generics 1999 http Awww doc ic ac uk caj9 7 projects course java generics report 15 Java Developer Connection Prototype for Adding Generics to the Java TM Programming Language http developer java sun com developer earlyAccess adding generics 16 Bracha G Odersky M Stoutamire D and Wadler P GJ extending the Java language with type parameters 1998 17 Pooley R Extended Bachus Naur Form http Awww cee hw ac uk rjp Coursewww Cwww EBNF html 18 Eckel B Thinking In Java 2 Edition 2000 Prentice Hall 19 Microsoft Visual Studio http msdn microsoft com vstudio 20 Sun Microsystems Forte http www sun com forte ffj overview html 71 21 Raskin J The Humane Interface 2000 Addison Wesley 22 Wall L et al Programming 2000 O Reilly 23 Sun Microsystems Java 2 Platform SE v1 3 http java sun com j2se 1 3 docs api 24 Microsoft MSDN Online Library http msdn microsoft com library 25 Imperial College Dept of Computing 1 year lab exercises http www doc ic ac uk lab cs 1 lab_specs html 72 Appendix A Grammar for Kenya The grammar for Kenya for use with SableCC looks like Package minija
8. bool_term factor bool_factor and bool_term and bool_factor bool_factor literal booleanliteral compare 1hs math expression comp operator rhs math expression negate not 1 parenthese bool expression parenthese Names ee field_access array name array_access scalar name 77 a simple name simple name qualified name qualified name simple name identifier qualified name field access dot identifier array access 1 bracket math expression 78 r bracket Appendix Some Example Kenya Programs and Their Translations to Java Hello World Kenya print Hello World Java package examples public class Program public static void main String args System out print Hello World Using Variables Kenya int a 2 int b 3 printin b string Hello printin h Java package examples public class Program public static void main String args int a 2 int b 3 System out println a b String h Hello System out println h 79 Using Arrays Kenya int a 1 print strin b 0 I print Java packa publi pub S 10 ln a 1 g b 2 2 ays are indexed from zero 0 Hello 1n 5101101 ge examples c class Program lic static void main String args nt a new int 10 1 5 ystem out pr
9. want to work with the Java They will end up with multiple files all called Program java If they change the name then the file will not compile but if they do not change the name then they cannot keep the files in the same directory The user could change the name of the class to something other than Program and change the name of the file accordingly but even if they do this there may be other problems When a Java program is compiled a class file is produced for each class that has been defined If two Kenya programs are written which each define a class Dog these will translate to two Java programs which each define a class Dog The user can change the name of the main class from Program to something else so that they can keep the two programs in the same directory for convenience However when they compile one of the programs using the Java compiler this will create a file called Dog class which will overwrite any Dog class files produced by previous compilation of the other program This will cause the first program to execute incorrectly unless the definitions of the Dog class in both programs were the same 45 These difficulties can be overcome using Java s package system By defining package corresponding to each Kenya program that is written all the java and class files generated from that Kenya program can be put in the same package In the filesystem this is implemented by creating a directory so it does not allow
10. Program public static void main String args int int b for 1 a lt 10 a for b 1 b lt b System out print System out println 87 Fibonacci This program generates the nth number in the Fibonacci sequence starting with the given terms Kenya int fibonacci int first int second int N if 0 return first TE Af return second else return call fibonacci second first second N 1 int number int first int second print Input a Fibonacci series position number readInt print Input the first and second starting values first readInt second readInt print Fibonacci number at position number in series call fibonacci first second number Java package fibonacci import kenya io IntReader public class Program public static void main String args int number int first int second System out print Input a Fibonacci series position number IntReader read System out print Input the first and second starting values first IntReader read second IntReader read 88 st System out print Fibonacci number at position number in series fibonacci first second number atic int fibonacci int first int second int N 0
11. SableCC will report that it cannot generate a parser as it cannot resolve precisely which production a set of tokens represents This is a shift reduce or reduce reduce error and is caused by an ambiguity in the grammar 38 SableCC produces so called bottom up parser This means that parser continues from left to right along a stream of tokens pushing the tokens on to a stack At each stage it tries to determine whether the elements at the top of the stack represent an entire production or whether more tokens need to be examined in order to make an unequivocal decision as to what the string of tokens represents If the top elements of the stack do represent a production those elements are reduced exchanging the set of tokens for a symbol representing the production as a whole If there is not yet enough information to make a decision then the next token in the stream is shifted on to the top of the stack and the process continues If at any point the parser is in a situation where it has multiple options and cannot make a decision based only on the contents of the stack and the next token then there is a shift reduce conflict if the parser is unsure whether to shift or reduce or a reduce reduce conflict if there are several possible reductions which could be made See the Dragon book 8 for more details on bottom up parsing techniques When SableCC generates a parser it checks whether either of these conflicts can ari
12. a language in which commercial software is developed as it does not tend to be used outside of educational institutions For this reason students are often displeased by having to learn Turing as they do not believe that it will be useful in the long run and feel that they would be better off learning a language which is more popular in industry such as C or Java If fact students will be much better off learning the principles of programming using a simple language It is far more important that students have a thorough understanding of how to solve problems and how to construct elegant and efficient algorithms in their programs than to know the syntax of a certain language Once the basic principles of good program design and implementation have been learned it is easy to take these principles and apply them in different languages Initially students need to be able to focus on the way that they are solving problems and not on remembering complex syntax This is why a less powerful language with a less complicated syntax like Turing makes a good tool for teaching and learning the principles of computer programming The Turing system that is currently used has an Integrated Development Environment IDE which can be used to edit code to compile it into a byte code which is then interpreted by a virtual machine to execute a program and to step through the execution an instruction at a time The ability to step through the executio
13. are not required in solving more everyday problems To achieve this 44 class declarations are added to a seperate list and code generator applied to them at end as is the case with function definitions Java programmers often define each class in a separate file The compiler insists that all public classes are defined in their own file and that the file is named after the class Programmers often take this to mean that all classes should be put in their own file This is also caused by programmers making any class that is not private public The public modifier is only necessary when a class needs to be accessed by a class in a different package Any classes that are only accessed by classes in the same package the majority of cases can be defined as friendly with no access modifer and can be defined in the same file It is my belief that particularly for simple classes it makes much more sense to include several classes in a file This saves having to constantly switch between files or have multiple editor windows open and allows the whole program to be seen at once I believe this is valuable when trying to learn how a program works Class Names As mentioned above each translation of a Kenya program to Java produces a public class called Program In order for the Java compiler to process this class it must be saved in a file called Program java This only becomes a problem when the user gets to the stage where they
14. caused it and then that line be mapped back to the line of Kenya which generated it This would associate any compiler errors directly with the Kenya This would be better than the current situation as Kenya users should not have to concern themselves with the Java stage unless they wish to Overall this project has yielded some insights into various aspects of teaching and learning programming A language and an initial version of a development environment have been designed and implemented Some areas have been identified on which further work would need to be done before the language could be used in a laboratory environment These are detailed in the next section 68 Further Work There are a number of extensions which could be made to this project These fall broadly into two categories small incremental improvements to the development environment and larger developments which might well constitute a project in their own right Small Developments e Improvements to the robustness of the IDE if Kenya were to be used in a classroom environment more work would have to be done to make sure that it stood up to the rigours of laboratory use e The provision of file handling in Kenya file handling is a fairly fundamental thing to want to do allowing programs to be run with larger amounts of data than can be easily input by hand e addition of graphical output to Kenya Java s Swing libraries make it relatively easy
15. considered to be good code A very experienced programmer may prefer code that is concise and therefore quicker for them to read and appreciate Other programmers may prefer code which provides a more verbose description of what is going on All the Java code that is generated by the Kenya system is self contained i e requires no extra classes to be imported apart from the code generated from input functions like readInt The Development Environment Novice programmers cannot be expected to work effectively with command line tools Their learning experience will be greatly enhanced if they are provided with a development environment which helps them to write programs effectively A development environment for Kenya was designed and implemented as part of this project The main goals for the design of the environment were it should provide everything the programmer needs to write and test programs e it should provide as much information as possible to users about problems with their code e it should be easy to use e it should not provide excessive or insufficient functionality In terms of the basic key components of a development environment the Kenya environment provides an editing area for writing code and a facility to compile code after Kenya code has been translated to Java and execute it all inside the same environment The code editor currently lacks features such as syntax highlighting automatically colouring
16. function call This is not done in the current Kenya system and it just allows through arithmetic with mixed integer and real terms and assumes the compiler will deal with it Testing the Development Environment The software developed in this project provides a development environment for writing Kenya programs checking their correctness and translating them into Java This tool was written in Java and the code comprises just over 30 000 lines of source code Approximately 20 000 lines of this code was generated by SableCC to create the lexical analyser the parser and the classes which represent the different nodes the abstract syntax tree 60 Responsiveness As the code for this tool is quite large I decided not to create all of the objects when the application starts up but to put off creating objects and loading classes until they were needed load on first use This is possible as Java supports dynamic class loading Doing this decreases the amount of time that the user has to wait when starting the application It does mean however that the user will experience a short delay the first time that they activate a certain feature for instance the first time they parse a piece of code or access the help This delay does not occur on repeated use of these features as the classes have already been loaded and initialised In no case is the delay more than a second or two and this is deemed acceptable for general use Running
17. is simply quoted verbatim in the Java program so it cannot be seen how the same functionality is achieved in Java This is not very useful as a step towards learning Java It would be much better if on parsing the JMac program some Java code was produced that was equivalent in function to the original JMac program and was self contained i e it should be the Java code that a programmer would write if they were trying to solve the same problem in Java as they have just solved using the simpler language BlueJ BlueJ 5 is a research project supported by Sun Microsystems in the Blue group at the School of Network Computing Monash University Australia The aim of BlueJ is to provide an easy to use teaching environment for the Java language that facilitates its teaching to first year students The designers of BlueJ have identified several problems with existing environments and aim to overcome these Alack of object orientation in the environment BlueJ is designed to teach Java and from an object oriented standpoint from the start Consequently a prime consideration in its design was to create an environment which supports and promotes the development of 12 object oriented programs This is different to aim of system to be designed in this project Here we are starting with a procedural programming approach rather than an object oriented one e The environment is under or overkill Considerable problems are cause
18. letters String If string is left uncapitalised then it may be confusing when moving to Java If it is capitalised it will be incongruous and require a complex explanation as to why it is that way At the current time I am of the opinion that string should not be capitalised Variables and Constants In Java variables are declared like this int int b 100 This seems good and simple and will be used in the introductory language The use of the semi colon as a statement separator will be considered later 18 In Java constants are declared like this public static final int a 100 This seems complicated and difficult to explain In the introductory language a const keyword will be provided which will map to static final in the translation to Java const int a 100 Records and User Defined Types Records structures containing a set of other types e g a Point containing an x and a y co ordinate both of a basic type are useful In Java this would be a class A large part of programming and software engineering is about finding a good abstraction model Writing a program to fit this model is helped a lot by the provision for user defined Abstract Data Types These are again implemented using classes in Java A point class in Java class Point int x xt ys In Java the contents of the record object are accessed in the following way assuming p is a Point 4 3
19. node Initially the approach taken was to simply generate the code public class Program public static void main String args and then traverse the tree generating the code for each statement and appending it to the program When the traversal was complete the program was completed by closing the method and the class I decided to call the main class Program in all cases Class names are generally decided by the purpose of the code being written It is not possible for a machine to infer from the Kenya code what the intent of the program is and summarise it with a suitable name so I decided to stick with Program Function Definitions For simple programs the above approach works well However when a function is defined in the Kenya program its definition cannot simply be written out in the flow of the Java code Doing this would put one method definition nested inside another Java does not allow this as all methods have to be members of a class even if they are static and can therefore be called 43 without creating an instance of class The code which needs to be generated is of following form public class Program public static void main String args Statements static void doSomething Statements To achieve this whenever a function definition is encountered instead of continuing down that branch of the tree and generating the code for the method definition the bra
20. or so the display should change to show the Java area containing the Java code which the system has generated Now click on the Compile button and after a few seconds you should see a Compiled OK message box Click ok in this window You can now run the program by clicking the Execute button An output window will pop up and you should see the words Hello World in the window Close the window and return to the Kenya code You can now save your Kenya program by clicking on the Save button You will be presented with a standard file selection dialog box Choose a suitable directory to save the file in type the name HelloWorld k into the selection box and press return 97
21. possible to write a program in Java without writing at least one class with a main method A better language for teaching with is something like Turing 2 which has a much simpler syntax and allows programmers to do simple things in simple ways The Hello World Program in Turing put Hello World The Hello World Program in Java public class HelloWorld public static void main String args System out println Hello World Things we have to explain to new programmers to write the Turing program 1 What put does 2 That strings need to go in double quotes Things we have to explain to new programmers to write the Java program What a class is What public means What static means What void means What a method is and how it takes parameters What library is 7 What the dot operator does 8 What System out println does 9 What curly brackets are for 10 What the semi colon is for 11 That strings need to go in double quotes It can be seen from this example that teaching introductory programming concepts is much easier with a language like Turing certainly in terms of getting people to the stage where they can write their first program However Turing is not a useful language in the real world People want to learn languages that they can actually use to write large applications and put on their CVs It can be argued that once the basic principles of good
22. the system translates the code into Java compiles and runs it However the translation between JMac and Java is not as clean as it perhaps could be For instance the following is the code for the Hello World program in JMac print Hello World This is fine and perfectly simple but asking the system to parse this code and create Java produces the following code A JMac Program aA class JMacProgram extends JMacLibrary Variable Declaration 1 Array Declaration LO A ABEL Constructor JMacProgram Bl K f Program Code print Hello World 11 Method Declaration main entry public static void main String args Creates a new JMacProgram JMacProgram myJMacProgram new JMacProgram Advanced Program Code It is not at all obvious to an introductory programmer what this does Consider this program in terms of the the kinds of concerns previously raised about the problems for an introductory programmer of understanding some of Java s complexities what is a class What does extends do What is a constructor Even an experienced Java programmer cannot understand from this code alone how the program works as no source code is shown for the JMacLibrary class and so it is not known what functionality this provides Also the JMac program
23. the Kenya system on a machine with a 300MHz Pentium II processor Redhat Linux v6 2 and the IBM Java Virtual Machine the time taken to parse type check and translate one of the programs from the exercises given above averages approximately 0 5 seconds This time is much smaller than the time taken to compile the Java code which averages around 5 seconds The delay added to producing running code by having to translate it from Kenya to Java is therefore negligible Usability Several students of the Department of Computing at Imperial College were given the system to test These students were actually quite experienced programmers rather than novices but their experience was considered valuable in assessing the usability of the system They were given the user manual and the set of example programs which are included in the appendices of this report None of the students experienced any difficulty in writing a simple program in Kenya translating it to Java compiling and running it The error messages produced by the type checker discussed further below seemed sufficient for the users to identify and correct simple errors in their programs It was mentioned that it would be nice to have the location of the error highlighted in the editing window The view was expressed that it would be nice to have automatic indentation in the editing windows and that indenting the Java code in a different way might make its function clearer These ar
24. the programmer will add a qualifier which will distinguish it from the global variable as in the following example class Dog int age Dog rover int age age 22 rover age 6 The possibility of having the same name refer to different variables in these situations must be taken into account when performing uniqueness checks Type Checks Type checking is the process of analysing a program to ensure that the types of expressions are consistent For instance if a variable is declared as being of type int then it should not be assigned a real value or a string or any other type To perform these checks the type checker needs to keep a record of the type associated with each name and check this type each time the name is referenced in the program Another check which should be made when a variable is declared is that it is assigned a type which exists If the programmer declares a variable of a user defined type then they must also have defined the type This check will also pick up simple typographical errors in type names 48 Checking and Inference There are two possible approaches which can be taken to verify that the types in a program are correct One is known as type checking and the other type inference These are best illustrated in terms of an example Say we are checking the following code fragment int a 4 5 7 Let us firstly take a type checking approach
25. user can swap between editing the Kenya or Java versions of their code which are displayed in tabbed panes of the operations available 52 opening a compiling code are given buttons tool bar At the bottom of window there is a box in which status messages are displayed giving the status of program compilation etc Love 5 16 Open File bel save File Parse Compile Execute Help Kenya Java Kenya Cade int a int p 1 for b print nn printin Messages When a program is executed another window is opened showing output which it produces I wanted to keep the execution of the program inside the Kenya system to give a more homogenous feel to development When developing with something like Sun s JDK for Java the code is edited in one program compiled using a command line compiler and then run either on the console or showing its own windows This means the programmer has to switch between different windows and tools depending on what they want to do Although this allows the more advanced programmer to use their own favourite tools for each part of the development process customising the way that they work it is less appropriate for a novice 53 This is the output window showing execution of program The box at bottom of the window allows data to be entered for programs that require user i
26. Abstract A fundamental part of the first year of a computing degree is learning programming In order to become good programmers students need to be able to concentrate on understanding the problems that they are solving techniques and algorithms This can be impeded by the need to learn the syntax of a complex programming language Teaching languages provide simple syntax but students are often not happy learning a language which they do not see as having commercial applications This report details the design of a teaching language Kenya and a translation system for converting code written in the teaching language into Java code This gives the progressing student a stepping stone to Java whilst letting the beginner concentrate on the principles of good programming The report also encompasses the design of a development environment for writing programs in this language Acknowledgements I would like to thank my supervisor Susan Eisenbach for her continued guidance throughout this project I would like to thank Christopher Anderson for his advice on constructing grammars compilers and type checkers I would like to thank Ashok Argent Katwala and Geoffrey Howie for the pertinent and some not so pertinent discussions that we have had over the course of this project ARAN iV n 1 ACKNOWLEDGEMENTIS 3 INTRODUCTION AND MOTIVATION e 444
27. My proposed syntax for the Kenya for loop is as follows for 0 9 print 1 for decreasing i 9 to 0 step 2 print i The second case gives a loop counter which is decremented by 2 at each iteration of the loop It may be possible to allow both this format and the Java style format of the loop in order to let the programmer or the teacher choose which they feel is more useful or applicable in a certain case Procedures and Functions Java calls procedures and functions methods Procedures are just functions which do not return a value they return void All methods are members of classes Unless a method is declared st atic it can only be called if an object of the class of which it is a member has been created As at the current time the introductory language will not support object oriented programming all methods should be members of the class in which main is defined and be declared st atic with package access so that they can be called from any part of the program Input and Output Textual output in Java on the console at least is most easily achieved by using the library functions System out println and System out print to print a line of text 24 with a newline at end in case of println To hide library teaching language will provide functions print and println which will translate to calls of System out print andSystem out printin
28. This method of defining class using a class keyword and use of dot operator to access member variables is fairly simple and widely used across a variety of languages so it seems a good idea to keep these for the teaching language The use of curly brackets to delimit the class definition seems an acceptable approach Taking this approach here suggests using curly brackets for delimiting all blocks of statements loops as they are used in Java In or C there is provision to provide a new name for a type For instance the name age could be assigned as a synonym for int This is done using a typedef Although this helps 19 with modelling problem there is provision for using typedefs in Java It would be difficult to write code in Java which a typedef would map to so unfortunately it will not be included in the introductory language at the moment Declaring Variables of User Defined Types In Java objects instances of classes are created using the new operator in the following way Point new Point 2 This creates an object on the heap assigning memory dynamically at runtime In C objects can be created on the heap in the same way or they can be created on the stack in the following way Point p pix 2 This uses the same syntax as declaring a variable of a basic type As the introductory programmer should not need to know the difference between creating an object on th
29. a most of the keywords and some of the syntax are substantially different For instance the designers decided to apply a command word rule to the language design This rule states that every command begins with a reserved keyword There are no semicolons each command must be on a separate line There are no curly brackets Multiline constructs are defined using If and EndIf Class and 13 EndClass Routine and EndRoutine The way in which code is indented is also important as corresponding If and EndIf pairs must be aligned to the same column JJ s designers have expressed the opinion that students find the word variable confusing and so have used the word box in its place Variable declarations are a case where JJ s syntax is substantially different to Java JJ uses Box name ofType type where Java would have type Although I agree with the technique of translating a simple language into a more complex one as an aid to teaching and will pursue this route in developing a teaching tool in this project I believe that the weaknesses in JJ are in the dissimilarities between the JJ language and Java During the time spent working with a language certain things become ingrained for instance the use of a certain character e g a semicolon as a line separator or the order of the terms in a variable declaration If a smooth transition between languages is to be ensured then in my opinio
30. atements if 4 statements In Java the case construct is called switch and has the following syntax switch 1 text first case break case 2 text second case break case default text no other cases match break If a does not match any of the cases the default case is selected The break statements are used to stop execution at the end of each case and jump to the closing curly bracket If the break statement was not included at the end of case 1 and case 1 was matched after case 1 s statements had been executed case 2 s statements would also be executed and so on until a break statement was reached Some programmers find it an annoyance to have to include a 22 break at end of each of their cases but it does allow for the possibility of leaving them out on purpose in order to let the execution drop through to the next case In order not to prevent programmers from using this technique if they want to it will be left to the programmer to put in the break statements rather than having the translator put them in automatically Loops There are four common types of loops while loops repeat until loops generalised loops and for loops The first two are quite similar With while the test for exiting the loop is done at the top so the body of the loop may not be executed and with repeat until the test is done at the end so the body is always executed at least once The T
31. ator or less accurately a compiler compiler The description of the language to be parsed is usually given in the form of a grammar which defines what constructs or productions can be used in the language and what tokens comprise these 15 There are many different parser generators available some of which are described in the following paragraphs Lex and Yacc Lex and Yacc 9 are fairly old UNIX programs They split the parsing problem into two parts Lex is essentially a lexical analyser or lexer It splits the source file into tokens Yacc Yet Another Compiler Compiler finds the hierarchical structure of the program The Yacc user specifies the structure of his input together with code to be invoked as each such structure is recognised ANTLR ANTLR 10 Another Tool for Language Recognition was written by Terence Parr who previously wrote PCCTS 11 but where PCCTS generates parsers written in the C language ANTLR produces parsers written in Java or SableCC SableCC 12 is another parser generator which produces Java code An advantage of SableCC is that the code which it generates uses the Visitor Pattern 13 This means that a new operation to be carried out on the AST can be added without changing the classes belonging to the AST by simply writing a new class containing methods which can be applied to each node in the AST An instance of this class is then passed to each node s apply method
32. ch people learn new pieces of software tends not to be by reading the manual but by trying things out and exploring the software Because of this some of the features of a piece of software are likely never to be discovered by users if they are hidden inside a number of levels of menus or option dialogs There is a compromise to be reached trading off the ease with which a user can find features against the possibility of presenting them with too much information and too many options causing them to become confused or daunted by the application s interface Jeff Raskin discusses such issues at length in his book The Humane Interface 21 In the design of the Kenya system I have tried not to include features which are unessential and to only provide one way of doing everything Larry Wall the inventor of Perl 22 has a famous adage There s always more than one way to do it Perl is a notoriously difficult language to master This should make it easier for the user to learn all of the features and never to find themselves thinking I wonder if I m doing this the right way perhaps another way would be better Having only a relatively small number of operations that the user can perform means that all of the features can be presented on the user interface without it becoming cluttered or confusing I designed the interface which is shown below to address these issues It has been created using components from the Java Swing classes The
33. code to make it easier to read and show its structure more clearly Using newlines to define new statements makes this difficult Adding white space may cause a working program not to function An alternative as used in C C and Java is to use a semicolon or some other special character to separate statements This allows arbitrary newlines to be inserted without affecting the function of the program although it does mean that an extra character has to be added after each statement and this does not look as tidy as it would without The fact that Java uses a semicolon as its statement separator swings the decision to use a semicolon in the teaching language also It is often the case that a lot of compiler errors in Java and even more in C C are caused by the programmer forgetting a semicolon at the end of a line If the programmer gets into the habit of putting semicolons in right from the start then this may be reduced 21 Conditionals Conditionals are a fundamental part of any programming language The most useful and generic construct is if then else The Java syntax for this seems straightforward enough to include in a language for teaching so the following syntax will be used if condition a boolean expression Statements else Statements Case Case is a useful construct to prevent programmers having to write if 1 statements if 2 statements if 3 st
34. d by an environment that provides tools that are too complicated too minimalistic or just the wrong tools e The environment focuses on user interfaces Many environments use graphics to allow the user to design graphical user interfaces BlueJ s environment concentrates on using graphics to illustrate the class structure of a program making it easier to understand BlueJ also overcomes two problems associated with teaching Java caused by the language itself namely the need to write a main function and the difficulties with textual input and output This project will also address these issues JJ JJ 6 is a programming language and environment designed for learning Java It was developed at Caltech by David Epstein and John Motil JJ is a subset of Java although it has syntactic differences and was designed to offer the ideal syntax for teaching programming to first year computer science students The designers had four main goals e JJ should be an ideal language for beginners e JJ should be an introduction to Java e A language for beginners should be easy to read close to natural languages and devoid of unnecessary punctuation e JJ should not have inheritance JJ is an introduction to Java and shares with this project the technique of translation from the teaching language to Java The translation is done line by line one line of JJ translating to one line of Java However although JJ is supposed to be a subset of Jav
35. ds relating to any functions defined in the Kenya program To create Java classes relating to any classes defined the Kenya program add any necessary import statements to allow access to code necessary for the correct compilation of the Java generate a correct translation of all statements 41 One of most difficult things to decide was how to write Java code in style of a human Different people have different coding styles and there is no one best style Style is concerned both the way that constructs are defined and combined and with the layout of keywords on the screen i e what the code looks like In a procedural language a constant would be declared just like any other variable possibly at the top of the file A Java programmer is more likely to define their constants as being static final members of the class which they are writing If a direct line by line translation was made from Kenya to Java then constants would be defined in the body of the main method The Java code would still compile but it would not be a good example of how to write Java programs To avoid this in the Kenya implementation all of the constant declarations are pulled out of a Kenya program and defined as attributes of the class In terms of layout a consistent layout was used always starting blocks with the curly bracket on a new line after an if ora class keyword In terms of the way in which the prog
36. ducation This examination along with consideration of the system currently being used to teach programming to first year students at Imperial College revealed that there were definitely some improvements which could be made in order to increase the ease with which a commercial language might be learnt whilst maintaining a focus on allowing the fundamental principles of programming to be taught These are essential if the student is to develop into a good 65 programmer who work with a number of different languages to express these underlying techniques and ideas Language Design The strengths and weaknesses of available teaching languages were examined in detail and the qualities which would be desirable in a new language defined The Kenya language was designed trying to incorporate as many of these features as possible and leaving out anything that seemed unduly complex or confusing for the novice programmer The other main consideration in designing the language was to maintain a similarity with Java where the Java syntax seemed sufficiently clear and simple When the student moves on to programming in Java at a later stage they should not have to learn a completely new set of keywords and syntax Tests were carried out to determine whether it is possible to write programs in Kenya to solve the sorts of problems given to students in an introductory programming class Although it was not easy to test how useful Kenya is in
37. e can choose not to use them When they do come to work with containers the concept of the parameterised type can be explained just as easily as the need to cast objects when they are extracted from containers 27 28 Issues with Generated Java The software developed in this project will translate the Kenya code written by the programmer into Java so that it can be compiled and run In fact in order to allow generics as detailed in the previous section the code produced will actually be GJ not Java but as GJ is a superset of Java I shall use the term Java to describe the code produced unless specifically talking about generics Qualities of the Code The main objective for the Java was that it should be as near as possible to the code that a Java programmer would write from scratch to solve a problem Often generated code is very obviously generated It is often more verbose and less elegant than code written by a human I wanted to make sure that the code generated from Kenya is clear and simple so that it is as easy as possible for the novice programmer to understand what the Java does whilst providing a good example of how to write good Java code I also wanted to show exactly what code was necessary A different approach would have been to provide a lot of library code which could be called allowing the generated code to be kept small However I wanted to avoid providing box functionality hiding the implem
38. e heap and creating it on the stack it seems sensible to use the C stack creation syntax for the creation of variables of all types The differences between basic and non basic types are thus made as transparent as possible to the programmer who can use them in the same way As objects cannot be created on the stack in Java a declaration such as Point would have to be converted to Point new Point by the translator Arrays Arrays are fairly fundamental data structures They are usually referenced by an identifier and an index For instance in C or Java a number or expression is used in square brackets after the identifier e g numbers 4 12 This seems a fairly straightforward notation In Turing a similar syntax is used but with round brackets instead of square To preserve consistency with Java square brackets will be used 20 In Java arrays are objects and need to be created using new operator e g int numbers new int 12 String names names new String 12 In the introductory language arrays will be declared without this in a C style int numbers 12 and converted to the correct Java by the translator Statement Separators To determine where one statement ends and another begins some sort of delimiter is needed It would be possible to use a newline for this putting each statement on a different line However it is quite often desirable to change the layout of the program
39. e slightly more important than just cosmetic changes but indentation is quite personal to individual programmers 61 Some concerns were raised over consistency between representations of program in system If the Kenya code is edited does it invalidate then Java version If the execute button is pressed which version of the code is run These issues would need to be addressed before deploying the system in a laboratory environment Error Reporting Some examples of errors detected by the type checker are shown below In the first case a variable called Name has been declared as a string but has had an integer assigned to it This is an example of a type check Kenya gt 2 Open File Save File Parse Campile Execute Help Kenya Java Kenya Cade string Name Name 4 Messages Error expression assigned to Name should type string 62 In the next example two variables are declared with same name This is not allowed as machine will not know which of the two variables the programmer is referring to This is an example of a uniqueness check 4 Kenya Ix 2 gt Open File Save File Compile Help int counter counter 1 real counter Messages A variable with the name counter has already been declared The information t
40. ects that programmers will want to store in vectors as well as Dogs in fact anything that is an Object and using this approach a different class would have to be written for each container for each type of object to be contained Every time a programmer defined a new type they would have to define a new set of containers to put them in Generics offer a solution to this problem by providing the possibility of having containers that are parameterised by type That is we say we want a Vector lt gt This means we want a Vector but that everything it contains will be of type A where A could be Dog Date String The parameterised container then deals with any type coercion necessary C offers generics in the form of templates At the moment 15 Java does not have generics but compilers are available which will compile a superset of Java including parameterised types GJ Generic Java 16 is such a compiler It would not be difficult to produce GJ code as the translation from Kenya rather than Java GJ is a superset of Java so the code would be pure Java if generics were not used in the Kenya code This would allow programmers to use the feature removing much of the need for casting one of the less elegant features of Java The use of parameterised types is quite an advanced concept and it is questionable whether they should be included in a teaching language However I think that they should be included as the novic
41. entation from the user The idea is to allow the programmer to see how things work so that they can do them for themselves Going against this is the fact that there is a Java paradigm of code re use Once you have written code to do something once you do not write it again but import it from the previous project This points towards the use of library code A compromise needed to be reached Library Routines I have decided to provide the routines for getting input into a program reading numbers and strings from the keyboard in a library Doing input in Java is not simple The simplest code to read an integer that I can come up with is still quite complex see the language design section 29 The code could be wrapped function static int read try java io BufferedReader stdin new java io BufferedReader new java io InputStreamReader System in String line stdin readLine return Integer parselInt line catch java io IOException System out printin e catch NumberFormatException System out printlin e return 0 A different function is required for each type of data to be read A Java programmer would probably write a wrapper class called something like IntReader with a static method read or they would write a Reader class which contained several methods readInt readReal etc They would then use this class in any program that
42. er This needs to be installed in order to use Kenya Starting Kenya To start the system ensure that the directory in which Kenya is installed is in your classpath and type java Kenya 95 Window The main window looks like this J Kenya oO X Open Save File Parse Compile Execute Help Kenya Java Kenya Code n The buttons in the toolbar allow you to do the following Open a Kenya file Save a Kenya file Parse Kenya code to generate Java Compile Java code Execute compiled Java code Open the online help In the middle of the window are two tabbed editing areas one for Kenya and one for Java You can switch between the areas by clicking on the tabs at the top 96 The messages box at bottom is where any messages will be displayed about errors in your code compiler messages etc Your First Kenya Program Traditionally the first program that people write in any language is the Hello World program This prints out Hello World on the screen Lets see how to write and run this in Kenya First make sure that the Kenya editing area is selected and type into it the following code print Hello World Now click on the Parse button You will be asked to enter a package name A package is what Java uses to keep all of the files associated with a particular program together Enter the name HelloWorld into the box and click ok After a second
43. erface 33 When the ParseListener detects a mouse click on parse button it creates a parser and a lexical analyser to construct an abstract syntax tree from the text in the KenyaArea The tree is formed from instances of subclasses of the class Node A TypeChecker object is then created and applied to the tree The type checker maintains a set of known types and a symbol table mapping names to types DepthFirstAdapter Node ParseListener TypeChecker apply SymbolTable HashSet KnownTypes The classes related to the type checker 34 After correctness of the program has been checked a Translator object is created and used to process the tree to produce Java code The translator adds each line of code that it generates to a Vector Later the ParseListener can access the code using the translator s getCode method It writes each line of Java code into the JavaArea DepthFirstAdapter Node ParseListener Translator applyO Vector code getCode The classes related to the translator Compiling the code causes the system create a directory and write the contents of the JavaArea into a file The compiler is then invoked in an external process and produces class files 35 When executing a compiled program
44. eviewed to see how far they fulfil the initial aims of the work The overall goals of the project were e To examine the teaching of programming to first year students e To consider the qualities of a language and a technique for teaching programming to novices e To design a language having such qualities To implement an environment for the development of programs in this language Examination of Teaching Languages Techniques In considering the way in which programming is taught to students at the beginning of their Computing degree it was noted that most important is that the students learn the fundamental principles of program design and think clearly about the techniques and algorithms that they are using to solve problems They should not be hindered unnecessarily by having to contend with complicated syntax in the language they are using At the same time it was recognised that students often become frustrated by being made to learn what they regard as toy languages with no real commercial application In view of this in designing an approach for teaching programming it must be taken into consideration how easy it is to progress from the teaching language to other more sophisticated and commericially oriented languages The majority of students studying Computing at Imperial College consider Java to be the language that they should be being taught in order for them to be employable Software Engineers on completion of their e
45. f node which may be encountered during the traversal of the tree The relevant case method is called depending on the type of the Node to which the DepthFirstAdapter is passed Most Nodes in the tree will have children What children they have depends on the grammar For example the grammar might contain the following definition variable_declaration type identifier initialiser semicolon Here we can expect a node of type AVariableDeclaration to have children representing exactly one type one identifier zero or one initialisers and a semicolon This might be processed in the following way public void caseAVariableDeclaration AVariableDeclaration node addToSymbolTable node getIdentifier node getType if node getInitialiser null node getInitialiser apply this control returns here after the tr representing the initaliser has been processed we see that get methods are provided for all possible children of node In case of the initialiser as it is optional a test is done to see if there is one If there is the visitor is passed on to process the tree representing the initialiser an initaliser will probably consist of at least an assignment operator and an expression which may in itself be a complicated set of nodes by calling node s apply method and passing it the DepthFirstAdapter i e this Programmers unfamiliar with the V
46. fee print I like println drink 55 56 Evaluation In this section the results of tests performed on the language and the development environment will be presented Testing the Language The Kenya language was designed to be used by novice programmers to solve the sorts of problems set in introductory programming courses Solutions to a selection of the exercises set for the first year computing laboratories 25 at Imperial College have been coded in Kenya The code for these solutions is given in Appendix C along with their translations to Java The fact that solutions to these exercises can be coded in Kenya shows that it would be possible to use it for teaching a first year programming course with similar exercises Exercises Star Triangle The object of this exercise is to draw a triangle of stars asterisks on the screen with one star at the top and ten at the bottom This demonstrates the use of textual output integer variables and for loops Fibonacci The object of this exercise is to calculate the nth term of a Fibonacci sequence where n and the two starting terms positions 0 and 1 are given by the user This demonstrates the definition and use of a recursive function conditional statements and getting input from the user Put Base The object of this exercise is to produce a program which outputs a given number in a given base This demonstrates the use of a void function and the mathematical modulu
47. hat the system provides about errors in programs is probably not sufficiently detailed at the moment to enable novice programmers to debug their programs easily and fully The lexical analyser and parser used in this project are generated automatically from the grammar Although this does not mean that it is not possible to change the way that they report errors if changes are made they are overwritten the next time that the parser generator is run this needs to be done every time that a change is made to the grammar Once the grammar has been completely finalised and there is no need to regenerate the analysis code changes could be made to provide more friendly error messages One of the major failings in terms of debugging messages is that it has not been possible to report the line numbers of semantic errors When the parser creates the abstract syntax tree representing the program it does not record in each node the line number on which the production which generated it was found The data stored in a Node could be altered and the parser changed to record line numbers as the code is analysed but this would require fairly large changes to the parser Again this would only be possible after the parser had been generated for the last time and would be difficult as the parser was designed by someone else and so some significant reverse engineering would be required 63 64 Conclusions In this section the results of the project will be r
48. ing introductory programs The difference between a float and a double or a short and a long is unlikely to be something that an introductory programmer should need to understand It would be useful to have a string type Strings in Java are not basic types but objects This can probably be made transparent to the introductory programmer 17 A more suitable set of basic types might be boolean e char e int real e string The term real has been used instead of double It is necessary to have a floating point data type but over complicated to have more than one at different precisions A double is more flexible than a float so it makes sense to use a double if only one of the two types is going to be included It would be confusing to use the word float to represent this as it would get translated to a Java double rather than a Java float However it does not seem sensible to call it double if there are no other floating point precisions available This will only lead to the question What is a single Calling the type real seems a good solution An introductory programmer is much more likely to be familiar with the concept of a real number than a double precision floating point number Should string have a capital letter In the introductory language a string is presented as a basic type None of the other basic types have capital letters In Java String is a class not a basic type and classes start with capital
49. intln a 1 tring b new String 2 2 01 0 Hello ystem out printin b 0 0 80 Using While Loop Kenya rnb a while i lt 10 printin i i i 1 Java package examples public class Program public static void main String args int i 1 while 1 lt 10 System out printin 1 i 1 81 Using a For Loop Kenya int X for 1 to 10 printin i for 1 2 to 10 step 2 printin i for decreasing i 10 to 2 printin i Java package examples public class Program public static void main String args Ent 35 for 1 1 i lt 10 i System out printin 1 for i 2 i lt 10 i 2 System out printin 1 for i 10 i gt 2 1 System out printin 1 82 Using a User Defined Data Kenya class Person string name int age Person me me name Robert me age 22 printin My name is me name println I am me age years old Java package examples public class Program public static void main String args Person me new Person me name Robert me age 22 System out println My name is me System out println I am me age class Person String name int age 83 name years old Using Conditional Kenya int a 2 if
50. isitor pattern may find it slightly counterintuitive that the this reference refers to the DepthFirstAdapter rather than the Node that is being processed After the tree representing the initialiser has been processed the flow of control 40 returns to this node as indicated by comment in the code fragment above showing that the tree is processed recursively Code Generation Once the abstract syntax tree has been generated it can be processed by the code generator to produce equivalent code in a different language in this case Java with GJ extensions see the language design section The main difference between a Kenya program and a Java program is that a Kenya program comprises a list of statements whereas a Java program consists of a set of class definitions and has the following structure e A program consists of a set of class definitions class definition consists of a list of attribute declarations and a list of method definitions method definition will contain a list of statements One of the methods defined must be called main This is the point of entry into the program In Kenya the point of entry is simply the top of the program The main issues of concern when generating Java code from a Kenya program are therefore To create at least one class insert the code for the main program operation into the main method of this class Other pertinent issues are create metho
51. keywords to distinguish them from variable and type names etc and auto indenting These would be nice features to have if the editor was going to be used for serious programming as 67 they make it much easier to see form and structure of code which is being worked The environment developed in this project was intended to demonstrate that development with the Kenya language is possible The fact that these features are missing at this stage is not a major failing but they should be added if further work is done on the project Other features which might be seen as being missing are the ability to load and save Java code rather than Kenya code or to change the package name after it has been set There will inevitably be extra features that people will want added to a development environment and these could be worked on further in future The type checker works quite well and will detect whether the type of the expression on the right hand side of an assignment matches the type of the variable to which the assignment is being made It also reports the use of types which have not been defined and the repeated use of type or variable names Currently any compiler messages generated by the GJ compiler are passed straight back to the user without being processed If it were possible to add line number tags then it would be much better if the line number reported in a compiler error could be matched to the line of Java which
52. le or a constant It should be ensured that assignments are not made to constant values Uniqueness checks are performed by checking that the key does not already appear in the hashtable before adding a new entry When a variable of a type which is a class is defined as well as noting the type of this variable in the symbol table the types of all of the attributes inside the class must also be recorded This could be done by including the fully qualified names of all accessible attributes in the symbol table but this is an inefficient implementation It should not be necessary to repeat the information about the types of attributes in the table every time a variable of a user defined type is declared as each of these will have the same set of attributes with the same types even if they are assigned different values To avoid this the approach which have taken is whenever a new class is defined to create a separate symbol table for the scope of that class and whenever a variable of that type is declared to insert a reference to that local symbol table into the global symbol table When a lookup is required the qualified name can be split where the dots appear as a qualified name will be of the form varname attribute the left hand side used to index the global symbol table and the right hand side used to index the class specific one to find the attribute information This indirection can be applied recursively so that classes may be nes
53. mmer to see how constructs from the teaching language map to Java code If this could be combined with a good Integrated Development Environment then this would be a very good tool for teaching introductory programming with a view to later moving on to Java The design of such a language and development environment is the aim of this project Naming the Language All programming languages need a name The programming language being designed in this project can serve as an introduction to Java something to try before hitting the hard stuff Taking a coffee analogy I have chosen the name Kenya for the language as this is a slightly milder coffee than Java see www starbucks com 10 Previous Attempts to Solve this Problem The fact that Java is not the easiest or best language for teaching programming has been recognised by a number of people and there have already been a few attempts to create languages and environments to help with this The JMac System A previous student of the Department of Computing at Imperial College worked on such a system for a project The result of their work was a system called JMac 4 The main feature of JMac is a very nice Integrated Development Environment created using the Java Swing classes This IDE is very well designed and it would be hard to find any substantial improvements to make upon it in terms of usability JMac also provides a simple programming language When a program has been written
54. multiple Program java s in the same directory but it does provide a neat organisation and certainly avoids the conflicts indicated by the Dog example above The best way to get a sensible package name before creating the directory is to ask the user to provide one package statement needs to be added to the top of the Java program to indicate which package the class is in Input Kenya provides the facility for obtaining input in programs by providing the functions readInt readReal and readString These translate to somewhat complex functions in Java as can be seen in the language design section Although in general in designing the Kenya system it has been my principle to present the user with all the code necessary to achieve the same functionality in Java in this case I have decided to place the functions into library classes and import them into the namespace This does not go against the principle of generating the code that a human Java programmer would write as a human programmer would not rewrite classes that they had written before but would reuse the code importing it from another package The code for the input routines will be supplied in the language reference for those who wish to know how they were written As the translator traverses the syntax tree every time it comes across a call to an input function it adds an entry to a set If only one type of input has been used e g just readInt that cla
55. n and also to examine the contents of variables during that execution makes debugging programs much easier as it allows programmer to observe the path of control through their programs A Java like Teaching Language The Turing system has many features which make it a good teaching language as have been described above There is no substitute for learning good techniques and thinking about structure and algorithms when it comes to learning programming The use of a teaching language makes it easier for students to develop these key skills without complex syntax getting in the way However programmers will soon want to move on to a more commercial language At the time of writing the language which is seen as most desirable by employers and on which most research and development of software engineering techniques is being based is Java The learning curve in moving from Turing to Java is quite steep as few pieces of the syntax are shared and some of the object oriented concepts in Java are hard to grasp quickly The severity of this learning curve could be reduced if a teaching language was developed which shared the syntax of Java or at least followed its general style for expressing common constructs Better than this would be a system which would take code written in a teaching language and automatically translate it into the Java code that a Java programmer would write to complete the same task This would allow a novice Java progra
56. n it would be more sensible to keep a common syntax and style between the two where this will not introduce overly complex constructs I also think that the need for the student to learn a whole new set of keywords when making the transition will present a barrier to moving easily to Java JJ enforces a certain style of indenting The way that code is laid out is very personal to individual programmers although perhaps more of an issue to the more experienced programmer and being forced to use a certain layout may prevent them from thinking about the problem as clearly as they might if it were possible to lay out their code with whatever spacing they wished 14 Compiler Compilers Parser Generation Computer programs written in high or medium level languages are most often represented as strings of characters in text files It is not possible for software to process raw sets of characters without some knowledge of what various combinations of characters forming keywords identifiers etc known as tokens mean in terms of the language in which the program has been written In order to do anything useful with the contents of these files the characters need to be analysed and built into some abstract representation of the program describing its structure which can be processed 7 8 Often this abstract representation is translated into some differing textual representation e g the conversion from C source code to binary object code
57. nch is copied into a list of function definitions and the code generator moves on to deal with the next branch of the tree At the end the main is closed but before the class definition is closed the translator object processes each function definition which has been accumulated in the list individually All the methods for which code is generated are declared to be st atic as an object of type Program will never be created However only main is declared as being public as it is the only method which will be called from outside the package by the Java Virtual Machine when the program is invoked All other methods will be called from inside the same class or the same package and so can and should be left friendly without an access modifier Class Definititions A similar issue to that raised by function definitions is raised by class definitions In Java it is permitted to nest one class definition inside another producing a so called inner class However it seems clearer to me at least to have all classes defined at the same level There are situations in which inner classes are useful These concern callbacks and cases in which multiple implementation inheritance is required see ch 8 of Eckel 18 for a discussion of these but these issues are far more complicated than anything that should concern an introductory programmer and I feel that the use of inner classes should be reserved for these more sophisticated uses as they
58. nctionality defined over the AST without having to modify any of the generated code by hand and also leads to an elegant solution 37 Writing the Grammar A grammar for SableCC is written using Extended Bachus Naur Form EBNF 17 The grammar has the following sections examples are given of the sorts of definition which might appear in each section Helpers digit 0 9 Tokens minus plus blank number digitt Ignored Tokens blank Productions expression plus expression plus number minus expression minus number number number Each of the constructs with an equals sign in the middle defines the name on the left hand side to represent whatever is on the right hand side In the case of digit this is the set of digits 0 to 9 The tokens give a name to each of the characters which may appear in the program source Operators such as can be used as in the case of number The use of says that a number is a sequence of one or more digits Productions represent the way in which tokens can be combined to write programs They can be defined recursively as shown in the example There are several alternatives for what an expression may be It may either be a number or an expression to which a number is added or subtracted Once a grammar has been written SableCC will try to generate a lexical analyser and a parser for the language Quite often during the writing of a grammar
59. ng with a survey was conducted of undergraduate students at different universities to discover their experiences of being taught programming A student from the University of Durham wrote hate to be thrown straight into object oriented programming It can be argued that object oriented programming is a viable introductory programming style depending on the method by which it is taught To make this an effective method would probably involve any actual programming being preceded by a course in object oriented design to give a good grounding in the principles of the structure of an object oriented piece of software A student from University College London wrote I think the best way of teaching modern languages is by showing them an abstract class diagram which is directly connected to whatever programming statements the user types in He advocates the object oriented approach to programming but with the caveat that pointers and references get very mind bending at times This perhaps presents a case for structured programming or the use of a language closer to Java than to C Java not having explicit pointer types therefore avoiding some of the confusion caused by the subtleties of pointers and references A student from the University of Edinburgh wrote J actually learned to program with Basic What made Basic so understandable was the very simply syntax of everything there were no braces no special characters at the end
60. nteraction Kenya Output sed X Program Output Although people are less inclined to read manuals than they once were there is still a requirement for good support documentation and this may more usefully be supplied in the form of online help Such documentation is particularly valuable in a programming environment as a language reference can be provided in which programmers can quickly look up syntax etc A Java developer will be likely to keep a web browser window open on the API documentation page 23 as they work and programmers working with Microsoft s libraries find the MDSN 24 references invaluable As the Kenya IDE is targeted toward a specific programming language such reference material can be provided for access inside the same interface meaning that a network connection is not necessary to access the documentation The only disadvantage with this is that any updates made to the documentation will be slow to propagate to users 54 This is an example of help window where users can find references on system and language Kenya Help a 06 Kenya language reference Types zi 3 Declaring Variables 3 Using Arrays 3 Text Output 3 User Input 3 Loops Conditional Statements Text Output Text can be printed on the screen using print or printin Using printin puts a carriage return on the end ofthe line println Hello println 6 println 7 3 string drink cof
61. of lines and the commands did pretty much what youd expect you have to agree PRINT HELLO is a bit more intuitive then printf 11 This points the need for an introductory language to have a simple syntax so that it is not difficult to understand and the programmer can concentrate on their algorithm A student from the University of Nottingham wrote The best language for teaching programming is Visual Basic I think due to the intellisense it finishes it for you so you dont have to remember every object s property names or methods to the letter it checks syntax as 93 20 can build GUIs and quick which look good with an intrinsically integrated IDE with good debugging features The merits of endorsing a Microsoft 3 product will not be argued here It is true that the Intellisense feature is an aid to programmers but it is a feature of the development environment rather than the language The same can be said of good debugging features and any sort of graphical builder Generating incomprehensible code from mouse clicks is probably not desirable in teaching programming 94 Kenya User Guide System Requirements Currently Kenya only runs on Linux As it is written entirely in Java a conversion to other platforms should be relatively easy and may be done in the future In order to take advantage of generics Kenya uses the GJ compiler rather than the standard Java compil
62. of the source language This checking called static checking to distinguish it from dynamic checking during execution of the target program ensures that certain kinds of programming errors will be detected and reported In the Kenya system two different static checks are considered uniqueness checks and type checks Uniqueness Checks Variables constants functions and types must have unique names in order that the compiler can tell which the programmer is referring to For instance the following Kenya code although syntactically correct should not be considered to be a valid program int a 4 int 6 print Here two variables with name have been declared This error should be reported Generating Java from this code blindly without checking for the uniqueness of identifiers would produce Java code that will not compile properly 47 It is possible to have variables with same name in different scopes local variable inside function may have the same name as a variable in the main body of the program Any use of the variable name inside the function body will refer to the local version leaving the previously declared version unaltered Once the program s execution has left the scope of the function references to that variable name will refer to the global version again It is also possible to give a field of a class the same name as a global variable When referring to the field
63. or from Java source code to Java byte code Parsing involves grouping the tokens of a source program into grammatical phrases The parser takes as an input the textual representation of the source program and constructs from it an abstract hierarchical representation of the same program usually in the form of a tree There are two stages to this conversion The first is lexical analysis which takes a stream of input characters from the textual representation of the program and converts them into a stream of tokens The second stage is syntax analysis where a parse tree is built from the stream of tokens which represents the program This tree can be processed by further stages either to perform semantic analysis for instance to check the correctness of the program or to generate code either machine code for a specific architecture in the case of a compiler or source code for another high level language in the case of a translator As the translation from text to an abstract program representation is a problem which has been encountered many times before it is now well understood how to construct programs to parse text into meaningful words and symbols and form so called Abstract Syntax Trees AST s representing the program In fact this problem is so common that software has been developed which will automatically generate a program to parse a text file into an AST given a description of the language Such software is called a parser gener
64. programming have been instilled through an introductory programming course it should be easy to learn a second language However students often complain about having to learn with toy languages and would rather be learning a real language The aim of this project is to design a language which is simple enough to use to teach introductory programming effectively but which the development environment will translate into the Java code which would be written to achieve the same task The syntax of the language will be kept simple but will borrow from Java where appropriate This should allow a programmer to progress easily onto writing Java at a later stage whilst allowing them to concentrate on the principles and structures behind their programs at earlier stages without getting bogged down in the overheads presented by writing the program in full Java The Current Situation Currently in the Department of Computing at Imperial College introductory programming is taught using a language called Turing 2 The Department would like to move to a more Java like system to make it easier for students to move on to programming in Java which is more applicable to real world problems The results of a brief survey of introductory programming techniques at other universities are included in Appendix D The Turing System The Turing language has a lot of features which make it a good choice for teaching programming Unfortunately it is not
65. r of the language is changed and the parser and AST classes regenerated then only code in the TypeChecker class needs to be altered to take account of this change rather than having to change code distributed throughout a number of different classes Each subclass of Node generated by SableCC has a method apply This method is void i e it does not return a value Initially this seems to present a problem when trying to write a 49 type checker as obvious approach would to process nodes of tree recursively calling a method which returned a type This is especially true if a type inference approach is taken However it is possible to overcome this when using a type checking approach by including an attribute in the TypeChecker class in which the expected type is stored The type checker object is passed to the node to be checked its type is compared to that which is expected and another boolean attribute is set depending on whether or not the types match A record needs to be kept of the type associated with each name referenced in the program This is done by constructing a symbol table and adding an entry to this table each time that a variable declaration is encountered while traversing the tree The symbol table is implemented using a hashtable and each entry is indexed by the name The symbol table contains information about the type associated with each name and also whether it was declared as a variab
66. ram itself is structured this is largely going to be due to the way in which the user has solved their problem However there are a few Java idioms which can be applied when generating the Java code The placement of different elements within a Java program is also a matter of style In the generated code the following pattern is followed class Classname constants main method other methods class Record attributes Kenya classes are just records and so cannot have methods included in them hence the class Record above has only attributes 42 Having an order for generating the code for different elements which is not necessarily the same as the order in which they are defined in the Kenya program means that the nodes of the abstract syntax tree cannot simply be processed in order Account must be taken of what the nodes represent and the tree must be rearranged to meet the required structure Implementing the Translator The translator was implemented by writing a visitor which could be passed to the apply method of each node of the syntax tree produced by the parser The class Translator extends the class DepthFirstAdapter created by SableCC The DepthFirstAdapter performs a depth first traversal of the tree doing nothing to the nodes as it walks over them By extending this class and overriding its methods a visitor was created which traverses the tree and generates the Java code relevant to each
67. rather than having to be left to be computed at runtime this will save on the required computation when the program is run This technique might be applied if the following code is encountered const int secondsInDay 60 60 24 const real pi 22 0 7 0 In both of these cases the value to be assigned to the constant can be calculated at compile time It is difficult to say whether it should be done at translation time In the first case the fact that the multiplication is written out helps to assure the programmer that the value is correct In the second case it is more debatable Most compilers will do constant folding when generating executable code and so there should not be a speed increase associated with calculation of the value at translation time However if the code was real diameter 23 7 real radius diameter 2 then not all of the operands of the division are real numbers and a type conversion needs to be applied to the 2 before the calculation can be done The compiler will either add in such a function so that the expression becomes something like radius diameter int_to_real 2 or it will rewrite the value 2 as 2 0 Perhaps this should be done at translation time as the division of a real by an integer is a type mismatch and maybe the programmer should know if the compiler is going to have to do something to fix their code so that it works especially if it has a computational cost as with a
68. rect Kenya programs The Kenya language is designed to be as simple as possible whilst still being useful This means that some shortcuts used by more experienced programmers for example the use of the increment operator are not provided As the translator produces Java code directly related to the Kenya code written the increment operator is not used in the Java code except in the one case of modifying the index variable in a for loop This may not be the code that a Java programmer would write but it is clear what the code does in relation to the Kenya code An interesting point to consider is whether the translator should perform any optimisations For instance in the above case the translator might come across in the Kenya code The current system would write out exactly the same statement in the Java code but Java has an increment operator so if the translator is to follow the rule of generating the code that a human Java programmer would write then it should possibly generate the shorter 1 even Taking account of these cases would add complexity to design of the translator as identifier used in the right hand side of the expression needs to be matched with that on the left This is something to be considered as a future improvement to the system 59 Another optimisation which might be made is that of constant folding i e if the result of an expression can be computed at compile time
69. red Tokens blank new line comment Productions statements list statement statements empty 74 lf statement declaration functioncall function_application semicolon assignment field_access assign expression semicolon if if l_parenthese bool expression r_parenthese block1 block else block2 block while while l_parenthese bool_expression r_parenthese block return return expression semicolon switch switch l_parenthes xpression r_parenthese switch_block for for decreasing name assign expl expression to exp2 expression step exp3 expression block break break semicolon print_str print expression semicolon println_str println expression semicolon block l_brace statements r brace function_application call name l_parenthese actual_param_list r_parenthese actual_param_list list actual_param_list comma expression exp expression empty switch block 1 brace possible case r brace possible case switch label block switch label cas xpression colon default default colon declaration class_dec klass identifier 1 declaration r_brace func_dec type identifier l_parenthese formal_param_list r parenthese block const dec const type identifier initialiser semicolon var dec type type param identifier array access initialiser semicolon
70. respectively in the Java Doing console input in Java is not simple The cleanest way found to read say an integer from the keyboard into an integer variable is to do something like the following try java io BufferedReader stdin new java io BufferedReader new java io InputStreamReader System in String line stdin readLine int Integer parselInt line catch java io IOException System out println e catch NumberFormatException e System out println e At the moment I think that the best way to deal with input is to provide functions with names like readInt and readSt ring If the user includes these in their program a function wrapping code similar to the above will be included in the Java source code and called at the relevant point Another option would be to have a generic read function which would translate to different Java functions depending on the type of the variable to which the result of the read is being assigned This is more complex to implement will be interactive but graphics will not be provided as these are not really a fundamental concept in programming and tend to be highly platform and language dependent Operators The following operators will be provided e _ e e ea 25 gt e lt pem or not Generics Generics 14 are a sophisticated concept Java is an objec
71. return first if N 1 return second else return fibonacci second 89 first second N 1 This program converts a given decimal number to given base 1 10 Kenya void putBase int number int base if number lt base print number else call putBase number base base printin number int number int base print Intput an integer in decimal number readInt print Input the base you want it expressed in base readInt print number in base base is call putBase number base Java package putbase import kenya io IntReader public class Program public static void main String args int number int base We System out print Intput an integer in decimal number IntReader read System out print Input the base you want it expressed in base IntReader read System out print number in base base is putBase number base static void putBase int number int base if number lt base 90 System out print number else putBase number base base System out println number base 91 92 Appendix 0 A Survey of People s Experiences of Learning Programming To find out which features of a language make it easy to learn programmi
72. riented but I would argue against this It is suggested that novice programmers should not be introduced immediately to the object oriented programming paradigm but that they should begin with a more traditional structured approach Because of this the Kenya system will generate the Java code that a programmer would write to solve a problem if they were approaching it in a structured way rather than an object oriented way There is not a straightforward mapping between a structured program and an object oriented program to solve the same problem as they are two different ways of thinking about solving the problem This makes the task of writing a mechanical translator from structured code to object oriented code extremely difficult It also means that were such a translation achievable a novice programmer looking at the generated code would not easily be able to see based on the way that they had been thinking about the problem how the Java code worked To carry out the translation first the text representing the Kenya program must be processed to form an abstract representation Then the abstract representation can be processed generating the relevant Java code from each construct and expression in the program Parsing The parser for Kenya was generated using SableCC 12 as it outputs Java code which is easy to incorporate with the rest of the code for the project Also the use of the Visitor pattern allows for easy extension of the fu
73. rking with the language The Language Implementation The implementation of the Kenya language produced in this project is not quite in line with the original specification A couple of extra restrictions were added in order to fix some of the shift reduce errors encountered while developing the grammar for the language The grammar had to be changed at several points during the development of the translator and the type checker in order that the necessary parts of the data structure were available from particular nodes in the tree mostly this was achieved by flattening productions With the pressure of time mounting towards the end of the project two compromises were necessary When a function call is made the keyword 11 must be placed in front of the 58 name of the function When logical not operator is applied expression to which it applies must be in brackets even if it only consists of one term Also unfortunately during the development of the grammar the unary minus operator was accidentally omitted and so is not present in this implementation If further development is done on an implementation of Kenya work should be undertaken to correct the grammar so that the language can be used exactly as it was designed without these restrictions Apart from these points the implementation allows full use of the language as it was designed In all the tests carried out the translator produced correct Java code from cor
74. s on the following pages show some of the more important classes and how they are linked together All of the different functional parts of the system are linked together by the user interface The rationale behind the design of the user interface is described in another section The user can enter code into the text areas and invoke other functions by pressing the buttons on the toolbar Each JButton has an ActionListener which listens for mouse clicks and then calls appropriate routines to parse the code compile it open a file etc 31 int Type Checker i Error Messages Translator public class gt File Directory Information flow through the Kenya system 32 Parser Vector CodeLines Process GJ Compiler i Error Messages public class gt Class Files KenyaUI KenyaArea MsgsArea KenyaToolbar JButton ActionListener ParseListener C ActionListener Parser Lexer The overall structure of the user int
75. s operator 57 comparison of the amount of code which needs to be written in terms of the number of characters between Kenya Turing and Java is given below Comments were removed from all the programs before measuring their size so that only executed code was taken into account Exercise Turing Java Star Triangle 89 chars 242 259 chars Fibonacci 519 520 734 400 507 641 The size of programs to solve different exercises in different languages In all cases it can be seen that Kenya is at least as concise as either of the other languages This is good as it points to Kenya being a simple language with no unnecessary syntax which was one of the key design goals The newly designed Kenya does posess many of the qualities of a good teaching language and fulfils the above aims to a high degree It is of course not possible to say that Kenya is a good teaching language without it being used to teach a novice programmer or an introductory programming class Unfortunately at this stage in the year there are hardly any novice programmers in the department due to the college s highly effective teaching and the threat of exams and so it was difficult to test it in the field Also the environment has not been developed to be sufficiently robust to give to a class for laboratory work It is intended only to demonstrate the possibilities of wo
76. se If they can then it will not complete the generation of the parser The productions in the grammar must be rewritten until the parser will no longer be able to enter such a conflict state Such conflicts are generally very frustrating and not straightforward to remove or prevent A great deal of time was spent during this project working on the grammar to remove conflicts Analysis SableCC takes a grammar definition and produces from it a Node class and a set of subclasses of it which represent each of the operators and constructs which can appear at each of the nodes in the syntax tree It produces a lexical analyser and a parser which together process the textual representation of the source code to produce this tree Everything which is a Node has a method with the signature public void apply Object o This allows visitor objects to be passed to each node to perform a particular type of analysis or processing on them SableCC generates a class called DepthFirstAdapter which will perform a depth first traversal of the parse tree without performing any operations on the nodes as it goes This class can be extended and its methods overridden to process the tree 39 The methods DepthFirstAdapter have signature of the following form public void caseAVariableDeclaration AVariableDeclaration node do something AVariableDeclaration is a subclass of Node There is caseXY method corresponding to each type o
77. section was developed the main goal of the project was to produce a good IDE It was originally my intention to use this IDE for Kenya simply plugging in the code relevant to parsing and translating the Kenya language where previously the code for the JMac language was However after spending some time analysing the code for the JMac IDE I felt that it would probably be easier to write my own graphical interface from scratch as it was difficult to work out exactly how the JMac IDE code worked This was not because the code was badly written The large number of different classes and the fact that the Graphical User Interfaces is event driven rather than following a step by step procedural execution so there is no real start or end to the program make it hard 51 to analyse Also the JMac had some features which I thought were unnecessary such as a class wizard By designing my own IDE I could provide exactly the features that I wanted The Kenya IDE I wanted to keep the IDE for Kenya as simple as possible The main features that are required are areas in which Kenya and Java code can be edited for accessing features either a menu or a toolbar some sort of display of status information awindow in which to view the output of a running program e a system to provide help to the user A lot of existing IDE s and other types of software contain a large number of features The way in whi
78. ss is imported explicitly by placing the following command at the top of the program import kenya io intReader If more than one type of input has been used then the whole of the kenya io package is imported into the namespace instead using the command import kenya io 46 Checking If a purely mechanical translation from Kenya to Java is carried out and resulting Java code presented to the Java compiler any mistakes in the Kenya code will be echoed or amplified in the Java code and cause compilation errors The errors reported by the Java compiler will be in relation to the Java code only and so will be useless and probably confusing to the novice programmer who is working with Kenya They may not be interested in the Java stage at all Errors during compilation will therefore signify nothing more useful than There is a bug in your program To aid this situation the system should make checks for the correctness of the Kenya program before carrying out the translation Syntactically incorrect Kenya programs i e programs that do not conform to the grammar will generate errors in the lexical analysis and parsing stages Error messages are returned to the user at this stage so incorrect code is not generated However there may be still be problems with a program which is syntactically correct The Dragon book 8 says A compiler must check that the source program follows both the syntactic and semantic conventions
79. t oriented language and almost everything is an object i e it extends the class Object When programs deal with large numbers of objects they tend to hold them in various kinds of containers like Vectors HashMaps etc Any sort of object can be put into a container and got out again later However if you put in say a Dog where Dog is a class that you or someone else has defined and try to get it out again you get out an Object This is because containers hold Objects They do not remember the more explicit type of each Object put into the container and so they can only give an Object back It is up to the programmer to remember the type of the objects they put into the container and convert them back to this type using a cast In Java this looks like Vector v new Vector Dog d new Dog d Dog Dog v elementAt 0 The cast is bracketed Dog after assignment operator on last line This coerces Object which comes out of the vector to a Dog so that it can be assigned to e It is somewhat annoying for the programmer to have to remember the type of the objects in a container and cast them whenever they are extracted A solution to this problem would be to have a class called DogVector which only contained Dogs We could then be sure that any object extracted from a DogVector would be a Dog and therefore no cast would be necessary 26 However there will be other types of obj
80. teaching first year programmers or fourth year programmers impersonating first year programmers to solve such problems example programs demonstrate that it is possible to solve the problems using the language Translation As well as a language design an approach for teaching with this language was developed This approach is to provide an automatic translation of the Kenya program written by the student into a Java program which performs the same function The goals for the Java code generated by the system were that it should be correct e and clean self contained not appear too mechanical i e it should appear as human generated as possible redundant code no extra semicolons brackets or braces For all the cases tested the translation of a valid Kenya program produced valid Java code which compiled and performed the intended function of the Kenya program However proof 66 by example is not a rigorous proof that all valid Kenya programs will be translated to valid Java programs by the software To verify that this is indeed the case would require specifying the operational semantics of both Kenya and Java and showing that both programs result in the same changes of state Unfortunately I currently have neither the skills knowledge nor time to undertake such an exercise Clarity and cleanliness of code is a somewhat subjective thing There are varying opinions on what is
81. ted to any depth desired by the programmer A set of known types is also maintained so that whenever a declaration is encountered a check can be made to ensure that the type to be associated with the name is valid 50 User Interface Although some programmers prefer to edit their code using a text editor like emacs or vi and run their compiler from the command line a novice programmer is likely to prefer a more friendly interface To this end an Integrated Development Environment should be provided for Kenya Integrated Development Environments There are many IDEs available for developing programs in languages such as Java Visual Basic They all have many different features but tend to share a few common key elements acode editor e the ability to run the compiler from inside the IDE the ability to test and debug the execution of the program Environments like Microsoft s Visual Studio 19 and Sun s Forte 21 offer many other features like class wizards and graphical resource or user interface editors These more advanced features are likely to be confusing for the novice programmer It would probably be better if an IDE targeted particularly at new programmers had only the essential features that are necessary to write compile debug and run a program without any other potentially confusing bells and whistles The JMac IDE When the JMac 4 system which was mentioned in a previous
82. they wrote in future using an import statement to include the class or just by putting a copy of it in the same package I have decided to write separate classes IntReader DoubleReader and StrReader which are in a package called kenya io The use of readInt readReal or readStr functions in a Kenya program causes the relevant class or the whole package to be imported into the generated Java program 30 Software Design The software designed in this project takes a Kenya program and processes it in several ways It builds an abstract representation of the program which can be checked for correctness and translated to Java code to be compiled and run The details of the different stages of processing are given in the following sections This section describes the overall structure of the system and how the different parts fit together The flow of information through the system is shown in the figure on the next page A textual representation of the Kenya source code is typed in by the user This is transformed into an abstract syntax tree by a lexical analyser and a parser The tree is processed by a type checker to ensure correctness of the program and by a translator to produce Java source code The Java source code is then saved in a file in a directory and the compiler is run to produce a class file In order to provide platform independence the software developed in this project has been written in Java The diagram
83. to produce graphical user interfaces for programs and it might be possible to provide the same sort of functionality in Kenya Larger Developments step through debugger allowing users to step through the execution of their program one instruction at a time and to examine the contents of variables as they do so is very valuable when debugging This would probably mean creating an interpreter for Kenya code rather than translating it to Java compiling and runnning on the Java virtual machine Improvements to the error reporting errors are currently reported in terms of unexpected tokens incorrect types etc It would be interesting to try and provide error reporting which identified the cause of the problem not just the symptoms 69 70 References 1 Sun Microsystems The Source for Java TM Technology http java sun com 2 Holtsoft Turing and Object Oriented Turing homepage http www holtsoft com turing 3 Microsoft homepage http www microsoft com 4 Lam Y K IDE for Beginning Java Programmers 2000 5 BlueJ BlueJ Teaching Java http www bluej org 6 Motil J and Epstein D JJ a Language Designed for Beginners Less 15 More available at http www publicstaticvoidmain com 7 Bailey R Language Processors 1998 Imperial College 8 Aho A V Sethi and Ullman J D Compilers Principles Techniques and Tools 1986 Addison Wesley 9 The Lex and Yacc page http www combo org lex yacc page
84. uring programming language provides a generalised loop in which the programmer can put the exit condition at the top or the bottom or anywhere in the middle to determine the way in which the loop works Most repeat until loops can be rewritten as while loops so to aid consistency only a while will be provided while condition boolean expression Statements The other sort of loop to be considered is the for loop This gives a number of iterations using an index variable For example for i from 1 to 100rfor i from 10 to 1 The syntax for this in Java is for i 1 i lt 10 i Statements This provides maximum flexibility from one construct without increasing complexity to do more sophisticated things e g increment in different steps or have complex loop termination conditions Turing takes a slightly less sophisticated approach using for i s 0 5 09 put i end for 23 The Java approach provides much more flexibility but the Turing syntax is far simpler It is a difficult decision which of these is the more important consideration I would argue for simplicity consistent with all of the features of this new language but at the same time it would be easy to make the language too restrictive therefore not allowing more sophisticated programs to be written While the language should be kept simple it is also important that a large number of problems can be solved and techniques applied using it
85. va Helpers Letter 2715 digit 0 9 er 13 10 point 45 55 not cr lf 32 127 cr 1 space Tokens boolean boolean char char int int real real string string void void klass class const const print print printin println fee Ui else while while return return switch switch case break break default default for for to to step step decreasing decreasing call call true true false false and and or xor j not not 73 readint readInt readreal readReal readstring readString identifier letter letter digit stringliterat 6 75 l parenthese parenthese l brace brace 1 bracket bracket semicolon colon e 2 phy comma dot point plus minus times divide power mod less lessequal greater gt greaterequal equal notequal assign intnumber digit fpnumber digit point digitt new_line cr 1 cr 1f blank space comment letter digit space cr Igno
86. variable has been declared as having type int We consequently examine the assignment statement We know that a has type int so we check that the assignment which is being assigned to a also has type int For an addition to have type int both of the addends must also be of type int First we check 4 which is an integer as we expect but then we check 5 7 which is a real number and therefore we have a type error A type inference approach works the other way around We expand the expression as far as possible and find the types of the end nodes in this case 4 and 5 7 int and real respectively These two are added and so we infer that the result must have type real This real value is assigned to a variable and so this variable should also have type real but we find that it does not and so again we have a type error I have chosen to take a type checking approach in implementing the Kenya type checker Implementation of the Kenya Type Checker The parser creates an abstract syntax tree representing the program as described in a previous section The code which SableCC produces promotes the use of the Visitor pattern This means that a TypeChecker object can be passed to the nodes of the abstract syntax tree in order to process it without writing any code inside the classes from which the tree is built This offers the advantage of being able to alter the implementation of the type checker independently of the tree If the gramma

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