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1 Introduction

1. Xl Y L y in Browse end end 1 2 M L First Y is introduced and initialized in the outer local in end Then in the inner local in end all variable on the L H S are introduced i e M Y X1 and L So the outer Y is invisible in the innermost local end statement The above st local Y in atement is equivalent to Y 1 local M X1 Y L in M M Y L X1 Y L 1 2 Browse M L end end If we want Y to denote the variable in the outer scope we have to suppress the introduction of the inner Y in the L H S by using the exclamation mark as follows is only meaningful in the L H S of an equality local Y 1 in local M f M Y X1 Tyers in Browse end end ty L 1 2 M L 5 3 Pattern Matching Let us consider a very simple example insertion of elements in a binary tree A binary tree is either empty represented by nil or is a tuple of the form tree Key Value Treel Value and 1 right subtree ments three argument Tr L TreeR where Key is a key of the node with the corresponding value reeL is the left subtree having keys less than Key and TreeR is the having keys greater than key The procedure Insert takes four argu of them are input arguments Key Value and TreeIn and one output eeOut to be bound to the resulting tree after insertion The program is shown in Figure 3 The symbol before TreeOut is a voluntary documentation 13 d
2. Another form of declaration is declare X Y Z in S This is an open ended declaration that makes X Y and Z visible globally in S including all statements the follows textually unless overridden again by another variable declaration of the same variables 3 Primary Oz Types Oz is a dynamically typed language Figure 1 shows the type hierarchy of Oz Any variable if it ever gets a value will be bound to a value of one of these types Most of the types seems familiar to experienced programmers except probably Chunk Cell Space FDint and Name We will discuss all of these types in due course For the impatient reader here are some hints The chunck data type allows users to introduces new abstract data types Cell introduces the primitive notion of cear Piero Figure 1 Oz Type Hierarchy state container and state modification Space will be needed for advanced problem solving and programming advanced search techniques including all you know of from an AI course FDint is the type of finite domain that is used frequently in constraint programming constraint satisfaction and operational research Name introduces anonymous unique unforgeable tokens we also think of security The language is dynamically typed in the sense that when a variable is introduced its type as well as its value is unknown Only when the variable is bound to an Oz value its type becomes determined Hello World Let us do like everybody else If you
3. The program shown in Figure 12 defines a concurrent Map function Notice that thread end is used here as an expression Let us discuss the behavior of this program If we enter the following statements 24 declare fun Fib X case X of 0 then 1 1 then 1 else thread Fib X 1 end Fib X 2 end end Figure 13 A Concurrent Fibonacci function declare FX Y 2 Browse thread Map X F end a thread executing Map is created which suspends immediately in the case statement because X is unbound If then enter the following statements X 1 2 Y fun F X X X end the main thread will traverse the list creating two threads for the first two arguments of the list thread F 1 end and thread F 2 end and then suspends again on the tail of the list Y Finally Y 3 2 Z nil will complete the computation of the main thread and the newly created thread thread F 3 end resulting in the final list 1 4 9 The program shown in Figure 13 is a concurrent divide and conquer program that is very inefficient way to compute the Fibonacci function This program creates an exponential number of threads See how it is easy to create concurrent threads You may use this program to test how many threads your Oz installation can create Try Fib 24 and use the panel program in your Oz menu If it works try a larger number The whole idea of explicit thread creation in Oz is to enable the programmer to structure his her application
4. suspend T suspends T resume T resumes T terminate T terminates T injectException T 41 raises exception E in T this T returns the current thread T setPriority T P sets T s priority setThisPriority P sets current thread s priority get priorities P gets system priority ratios set priorities high X medium Y sets system priority ratios Table 1 Thread Operations The system procedure System set priorities high X medium yY sets the processor time ratio to X 1 between high priority threads and medium priority thread It also sets the processor time ratio to Y 1 between medium priority threads and low priority thread X and Y are integers So if we execute System set priorities high 10 medium 10 each for 10 time slices allocated to runnable high priority thread the system will allocate one time slice for medium priority threads and similarly between medium and low priority threads Within the same priority level scheduling is fair and round robin Tronically in the research project Perdio developing an Internet wide distributed Oz system stream communication across machines works better because of designed flow control mechanisms that suspend producers with the network buffers are full 29 Now let us make our producer consumer program work We give the producer low priority and the consumer high We also set the priority ratios to 10 1 and 10 1 L in Syste
5. 1 2 3 4 fun X X X end andthen and orelse After all we have been doing a lot of work for nothing Oz already provides the Boolean lazy non strict version of the Boolean function 7A strict function evaluates all its argument first before executing the function 21 fun BinaryTree T case T of nil then true tree K V Tl T2 then BinaryTree Tl andthen BinaryTree T2 else false end end Figure 11 Checking a Binary Tree lazily And and Or as the Boolean operators andthen and orelse respectively The former behaves like the function AndThen and the latter evaluates its second argument only if the first argument evaluates to false As usual these operators are not primitives they are defined in Oz Figure 11 defines the final version of BinaryTree To Function or not to Function The question is when to use functional nota tion and when not The honest answer is that it is up to you I will tell you my personal opinion Here are some rules of thumb e First what I do not like Given that you defined a procedure P do not call it as a function i e do not use functional nesting for procedures Use instead procedural nesting with nesting marker as in the Merge example And given that you defined a function call it as function e I tend to use function definitions when things are really functional i e there is one output and possibly many inputs and the output is a mathematical function of the input arguments
6. BinaryTree T1 BinaryTree T2 B 17 o What is a binary tree declare BinaryTree local proc And B1 B2 B case B1 then case B2 then B true else B false end else B false end end in proc BinaryTree T B case T of nil then B true tree K V T1 T2 then And BinaryTree T1 BinaryTree T2 B else B false end end end Figure 7 Checking a Binary Tree It is certainly doing unnecessary work according to our nesting rules it evaluates its second argument even if the first is false One can fix this problem by making an new procedure AndThen that takes as its first two arguments two procedure values and calls the second argument only if the first returns false Thus getting the effect of delaying the evaluation of its arguments until really needed The procedure is shown in Figure 9 AndThen is the first example of a higher order procedure i e a procedure that takes other procedures as argument and may return other procedures as results In our case AndThen returns just a Boolean value but in general we are going to see other examples where procedures return procedures as result As in functional languages higher order procedures are invaluable abstraction devices that helps creating generic reusable abstractions Control Abstractions Higher order procedures are used in Oz to define various control abstractions In modules Control and List as well as many others you will find many control abstractions Here are
7. I tend to use procedures in most of the other cases i e multiple outputs or nonfunctional definition due to stateful data types or nondeterministic defini tions e One may relax the previous rule and use functions when there is a clear di rection of information flow although the definition is not strictly functional Hence by this rule we can write the binary merge in Figure 6 as a function although it is definitely not After all functions are concise 7 Modules and Interfaces Modules also known as packages are collection of procedures and other values that are constructed together to provide certain related functionality A module typically has a number of private procedures that are not visible outside the module and a number of interface procedures that provides the external services of the module The interface procedures provide the interface of the module In Oz there are no syntactic support for modules or interfaces Instead the lexical scoping of In fact I use mostly objects except for typical constraint and logic programming applications Classes objects etc 22 the language and the record data type suffices to construct modules The general construction looks as follows Assume that we would like to construct a module called List that provides a number of interface procedures for appending sorting and testing membership of lists This would look as follows declare List in local proc Append end pr
8. end 6 Functional Notation Oz provides functional notation as syntactic convenience We have seen that a procedure call P Xi Xn R could be used in a nested expression as a function call P Xit Xn Oz also allows functional abstractions directly as syntactic notation for procedures So the following function definition fun F Xi Xna S E end where S is a statement and E is an expression corresponds to the following procedure definition proc F X Xn R S R E end The exact syntax for functions as well as their unfolding into procedure defini tions is defined in 3 Here we rely on the reader s intuition Roughly speaking the general rule for syntax formation of functions looks very similar to how procedures are formed with the exception that whenever a thread of control in a procedure ends in a statement the corresponding function ends in an expression The program shown in Figure 10 is the functional equivalent to the program shown in Figure 8 Notice how the function AndThen is unfolded into the procedure AndThen Here are a number of steps that give some intuition of the transformation process All the intermediate forms are legal Oz programs fun AndThen BP1 BP2 case BP1 then case BP2 then true else false end else false end end Make a procedure by introducing a result variable B proc AndThen BP1 BP2 B B case BP1 then case BP2 then true else false end else false end end Move
9. end This does not automatically lock the object when one of its methods are called instead we have to use the construct lock S end inside a method to guarantee exclusive access when S is executed Remember that our locks are thread reentrant This implies that e if we take all objects that we constructed and enclose each method body with lock end and e execute our program with only one thread then e the program will behave exactly as before Of course if we use multiple threads we might deadlock if there is any cyclic dependency Writing nontrivial concurrent program needs careful understanding of the dependancy patterns between threads In such programs deadlock may occur whether locks are used or not It suffices to have a cyclic communication pattern for deadlock to occur The program in Figure 23 can be refined to work in concurrent environment by refining it as follows declare class CCounter from Counter prop locking meth inc Value lock Counter inc Value end end meth init Value lock Counter init Value end end end Let us now study a number of interesting examples where threads not only perform atomic transactions on objects but also synchronize through objects The first example shows a concurrent channel which is shared among an arbitrary number of threads A producing thread may put information in the channel asynchronously A consuming thread has to wait until information exists in the channel Waiti
10. gt 0 then N Generator N 1 else nil end end fun Sum L fun Suml L A case L of nil then A X Xs then Suml Xs A X end end in Suml L 0 end Figure 16 Summing the Elements in a List stream is a list that is created incremently by one thread the producer and subse quently consumed by one or more threads The threads consume the same elements of the stream For example the program in Figure 16 is an example of stream com munication where the producer generates a list of numbers and the consumer sums all the numbers Try the program in Figure 16 by running the following program local L in thread L Generator 150000 end thread Browse Sum L end It should produce the number 11250075000 Let us understand the working of stream communication A producer incremently create a stream a list as in the following where it is producing a volvo This happens in general in an eager fashion fun Producer volvo Producer end The consumer waits on the stream until items arrives then it is consumed as in proc Consumer Ls case Ls of volvo Lr then Consume volvo end Consumer Lr end The data flow behavior of the case statement lets the consumer wait until the arrival of the next item of the stream The recursive call allows the consumer to iterate the action over again The following pattern avoids the use of recursion by using an iterator instead proc Consumer Ls ForAll Ls pr
11. had to program a signalling mechanism for producers in a way similar to traditional monitors Observe the pattern in the put method First waiting has to be done outside the exclusive region This is done by using an auxiliary variable X which gets bound to wait if waiting has to be done The signaling mechanism is using a channel stored in the attribute prodg The get method notifies one producer at a time by seting the empty flag and notifying one producer if any This is done as an atomic step Notice that this example uses reentrant locking in an essential way Do you how The example discussed above was an illustration of how to write traditional mon itors There is a more simple way to write a unit buffer class that is not traditional This is due to the combination of objects and logic varaible Here follows in Fig ure UnitBuffer2 a simple definition of the UnitBuffer class no locking is needed directly Simple generalization of this leads to an arbitrary size bounded buffer class This is shown in Figure 34 53 declare class UnitBuffer from Channel prop locking attr empty true prodg meth init Channel init pq lt New Channel init end meth put I X in lock case empty then Channel put I empty lt false X nowait else X wait end end case X wait then prodq get _ self put TI else skip end end meth get I Channel get I lock empty lt true prodq put unit end end end Figure 32 A Unit Buffer
12. 20 defines a thread that acts as FIFO queue server Using logic variables makes the server insensitive to the arrival order of the get messages relative to put messages A server is created by NewQueueServer Q This statement returns back a port Q to the server A client thread having access to Q can request services by sending message on the port Notice how results are returned back through logic variables A client requesting an Item in the queue will send the message Send Q get I The server will eventually answer back by binding I The following sequence of statement illustrates the working of the program 32 declare NewQueueServer in local fun NewQueue X in q front X rear X number 0 end fun Put Q I X in Q rear I X AdjoinList Q rear X number Q numbert 1 end proc Get Q I NQ X in Q front I X NQ AdjoinList Q front X number Q number 1 end proc Empty Q B NQ B Q number 0 NQ Q end proc QServer Xs Q case Xs of X Xr then NQ in case X of put I then NQ Put Q I get I then Get Q I NQ empty B then Empty Q B NQ else NQ Q end QServer Xr NQ nil then skip end end S P NewPort S in fun NewQueueServer thread QServer S NewQueue end P end end Figure 20 Concurrent Queue Server 33 Q NewQueueServer Send Q put 1 Browse Send Q get S Browse Send Q get S Browse Send Q get S Send Q
13. Here the use of parallel case is essential 15 proc Merge Xs Ys Zs case Xs Ys of nil Ys then Zs Ys Xs nil then Zs Xs X Xr Ys then Zr in Zs X Zr Merge Ys Xr Zr Xs Y Yr then Zr in Zs Y ar Merge Yr Xs Zr end end Figure 5 Binary Merge of Two Lists 5 4 Nesting Let us use our Insert procedure as defined in Fig 4 The following statement inserts a few nodes in an initially empty tree Note that we had to introduce a number of intermediate variables to perform our sequence of procedure calls local TO T1 T2 T3 in Insert seif 43 nil TO Insert eeva 45 TO T1 Insert rebecca 20 T1 T2 Insert alex 17 T2 T3 Browse T3 end Oz provides syntactic support for nesting one procedure call inside another state ment at an expression position So in general local Y in Pee Ne 3a OMe gece TY end could be written as COP sa S eA A Gass od is called a nesting marker and thus the variable Y is eliminated The rule to revert to the flattened syntax is that a nested procedure call inside a procedure call is moved before the current statement and a new variable is introduced with one occurrence replacing the nested procedure call and the other occurrence replacing the nesting marker Functional Nesting Another form of nesting is called functional nesting a pro cedure P X R could be considered as a function its result is the argument 16
14. Xs N case Xs of nil then N 0 _ Xr then N1 in ListC length Xr N1 N N1 1 end end meth nrev Xs Ys case Xs of nil then Ys nil X Xr then Yr in ListC nrev Xr Yr ListC append Yr X Ys end end end Figure 24 List Class 39 or B is a superclass of a class appearing in the from declaration of A Inheritance is a way to construct new classes from existing classes It defines what attributes features and methods are available in the new class We will restrict our discussion of inheritance to methods However the same rules apply to features and attributes Which methods are available in a class is defined through a precedence relation on the methods that appear in a class hierarchy We call this relation the overriding relation e A method in a class overrides any method with the same label in a super class e A method defined in a class declared in a from declaration overrrides any method with the same label defined in a class to the left in the from decla ration Now a class hierarchy with the super class relation can be seen as a directed graph with the class being defined as the root There are two requirements for the inher itance to be valid First the inheritance relation is directed and acyclic So the following is not allowed class A from B end class B from A end Second after striking out all overriden methods each remaining method should have a unique label and is define
15. a case statement is defined in terms of an if statement it is merely a notational convenience 5 2 Procedural Abstraction Procedure Definition Procedure abstraction is a primary abstraction in Oz A procedure can be defined passed around as argument to another procedure or stored in a record A procedure definition is a statement that has the following structure proc P X Xn S end Assume that the variable P is already introduced executing the above statement will create a procedure consisting of a unique name a and an abstraction A X Xn S This pair is stored in the cell labeled by P A procedure in Oz has a unique identity given by its name and is distinct from all other procedures Two procedure definitions are always different even if they look similar Procedures are the first Oz values that we encounter whose equality is based on name or token equality others include objects classes and chunks 11 On Lexical Scoping In general the statement S in a procedure definition will have many syntactic variable occurrences Some occurrences are syntactically bound and others are free A variable occurrence X in S is bound if it is in the scope of the procedure formal parameter X or in the scope of a variable introduction statement that introduces X Otherwise the variable occurrence is free Each variable occurrence in an Oz is eventually bound by the closest statically surrounding variable binding construct
16. are unfamiliar with Oz here is your first Oz program If you feed it it will print the string Hello World ina window that appears in your screen Browse Hello World In fact the main reason for showing this program is to get you accustomed with one of the unconventional syntactic aspects of Oz namely the syntax of procedure calls applications Browse Hello World is a procedure application of Browse on the single string argument Hello World Browse is actually a pre declared global variable that got bound to the browsing procedure when the browsing module was loaded into the system Oz follows in What you see is an oz application called the browser You have to switch it to the virtual string mode to enable string visualization this syntactic aspect other functional languages e g SCHEME and ML with the exception of using braces instead of parentheses 3 1 Adding Information In Oz there are few ways of adding information to the store or said differently of binding a variable The most common form is using the equality infix operator For example X 1 will bind the unbound variable X to the integer 1 and add this information to the store Now if X is already assigned the value 1 the operation is considered as performing a test on X If X is already bound to an incompatible value i e any other value different from 1 a proper exception will be raised 3 2 Data Types with Structural Equality The hierarch
17. in a modular way Therefore create threads only when the application need it and not because concurrency is fun Time In module Time we can find a number of useful real time procedures Among them are Alarm I U which creates immediately its own thread and binds U to unit after I milliseconds Delay I suspends the executing thread for a least I milliseconds and then reduces to skip The program shown in Figure 15 starts two threads one displays Ping periodi cally after 500 milliseconds and the other Pong after 600 milliseconds Some Pings will be displayed immediately after each other because of the periodicity difference Stream Communication The data flow property of Oz easily enables writing threads that communicate through streams in a producer consumer pattern A 25 itreaas Memory Problem Solving Runtime Run 7 31 s E Garbage Collection 0 88 s E Copy 0 00 s E Propagation 0 00 s E Load 1 01 s Threads Created 75108 Runnable 1 E Figure 14 The Oz Panel Showing Thread Creation in Fib 26 X local proc Ping N case N 0 then Browse ping terminated else Delay 500 Browse ping Ping N 1 end end proc Pong N For 1 N 1 proc I Delay 600 Browse pong end Browse pong terminated end in Browse game started thread Ping 50 end thread Pong 50 end end Figure 15 A Ping Pong Program 26 declare fun Generator N case N
18. is made private to the class Moreover it is made available for subclasses i e protected by storing it in the the attribute displayHistory You can now try the class by the following statements declare P P New HistoryPoint init 2 0 P display P move 3 2 10 8 Default Argument Values A method head may have default argument values So in the following example meth a X Y d1 2Z lt 0 d2 W lt 0 end a method call or object application of a may leave the arguments of features d1 and d2 unspecified In this case these arguments will assume the value 0 46 declare class HistoryPoint from Point attr history nil displayHistory DisplayHistory meth init X Y Point init X Y call your super history lt 1 X Y end meth move X Y Point move X Y history lt 1 X Y history end meth display Point display self DisplayHistory end meth DisplayHistory private method Browse with location history Browse history end end Figure 27 The class HistoryPoint We can go back to our Point example by specialized Point in a different di rection We define the class BoundedPoint as a point that moves in a constrained rectangular area Any attempt to move such a point outside of the area will be ignored The class is shown in Figure 28 Notice that the method init has two default arguments that gives a default area if not specified in the initialization of an instance of BoundedPoint We conc
19. meth A X end meth a self A 5 end end end where A is bound to a new name in the lexical scope of the class definition Some object oriented languages have also the notion of protected methods A method is protected if it is accessible only in the class it is defined or in descendant classes i e subclasses and subsubclasses etc In Oz there is no direct way to define a method to be protected However there is a programming technique that gives the same effect We know that attributes are only visible inside a class or to descendants of a class by inheritance We may make a method protected by firstly making it private and secondly storing it is in an attribute Consider the following example class C from attr pa A meth A X end meth a self A 5 end end Now we subclass C and access method A as follows class Cl from C meth b self pa 5 end end Method b access method A through the attribute pa Let us continue our simple example in Figure 26 by defining a specialization of the class that in addition of being a point it stores a histroy of the previous movement This is shown in Figure 27 There are a number of remarks on the class definition HistoryPoint First observe the typical pattern of method refinement The method move specializes that of class Point It first calls the super method and then does what is specific to being a history point class Second method DisplayHistory
20. put 2 Send Q put 3 Browse Send Q empty Explain the use of logic variables as a mechanism to returns value Explain the common pattern of using nesting marking in message passing 9 2 Chunks Ports are actually stateful data structures A port keeps a local state internally tracking the end of its associated stream Oz provides two primitive devices to construct abstract stateful data types chunks and cells All others subtypes of chunks can be defined in terms of chunks and cells A chunk is similar to a record except that the label of a chunk is an oz name and there is no arity operation available on chunks This means one can hide certain components of a chunk if the feature of the component is a name that is visible only by lexical scoping to user define operations on the chunk A chunk is created by the procedure NewChunk Record This create a chunk with the same arguments as the record but having a unique label The following program creates a chunk local x in Browse X NewChunk f c 3 a 1 b 2 Browse X c end It will display the following lt Ch gt a 1 b 2 c 3 3 9 3 Cells and State A cell could be seen as a chunk with a mutable single component A cell is created as follows NewCell X C A cell is created with an initial content referring to the variable x The variable c contains the cell The Table 2 shows the operations on a cell Check the following program The last statement
21. the result variable into the outer case expression to make it a case statement proc AndThen BP1 BP2 B case BP1 then B case BP2 then true else false end else B false end end 20 o Syntax Convenience functional notation local fun AndThen BP1 BP2 case BP1 then case BP2 then true else false end else false end end fun BinaryTree T case T of nil then true tree K V Tl T2 then AndThen fun BinaryTree Tl end fun BinaryTree T2 end else false end end end Figure 10 Checking a Binary Tree lazily Move the result variable into the inner case expression to make it a case statement and we are done proc AndThen BP1 BP2 B case BP1 then case BP2 then B true else B false end else B false end end If you are a functional programmer you can cheer up You have your functions including higher order ones and similar to lazy functional languages Oz allows cer tain forms of tail recursion optimizations that are not found in certain strict functional languages including Standard ML SCHEME and the concurrent language Erlang However functions in Oz are not lazy This property is easily programmed using the concurrency constructs and the variables Here is an example of the well known higher order function Map It is tail recursive in Oz but not in Standard ML nor in SCHEME declare fun Map Xs F case Xs of nil then nil X Xr then F X Map Xr F end end Browse Map
22. us the ability to perform coarse grain atomic operation of a set of objects A very important requirement in distributed database system The basic mechanism in Oz to getting exclusive access is through locks 11 1 Locks The purpose of a lock is to mediate exclusive access to a shared resource between threads Such a mechanism is typically made safer and more robust by restricting this exclusive access to a critical region on entry into the region the lock is secured and the thread is granted exclusive access rights to the resource and when execution leaves the region whether normally or through an exception the lock is released Typically a concurrent attempt to secure the same lock will block until the thread currently holding it has released it 49 Simple Locks In the case of a simple lock a nested attempt by the same thread to secure the same lock during the dynamic scope of a critical section guarded by it will block reentrancy is not supported Simple locks can be modelled in Oz as follows where Code is a nullary procedure encapsulating the computation to be performed in the critical section proc NewSimpleLock Lock Cell NewCell unit in proc Lock Code Old New in Exchange Cell Old New Wait Old try Code finally New unit end end end Thread Reentrant Locks In Oz the computational unit is the thread There fore an appropriate locking mechanism should grant exclusive access rights to threads As a co
23. A Tutorial of Oz 2 0 Seif Haridi SICS November 28 1996 Abstract We introduce in this tutorial the programming language Oz 2 0 Oz 2 0 is a multiparadigm programming language It enables users to write significant applications in a fraction of time compared to other existing languages The document is deliberately informal and thus complements other Oz 2 0 docu mentations 1 Introduction A very good starting point is to ask why Oz Well one rough short answer is that compared to other existing languages it is magic It provides the programmer or using the nicer term application developer with a wide range of program ming system abstractions to develop quickly and robustly advanced applications And yet it is a simple and coherent design Oz tries to merge several directions of programming language designs into a single coherent one Most of us know the benefits of the various programming paradigms whether object oriented functional or logic constraint programming Yet when we start writing programs in any ex isting language we quickly find ourselves confined by the concepts of the underlying paradigm Oz tries to attack this problem by a coherent design of a language that combines the programming abstractions of various paradigms in a clean and simple way So before answering the above question let us see what Oz is This is again a difficult question to answer in a few sentences So here is the first shot It is a high lev
24. Ind Xs P which is similar to ForAll but P 2 is a binary procedure that takes the index of the current element of the list starting from 1 as its first argument and the element as its second argument Here is the program Thread Priority and Real Time Try to run the program using the following statement Consumer thread Producer 5000000 Switch on the panel and observe the memory behavior of the program You will quickly notice that this program does not behave well The reason have to do with the asynchronous message passing If the producer sends messages i e create 28 proc Consumer Ls fun IsVolvo X X volvo end Lsl in thread Lsl Filter Ls IsVolvo end List forAllInd Ls1 proc N X case N mod 1000 0 then Browse riding a new volvo else skip end end end new elements in the stream in a faster rate than the consumer can consume more and more buffering will be needed until the system starts to break down There are a number of ways to solve this problem one is to create a bounded buffer between producers and consumers which we will be discussed later Another way is to experimentally control the rate of thread execution so that the consumers gets a larger time slice than the producers The modules Thread in 2 and Systen in 4 provide a number of operations pertinent to threads Some of these are summarized in Table 1 Procedure Description state T A returns current state of T
25. Modules Static method calls have in general the same efficiency as procedure calls This allows classes to be used as modules This is advantageous because classes can be built incremently by inheritance The program shown in Fig ure 24 shows a possible class acting as a module The class Listc defines some com mon list procedures as methods Listc defines the methods append 3 member 2 length 2 and nrev 2 Notice that a method body is similar to any Oz statement but in addition method calls are allowed We also see the first instance of inheritance in class ListC from BaseObject Here the class ListC inherits from the predefined class BaseObject BaseObject has only one trivial method meth noop skip end To use a class as a module one need to create an object from it This is done by declare ListM New ListC noop ListM is an object that will act as a module We can try this module by performing some method calls Browse ListM append 1l 2 3 4 5 Browse ListM length 1l 2 3 Browse ListM nrev 1l 2 3 10 3 Inheritance Classes may inherit from one or several classes appearing after the keyword from A class B is a superclass of a class A if B appears in the from declaration of A 38 declare ListC in class ListC from BaseObject meth append Xs Ys Zs case Xs of nil then Ys Zs X Xr then Zr in Zs X Z2r ListC append Xr Ys Zr end end meth member X L B Member X L B end meth length
26. Monitor declare class UnitBuffer from BaseObject attr putq getq meth init putq lt New Queue init getq lt New Queue init getq put unit end meth put I getq get _ putq put 1 end meth get T putq get I getq put unit end end Figure 33 A Unit Buffer Class 54 begin ozdisplay declare class BoundedBuffer from BaseObject attr putq getq meth init N putq lt New Queue init getq lt New Queue init For 1 N 1 proc _ getq put unit end end meth put I getq get _ putgq put I end meth get T putgq get I getq put unit end end Figure 34 A Bounded Buffer Class 12 Distributed Programming See the paper on mobile objects 13 Encapsulated Search To be completed 14 Logic Programming To be completed 15 Constraint Programming See the Oz primer References 1 V Saraswat Concurrent Constraint Programming 2 The Oz Standard Modules 3 The Oz Notation 4 The Oz User Manual 55
27. We have already seen how to apply procedures Let us now show our first procedure definition In Figure 2 we have seen how to compute the maximum of two numbers or literals We abstract this code into a procedure local Max X Y Z in proc Max X Y 2 case X gt Y then Z X else Z Y end end X 5 Y 10 Max X Y Z Browse Z end Anonymous Procedures and Variable Initialization One could ask why a variable gets bound to a procedure in a way different from the normal way where an equality is used X Value The answer is that what you have seen is just a syntactic variant to the equivalent form P proc X Y S end where the R H S defines an anonymous procedural value This is equivalent to proc P X Y S end In Oz we can initialize a variable immediately while it is being introduced by using an equality between local and in in the statement local in end So the previous example could be written as follows where we also use anony mous procedures local Max proc X Y Z case X gt Y then Z X else Z Y end end X 5 Y 10 Z in Max X Y Z Browse Z end Now let us understand variable initialization in more detail The general rule says that in a variable initialization equality only the variables occurring on the L H S of the equality are being defined Consider the following example This rule is approximate Methods also bind variable occurrences as well as patterns 12 in
28. actually the integer 48 and amp a is 97 Some control Exception handling is described later characters have also a representation e g amp n is newline Operations on characters integers and floats can be found in the library modules Char Float and Int More generic operations on all numbers are in module Number 3 2 2 Literals Another important category of atomic types i e types whose members have no internal structure is literals Literals are divided into atoms and names Atoms are symbolic entities that have an identity made up of a sequence of alphanumeric characters starting with a lower case letter or arbitrary printable characters inclosed in quotes e g a foo OZ 2 0 Hello World Atoms have an ordering based on lexicographic ordering Another category of el ementary entities are names The only way to create a name is by calling the procedure NewName X where X is assigned a new name that is guaranteed to be world wide unique Names cannot be forged or printed As will be seen later names play an important role in the security of Oz programs A subtype of Name is Bool which consists of two names protected from being redefined by having the reserved keywords true and false Thus a user program cannot redefine them and mess up all programs relying on their definition There is also the type Unit which consists of the single name unit This is used as a synchronization token in many concurrent program
29. airly However there are 23 declare fun Map Xs F case Xs of nil then nil X Xr then thread F X end Map Xr F end end Figure 12 A Concurrent Map function three priority levels high medium and low that determines how often a runnable thread is allocated a time slice So a high priority thread cannot starve a low priority one Priority determines only how large piece of the processor cake a thread can get One can also get a reference to a new thread by asking for a reference when a new thread is spawned as follows thread X runs S end Having a reference to a thread enables operations on threads such as terminating it or raising an exception in it Thread operations are defined module Thread 2 Let us see what we can do with threads First remember that each thread is a data flow thread that blocks on data dependency Consider the following program declare X0 X1 X2 X3 thread local Y0 Y1 Y2 Y3 in Browse YO Yl Y2 Y3 YO X0 1 Y1 X1 Y0 Y2 X2 Y1 Y3 X3 Y2 Browse completed end end Browse XO X1 X2 X3 If you input this program and watch the display of the Browser tool the variables will appear unbound If you now input the following statements one at a time x0 0 XLS X2 2 XIS you will see how the thread resumes and then suspends again First when XO is bound the thread can execute YO X0 1 and suspends again because it needs the value of x1 while executing Y1 X1 Y0 and so on
30. alk Java etc they are just descriptions of how the objects of the class should behave Classes from First Principles The Figure 21 example shows how a class is constructed from first principles Here we construct a CounterClass It has a single attribute accessed by the atom val It has a method table that has three methods accessed through the record features browse init and inc A method is a procedure that takes a message always a record an extra parameter representing 35 declare Counter local Attrs val MethodTable m browse MyBrowse init Init inc Inc proc Init M init Value S Self Assign S val Value end proc Inc M inc Value S Self local X in Access S val X Assign S val X Value end end proc MyBrowse M browse S Self Browse Access S val end in Counter NewChunk c methods MethodTable attrs Atts end Figure 21 A Example of Class Construction the state of the current object and the object itself known internally as self Talk about the example Explain M init Objects from First Principles Figure 22 shows a generic procedure that cre ate an object from a given class This procedure creates an object state from the attributes of the class It initializes the attributes of the object each to a cell with unbound initial value We use here the iterator Record forAll that iterates over all fields of a record NewObject returns a procedure Object that identifies the object Notice that th
31. as the incremental tell operation e x and Y are labels of the same cell the operation is completed e x Y is free merge X Y to the cell of y x and discard the original cell of x Y we have a situation where a cell is labeled by a set of variables e x and Y are labels of different cells containing the records Rx and Ry respec tively Rx and Ry have different labels arities or both the operation is com pleted and an exception is raised Otherwise arguments of Rx and Ry with the same feature are pair wise merged in arbitrary order Here are some examples of successful equality operations local X Y Z in f 1 X 2 b f a Y f Z a Z Browse X Y Z end will show a b R14 f R14 a in the browser R14 f R14 a is the external representation of a cyclic graph The following example shows what happens when variables with incompatible values are equated declare X Y Z X f c a Y Z b xX Y The incremental tell of x y will assign z the value c but will also raise an exception that is caught by the system when it tries to equate a and b In general the two graphs to merge could have cycles however any correct implementation of the merge operation will consider cell pairs for which an attempt to merge has been made as successfully merged Now we discuss the equality test operator The basic procedure X Y R tries to test whether X and Y are equal or not and returns the
32. ationals declare class Int meth less X Y B B X lt Y end end class Rat from Object meth less X Y B local P Q X R S Y in B P S lt Q R end end end Thereafter we can execute the following statements Browse New SortClass Int noop qsort 1l 2 5 3 4 Browse New SortClass Rat noop asort 23 3 7 34 11 A 47 17 Self Application The program in Figure 25 shows in the method gsort an object application using the keyword self meth qsort Xs Ys case Xs self partition Xr P S L We use here the phrase object application instead of the commonly known phrase message sending because message sending is misleading in a concurrent language like Oz When we use self instead of a specific object as in self partition Xr P S L we mean that we dynamically pick the method partition that is defined in the current object Thereafter we apply the object as a procedure on the message This is a form of dynamic binding common in all object oriented languages 10 6 Attributes We have touched before on the notion of attributes Attributes are the carriers of state in objects Attributes are declared similar to features but by using the keyword attr When an object is created each attributes is assigned a new cell as its value These cells are initialized very much the same way as features The difference lies in the fact that attributes are cells that can be assigned reas
33. cialized for specific purposes The common way in object oriented programming is to define first an abstract class where some methods are left unspecified Later these methods are defined in subclasses of this class Suppose you defined a generic class for sorting where the comparison operator lessThan is needed This operator depends on what kind of data are being sorted Different realizations are needed for integer rational or complex numbers In this case by subclassing we can specialize the abstract class to a class for a concrete type In Oz we have also another natural method for creating generic classes Since classes are first class values we can instead define a function the takes some type argument s and return a class that is specialized for the type s In Figure 25 a function SortClass is defined that takes a class as its single argument and returns a sorting class specialized for the argument 42 declare fun SortClass Type class C from BaseObject meth qsort Xs Ys case Xs of nil then Ys nil P Xr then S L in self partition Xr P S L ListC append C qsort S P C qsort L Ys end end meth partition Xs P Ss Ls case Xs of nil then Ss nil Ls nil X Xr then Sr Lr in case Type less X P then Ss X Sr Lr Ls else Ss Sr Ls X Lr end C partition Xr P Sr Lr end end end in C end Figure 25 Parameterized Classes 43 We can now define two classes for integers and r
34. d locally that overrides the conflict causing methods e There is another problem with multiple inheritance when sibling super classes share directly or indirectly a common ancestor class that is stateful i e has attributes One may get replicated operations on the same attribute This typically happens when initializing a class one has to initialize its super classes The only remedy here is to carefully understand the inheritance hier archy to avoid such replication Or inherit from multiple classes that do not share common ancestor This problem is known as the implementation sharing problem 10 4 Features Objects may have features similar to records Features are components that are specified in the class declaration class C from feat al an end As in a record a feature of an object has an associated field The field is a logic variable that can be bound to any Oz value including cells objects classes etc Features of objects are accessed using the infix operator The following shows an example using features declare class ApartmentC from BaseObject meth init skip end end class AptC from ApartmentC feat streetName york streetNumber 100 wallColor white floorSurface wood end Feature initialization The example shows how features could be initialized at the time the class is defined In this case all instances of the class Aptc will have the features of the class with the same values of the feat
35. d only in one class in the hierarchy Hence class C in the following example is not valid because the two methods labeled m remain class A meth m end end class B meth m end end class B from B1 end class A from Al end class C from A B end Whereas in the following class C is valid and the method m available in C is that of B class A meth m end end class B meth m end end class C from A B end Notice that if you run a program with an invalid hierarchy the system will not complain until an object is created that tries to access an invalid method Only at this point of time you are going to get a runtime exception The reason is that classes are partially formed at compile time Rather they are completed by demand using method caches at execution time Multiple inheritance or not My opinion is the following e In general to really use multiple inheritance correctly one has to understand the total inheritance hierarchy Which is sometimes worth the effort e I do not like the existance of any overriding rule that depends on the order of classes in the from construct So I would consider the later example as invalid l4to be defined shortly 40 as the former one The reason is that if you take A and B in the later example and refine them to get the first example your working program will suddenly cease to work If there is a name conflict between immediate super classes I would define a metho
36. e state of the object is visible only within Object One may say that Object is a procedure that encapsulates the state We can try our program as follows declare C NewObject Counter init 0 C C inc 6 C inc 6 C browse Try to execute the following statement local X in C inc X X 5 end C browse You will see that nothing happens The reason is the object call C inc X sus pends inside the inc method Do you know where exactly If you on the other hand execute the following statement things will work as expected local X in thread C inc X end X 5 end C browse Tn fact classes may have some invisible state In the current implementation a class usually has method cache which is stateful 13This is a simplification an object in Oz is a chunk that has the above procedure in one of its fields other fields contain the object features 36 declare proc NewObject Class InitialMethod 0b ject local State O in State MakeRecord s Class attrs Record forAll State proc A NewCell _ A end proc O M Class methods Label M M State O end O InitialMethod Object O end end Figure 22 Object Construction 10 1 Objects and Classes for Real Oz supports object oriented programming following the methodology outlined above There is also syntactic support and optimized implementation so that object calls calling a method in objects is as cheap as procedure calls The class Counter defined earlie
37. e used as well as numbers strings or labeled tuples with virtual strings as argument Here is one example 123 23 is 100 represents the string 123 23 is 100 4 Equality and the Equality Test Operator We have so far shown simple examples of the equality statement le W tree I Y LT LR These were simple enough to intuitively understand what is going on However what happens when two unbound variables are equated X Y or when two large data structures are equated Here is a short explanation We may think of the store as a dynamically expanding array of cells each cell is labeled by a variable When a variable X is introduced a cell is created in the store labeled by X and its value is unknown At this point the cell does not possess any value it is empty as a container that might be filled later A variable with an empty cell is a free variable The cell as a container is flexible enough to contain any arbitrary Oz value The operation W tree 1 I 2 Y 3 LT 4 LR stores the record structure in the cell associated with w Notice that we are just getting a graph structure where arcs emanating from the w labeled cell are pointing to the cells of IT Y LT and LR respectively Each arc in turn is labeled by the corresponding feature of the record Given two variable X and Y X y will try to merge their respective cells Now we are in a position to give a reasonable account for the merge operation of X Y known
38. eclare proc Insert Key Value TreeIn TreeOut if TreeIn nil then TreeOut tr Key Value nil nil K1 V1 T1 T2 in TreeIn tree Kl V1 T1 T2 then case Key K1 then TreeOut tree Key Value T1 T2 elsecase Key lt K1 then local T in TreeOut tree Kl V1 T T2 Insert Key Value Tl T end else local T in TreeOut tree Kl V1 T1 T Insert Key Value T2 T end end end end Figure 3 Insertion in a Binary Tree to denote that the argument plays the role of an output argument The proce dure works by cases as obvious First depending on whether the tree is empty or not and in the latter case depending on a comparison between the key of the node encountered and the input key Notice the use of case then elsecase else end withthe obvious meaning Pattern Matching In Figure 3 it is tedious to introduce the local variables in the clause Kl V1 Tl T2 in TreeIn tree Kl V1 Tl T2 then Oz provides a pattern matching case statement that allows implicit introduction of variables in the patterns Two forms exist for a pattern matching case statement case X of Pattern then S elseof Patterna then S elseof else S end and case X of Pattern then S Patterng then S Pl else S end All variables introduced in a Pattern are implicitly declared and have a scope stretching over the corresponding clause In the first case matching statement the patterns are tested in order In the s
39. econd called parallel case matching statement 14 case for pattern matching declare proc Insert Key Value TreeIn TreeOut case TreeiIn of nil then TreeOut tr Key Value nil nil tree Kl V1 T1 T2 then case Key K1 then TreeOut tree Key Value T1 T2 elsecase Key lt K1 then T in TreeOut tree Kl V1 T T2 Insert Key Value Tl T else T in TreeOut tree Kl V1 T1 T Insert Key Value T2 T end end end Figure 4 Insertion in a Binary Tree the order is indeterminate The else part may be omitted in which case an exception is raised if all matches fail Again in each pattern one may suppress the introduction of new local variable by using For example in case f X1 X2 of f Y Z then else end matching of X1 is against the value of the external variable y Now remember again that the case statement and its executing thread may suspend if current value of x1 is insufficient to decide the result of the matching Have all this said Figure 4 shows the tree insertion procedure using a matching case statement We have also reduced the syntactic nesting by abbreviating local T in TreeOut tree JT Insert T end into T in TreeOut tree JT Insert T The expression we may match against could be any record structure and not only a variable This allows multiple argument matching as shown in Figure 5 which depicts a classical stream merge procedure
40. el programming language that is designed for modern advanced concurrent in telligent networked soft real time parallel interactive and pro active applications As you see it is still hard to know what all this jargon means Oz combines the salient features of object oriented programming by providing state abstract data types classes objects and inheritance It provides the salient features of functional programming by providing a compositional syntax first class procedures and lexical scoping In fact every Oz value is first class including procedures threads classes methods and objects It provides the salient features of logic and constraint pro gramming by providing logical variables disjunctions flexible search mechanisms and constraint programming It is also a concurrent language where users can cre ate dynamically any number of sequential threads that can interact with each other Most of this work was done while on sabbatical at the programming Systems group at DFKI However in contrast to conventional concurrent languages each Oz thread is a data flow thread Executing a statement in Oz proceeds only when all real data flow dependencies on the variables involved are resolved Oz has its roots in a paradigm that is known as concurrent constraint programming 1 2 Variables Declaration We will restrict ourselves initially to the sequential programming style of Oz You can think of Oz computations as performed by one
41. exist And a member ceases to exist when it is no longer accessible 9 1 Ports Ports is such an abstract data type A Port P is an asynchronous communication channel that can be shared among several sender A port has a stream associated 11 For examples objects are implemented in a totally different way depending on the optimization level of the compiler 31 declare proc Conc Ps proc Conc Ps I 0O case Ps of P Pr then M in thread P M I end Conc Pr M 0 nil then O I end end in Wait Conc Ps unit end Figure 19 Concurrent Composition with it The operation NewPort S P creates a port P and initially connect to the variable S taking the role of a stream The operation Send P M will append the message M to the end of the stream The port keeps track of the end of the stream as its next insertion point The operation IsPort P B checks whether P is a port The following program shows a simple example using ports declare S P P Port new S Browse S Send P 1 Send P 2 If you enter the above statements incremently you will observe that S gets incre mently more defined S i Tre Ports are more expressive abstractions than pure stream communication discussed in Section 8 since they can shared among multiple thread and can be embedded in other data structures Ports are the main message passing mechanism between threads in Oz Server Clients Communication The program shown in Figure
42. form if X Xn in C then S else So end where is a sequence of equalities is called a simple conditional The subex pression if X Xn in C then Si 5 S gt may not be executed at all if an exception is raised in 5 is usually called a clause and Xi Xn in C is the condition of the clause A thread executing such as conditional will first check whether the condition There are X1 Xn such that C is true is satisfied or falsified by the current state of the store If the condition is satisfied statement S4 is executed if it is falsified the thread executes So and if neither the thread suspends The most common types of conditions are simple equalities as X true X is bound to true in the store Y Z in X Y Z 3 X is bound to a tuple of 2 arguments X Y X and Y label the same cell in the store Comparison Procedures Oz provides a number of built in tertiary procedures used for comparison These include that we have seen earlier as well as lt lt gt gt andthen and orelse Common to these procedures is that they are used as boolean functions in an infix notation The following example illustrates the use of a conditional in conjunction with the greater than operator gt In this example Z is bound to the maximum of X and Y Le to Y local X Y F Z in X 5 Y 10 F X gt Y if F true then Z X else Z Y end end Parallel Conditional A parallel condi
43. iables X Y will be merged together labeling a single box that contains the value unit Wait X suspends the current thread until X is bound In Figure 19 refines a higher order construct combinator that 30 local proc Producer L case L of X Xs then X volvo Producer Xs nil then Browse end of line end end proc Consumer N L case N 0 then L nil else X Xs L in case X of volvo then case N mod 1000 0 then Browse riding a new volvo else skip end Consumer N 1 Xs else Consumer N Xs end end end in Consumer 10000000 thread Producer end end Figure 18 Producing volvo s implements a concurrent composition control construct It takes a single argument that is a list of nullary procedures When it is executed the procedures are forked concurrently The next statement is executed only when all procedures in the list terminate 9 Stateful Data Types Oz provides set of stateful data types These include ports objects arrays and dictionaries hash tables These data types are abstract in the sense that they are characterized only by the set of operations performed on the members of the type Their implementation is always hidden and in fact different implementations exist but their corresponding behavior remains the same Each member is always unique by conceptually tagging it with an Oz name upon creation A member is created by an explicit creation operation A type test operation always
44. increments the cell by one If we leave out thread end the program deadlocks Do you know why local I O X in I NewCell a Browse Access I Assign I b Browse Access I Assign I X Browse Access I X 5 5 Exchange I O thread O 1 end Browse Access I 34 Procedure Description NewCell X C creates a cell c with content X Access C X returns the content of C in X Assign C Y modifies the content of C to Y IsCell C tests if C a cell Exchange C X Y swaps the content of c from xX to Y Table 2 Cell Operations end Cells and higher order iterators allow conventional assignment based program ming in Oz The following program accumulates in the cell J the sum 1 2 3 10 declare J in J NewCell 0 For 1101 proc I O N in Exchange J O N N O I end Browse Access J Explain how ports can be implemented by chunks and cells 10 Classes and Objects A Class in Oz is a chunk that contains e A collection of methods in a method table e A description of the attributes that each instance of the class will possess An attribute is statefull cell that is accessed by the attribute name which is a literal e A description of the features that each instance of the class will possess A feature is an immutable component a variable that is accessed by the feature name which is a literal Class are stateless oz values Contrary to languages like SmallT
45. key seif value 43 left nil right nil tree seif 43 nil nil Operations on records We discuss some basic operations on records Most operations are found in the module Record To select a record component one uses the infix operator Record Feature So o Selecting a Component Browse T key Browse W 1 will show seif twice on the display seif seif The arity of a record is a list of the features of the record sorted lexicographically To display a record s arity we use the procedure Arity The procedure application Arity R X will execute once R is assigned a record and it will bind x to the arity of the record o Getting the Arity of a Record local X in Arity T X Browse X end local X in Arity W X Browse X end will show key left right value 1 2 3 4 Another useful operation is conditionally selecting a field of a record The operation CondSelect takes a record R a feature F and a default field value D and a result argument X If the feature F exists in R R F is assigned to X otherwise the default value D is assigned to X CondSelect is not really a primitive operation It is definable in Oz The following statements Q Selecting a component conditionally local X in CondSelect W key eeva X Browse X end local X in CondSelect T key eeva X Browse X end will display eeva seif A common infix tuple operator used in Oz is So 1 2 is a tuple of two elements and obser
46. locks until is determined If E is not a lock then a type error is raised NewLock L allocates a new lock L IsLock E returns true iff E is a lock Arrays Oz has arrays as primary chunk type Operation on arrays are defined in module Array 2 To create an array the procedure NewArray L H I A is used where A is the resulting array L is the lower bound index H is the higher bound index and I is the initial value As a simple illustration of locks consider the program in Figure 30 Assume that we want to perform the procedures Zero and Switch each atom ically but in an arbitrary order To do this we can use locks as in the following example local X Y in thread Delay 100 lock L then Zero A end X unit end thread lock L then Switch A end Y X end Wait Y For 1 10 1 proc I Browse Get A I end end By Switching the delay statement above between the the first and the second threads we observe that all the elements of the array either will get the value zero or 0 We have no mixed values Write an example of an atomic transaction on multiple objects 51 11 2 Locking Objects To guarantee mutual exclusion on objects one may use the locks described in the pervious subsection Alternatively we may declare in the class of the objects that its instances can be locked with a default lock existing in the object when it is created A class with a lock is declared as follows class C from prop locking
47. lude this section by finishing our example in a way that shows the multi ple inheritance problem We would like now a specialization of both HistoryPoint and BoundedPoint as a bounded history point A point that keeps track of the history and moves in a constrained area We do this by defining the class BHPoint that inherits from the two previously defined classes Since they both share the class Point which contains stateful attributes we encounter the implementation sharing problem Since we in any way anticipated this problem we created two protected methods stored in boundConstraint and displayHistory to avoid repeating the same actions In any case we had to refine the methods init move and display since they occur in the two sibling classes The solution is shown in Figure 29 Notice how we use the protected methods We did not care avoiding the repetition of initializing the attributes x and y since it does not make so much harm Try the following example declare P P New BHPoint init 2 0 P display AT declare class BoundedPoint from Point attr xbounds 0O 0 ybounds O 0 boundConstraint BoundConstraint meth init X Y xbounds XB lt 0 10 ybounds YB lt 0 10 Point init X Y 3 call your super xbounds lt XB ybounds lt YB end meth move X Y case self BoundConstraint X Y then Point move X Y else skip end end meth BoundConstraint X Y B B X gt xbounds 1 andthen X l
48. m set priorities high 10 medium 10 thread X in Thread setThisPriority low L Producer 5000000 end thread Thread setThisPriority high Consumer L end end Demand driven Execution An extreme alternative solution is to make the pro ducer lazy Only producing an item when the consumer requests one A consumer in the case construct the stream with empty boxes unbound variables The pro ducer waits for the empty boxes unbound variables appearing of the stream It then fills the boxes binds the variables The general pattern of the producer is a follows proc Producer Xs case Xs of X Xr then I in Produce I X I Producer Xr end end The general pattern of the consumer is as follows proc Consumer Xs X Xr in Xs X Xr Consume X Consumer Xr end The program shown in Figure 18 is a demand driven version of the program in Figure 17 You can run with very large number of Volvo s Thread Termination Detection We have seen how thread are forked using the statement thread S end A natural question that arises is how to join back back into the original thread of control In fact this is a special case of detecting termination of multiple threads and making another thread wait on that event The general scheme is quite easy since Oz is a data flow language thread T X unit end thread T X X end thread T Y Xyn_1 end Wait Y MainT hread When All threads terminate the var
49. ng threads are served fairly Figure 31 shows one possible realization This program relies on the use of logical variables to achieve the desired synchronization The method put inserts an element in the channel A thread executing the method get will wait until an element is put in the channel Multiple consuming threads will 52 declare class Channel from BaseObject prop locking attr fr meth init X in f lt X r lt X end meth put I X in lock r I X r lt X end end meth get T X in lock f I X f lt X end Wait I end end Figure 31 An Asynchronous Channel Class reserve their place in the channel thereby achieving fairness Notice that Wait I is done outside an exclusive region If waiting was done inside lock end the program would deadlock So as a rule of thumb do not wait inside an exclusive region if the waking up action has to acquire the same lock The next example shows a traditional way to write monitors In Oz we do not use the normal signaling mechanism of monitors currently known as notify event and waitForEvent event Instead logic variables gives the same effect We show here an example of a unit buffer The unit buffer behaves in a very similar way as channel when it comes to conumers In the case of producers only one is allowed to insert an item in the an empty buffer Other producers have to suspend until the item is consumed The program is Figure 32 shows a single buffer monitor Here we
50. nsequence the non reentrant simple lock mechanism presented above is in adequate A thread reentrant lock allows the same thread to reenter the lock i e to enter a dynamically nested critical region guarded by the same lock Such a lock can be secured by at most one thread at a time Concurrent threads that attempt to secure the same lock are queued When the lock is released it is granted to the thread standing first in line etc Thread reentrant locks can be modelled in Oz as follows proc NewLock Lock Token NewCell unit Current NewCell unit in proc Lock Code ThisThread Thread this in case ThisThread Access Current then Code else Old New in Exchange Token Old New Wait Old Assign Current ThisThread try Code finally Assign Current unit New Old end end end end Locks in Oz are Thread reentrant locks that are given syntactic and implemen tational support in Oz They are implemented as subtype chunks Oz provides the following syntax for guarded critical regions 50 declare A L in A NewArray 1 10 5 L NewLock proc Switch A For Array low A Array high A 1 proc I X in X Get A I case X lt 0 then Put A I 7X elsecase 0 then Put A I zero else skip end Delay 100 end end proc Zero A For Array low A Array high A 1 proc I Put A I 0 Delay 100 end end Figure 30 Locking lock E then S end where F is an expression and the construct b
51. oc Item case Item of volvo then Consume volvo end end end 27 declare fun Producer N case N gt 0 then volvo Producer N 1 else then nil end end proc Consumer Ls proc Consumer Ls N case Ls of nil then skip volvo Lr then case N mod 1000 0 then Browse riding a new volvo else skip end Consumer Lr N 1 else Consumer Lr N end end in Consumer Ls 1 end Figure 17 Producing volvo s Figure 17 shows a simple example using this pattern The consumer counts the cars received Each time it receives 1000 car it prints a message on the display of the Browser You may run this program using Consumer thread Producer 10000 end Notice that the consumer was written using the recursive pattern Can we write this program using the iterative ForAll construct This is not possible because the consumer carries an extra argument N that accumulates a result which is passed to the next recursive call The argument corresponds to some kind of state In general there are two solutions either introduce the concept of stateful data structures which we will do in Section or define another iterator that passes the state around In our there case there are some iterators that fit our needs in the module List in 2 First we need an iterator that filter away all items except volvos We can use Filter Xs P Ys which outputs in Ys all elements that satisfies P 2 as a Boolean function The second is List forAll
52. oc Sort MergeSort end proc Member end proc MergeSort end in List list append Append sort Sort member Member end To access Append procedure outside the module List by List append Notice that the procedure MergeSort is private to the list module Oz standard modules 2 follows the above structure Often some of the procedures in the a module are made global without the module prefix The are several ways to do that Here is one way to make Append Externally visible declare List Append in Append List append local proc Append end in List list append Append sort Sort member Member end Modules are often stored on files for example the List module could be stored on the file list oz This file could be inserted into another file by using the directive insert list oz Consult the user manual 4 for details Modules in Oz could be nested This is apparent from compositional syntax of Oz 8 Concurrency So far we seen only one thread executing It is time to introduce concurrency In Oz an new concurrent thread of control is spawned by thread S end Executing this statement a thread is forked that runs concurrently with the current thread The current thread resumes immediately with the next statement Each nonterminating thread that is not blocking will be eventually allocated a time slice of the processor This means that threads are executed f
53. proc Merge Xs Ys Zs case Xs Ys of nil Ys then Zs Ys Xs nil then Zs Xs X Xr Ys then Zs Xs Y Yr then Zs X Merge Ys Xr Y Merge Yr Xs end end Figure 6 Binary Merge of Two Lists in Nested Form R and therefore P X could considered as a function call that can be inserted in any expression instead of the result argument R So TO AP X wee E wee 1 is equivalent to local R in iP E R POR neta te sf end Now back to our example a more concise form using functional nesting is Brows Insert alex 17 Insert rebecca 20 Insert eeva 45 Insert seif 43 nil There is one more rule to remember It has to do with a nested application inside a record or a tuple as in Zs X Merge Xr Ys Here the nested application goes after the record construction statement Do you know why So we have local Zr in Zs X Zr Merge Xr Ys Zr end We can now rewrite our Merge procedure as shown in Figure 6 where we use nested application 5 5 Procedures as Values Since we have been inserting elements in binary trees let us define a program that checks if is data structure is actually a binary tree The procedure BinaryTree shown in FigureBinaryTree checks a structure to verify whether it is a binary tree or not and accordingly returns true or false in its result argument B Notice that we also defined the auxiliary local procedure And Consider the call And
54. r has the syntactic form shown in Figure 23 A class X is defined by class X end Attributes are defined using the attribute declaration part before the method declaration part attr A n Then follows the method declarations each has the form meth E S end where E evaluates to a method head which is a record whose label is the method name An attribute A is accessed using the expression A It is assigned a value using the expression A lt The following shows how an object is created from a class using the procedure New whose first argument is the class the second is the initial method and the result is the object New is a generic procedure for creating objects from classes declare C in C New Counter init 0 C browse C inc 1 local X in thread C inc X end X 5 end 10 2 Static Method Calls Given a class Class and a method head m a method call has the following form Class m 37 declare class Counter attr val meth browse Browse val end meth inc Value val lt val Value end meth init Value val lt Value end end Figure 23 Counter Class A method call invokes the method defined in the class argument A method call can only be used inside method definitions This is because a method call takes the current object self as implicit argument The method could be defined there or inherited from super classes Inheritance will be explained shortly Classes as
55. result in R It returns true if the graphs starting from the cells of X and Y have the same structure with each pair wise corresponding cells having identical Oz values or are the same cell It returns false if the graphs have different structure or some pair wise corresponding cells have different values It suspends when it arrives at pair wise corresponding cells that are different but at least one them is free Now remember if a procedure suspends the whole thread suspends This does not seem useful however as you will see it becomes a very useful operation when multiple thread start interacting with each other The equality test is normally used as a functional expression rather than a statement As shown is the following example See lists are just tuples which are just records local L1 L2 L3 Head Tail in L1 Head Tail Head 1 Tail 2 nil L2 1 2 Browse L1 L2 L3 1 1 2 2 nil Browse L1 L3 end 5 Basic Control Structures We have already seen basic statements in Oz Introducing new variables and se quencing of statements S1 S2 Reiterating again a thread executes statements in a sequential order However a thread contrary to conventional languages may suspend in some statement so above a thread has to complete execution of S1 before starting 52 Skip The statement skip is the empty statement 5 1 Conditionals Simple Conditional A statement having the following
56. s local X Y B in x foo NewName Y B true Browse X Y B end 3 2 3 Records and Tuples Records are structured compound entities A record has a label and a fixed number of components or arguments There are also records with a variable number of arguments that are called open records For now we restrict ourselves to closed records The following is a record tree key I value Y left LT right RT It has four arguments and the label tree Each argument consists of a pair Feature Field so the features of the above record is key value left and right The corresponding fields are the variables I Y LT and RT It is possible to omit the features of a record reducing it to what is known from logic programming language as a compound term In Oz this is called a tuple So the following tuple has the same label and fields as the above record tree I Y LT RT which is just a syntactic notation for the record tree 1l I 2 Y 3 LT 4 RT 3 All characters can be written as ooo where o is an octal digit where the features are integers starting from 1 up to the number of fields in the tuple The following program will display a list consisting of two elements one is a record and the other is tuple having the same label and fields declare T I Y LT RT W in T tree key I value Y left LT right RT I seif Y 43 LT nil RT nil W tree I Y LT RT Browse T W The display will show tree
57. sequential process that executes one statement after the other We call this process a thread This thread has access to a memory called the store It is able to manipulate the store by reading adding and updating information Information is accessed through the notion of variables i e a thread can access information only through the variables visible to it directly or indirectly Oz variables are single assignment variables This notion is known from many languages including data flow and concurrent logic programming languages A single assignment variable has a number of phases in its lifetime Initially it is introduced and later it might be assigned a value in which case the variable becomes bound Once a variable is bound it cannot itself be changed However this does not mean that you cannot model state because a variable as you will see later could be bound to an object or a cell which is statefull i e the object can change its content A thread executing the statement local X Y Z in S end will introduce three variables X Y and Z and execute S in the scope of these variables A variable normally starts with an upper case letter possibly followed by an arbitrary number of alphanumeric characters Before the execution of S the variables declared above will not have any associated values i e the variables are unbound Any variable in an Oz program must be introduced except for certain pattern matching constructs to be shown later
58. signed an accessed at will However attributes are private to their objects The only way to manipulate an attribute from outside an object is to force the class designer to write a method that manipulates the attribute In the Figure 26 we define an the class Point Note that the attributes x and y are initialized to 0 before the initial message is applied The method move uses 44 declare class Point from BaseObject attr x 0 y 0 meth init X Y X lt a K Vom attribute update end meth location L L 1 x x y y attribute access end meth moveHorizontal X xr SSK end meth moveVertical Y VS end meth move X Y self moveHorizontal X self moveVertical Y end meth display Browse StringToAtom VirtualString toString point at x y yh 4 end end Figure 26 The class Point self application internally Try to create an instance of Point and apply some few messages declare P P New Point init 2 0 P display P move 3 2 10 7 Private and Protected Methods Methods may be labelled by variables instead of literals These methods are private to the class in whicg they are defined as in class C from meth A X end meth a self A 5 end end The method A is visible only within the class C In fact the notation above is just an abbreviation of the following expanded definition local A NewName in 45 class C from
59. some examples The procedure For From To Step P is an iterator abstraction that applies the unary procedure P normally saying the procedure P 1 instead to integers from From to To proceeding in steps Step Notice that use of the empty statement skip Executing For 1 10 1 Browse will display the integers 1 2 10 Another control abstraction that is often used is the ForAl1 iterator defined in the List module ForAl1 applies a unary procedure on all the elements of a list in the order defined by the list Think what happens if the list is produced inclemently by another concurrent thread declare proc ForAll Xs P case Xs of nil then skip 18 declare BinaryTree local proc AndThen BP1 BP2 B case BP1 then case BP2 then B true else B false end else B false end end in proc BinaryTree T B case T of nil then B true tree K V Tl T2 then AndThen proc Bl BinaryTree T1 Bl end proc B2 BinaryTree T2 B2 end B else B false end end end Figure 8 Checking a Binary Tree lazily local proc HelpPlus C To Step P case C lt To then P C HelpPlus C Step To Step P else skip end end proc HelpMinus C To Step P case C gt To then P C HelpMinus C Step To Step P else skip end end in proc For From To Step P case Step gt 0O then HelpPlus From To Step P else HelpMinus From To Step P end end end Figure 9 the For Iterator 19 X Xr then P X ForAll Xr P end
60. t xbounds 2 andthen Y gt ybounds 1 andthen Y lt ybounds 2 end meth display Point display self DisplayBounds end meth DisplayBounds XO X1 xbounds YO Y1 ybounds S in S xbounds X0 X1 ybounds YO Y1 Browse StringToAtom VirtualString toString S end end Figure 28 The class BoundedPoint 48 declare class BHPoint from HistoryPoint BoundedPoint meth init X Y xbounds XB lt 0 10 ybounds YB lt 0 10 HistoryPoint init X Y BoundedPoint init X Y xbounds XB ybounds YB repeats init end meth move X Y L in L boundConstraint case self L X Y then HistoryPoint move X Y else skip end end meth display BoundedPoint display self displayHistory end end Figure 29 The class BHPoint P move l1 2 This pretty much covers the object system What is left is how to deal with concurrent threads sharing common space of objects 11 Concurrent Objects As we have seen objects in Oz are stateful data structures Threads are the active computation entities Threads can communicate either by message passing using ports or through common shared objects Communication through shared objects requires the ability to serialize concurrent operations on objects so that the object state is kept coherent after each such operation In Oz we separate the issue acquiring exclusive access of an object from the object system This gives
61. tional is of the from if C then S Co then S else Sp end A parallel conditional is executed by evaluating all conditions C1 Co in an ar bitrary order possibly concurrently If one of the conditions say C is true its corresponding statement 5 is chosen If all conditions are false the else statement Sn is chosen otherwise the executing thread suspends Parallel conditionals are useful mostly in concurrent programming e g for programming time out on certain events However it is often used in a deterministic manner with mutually exclusive conditions So the above example could be written as local X Y F Zin X 5 Y 10 10 local X Y Z in X 5 Y 10 case X gt Y then Z X else Z Y end end Figure 2 Using a Case Statement F X gt Y if F true then Z X F false then Z Y end end Notice that the else part is missing This is just a short hand notation for an else clause that raises an exception Case Statement Oz provides an alternative conditional syntax that encourages the use of a very simple form of conditionals This is called the case statement The following is the simplest form case B then S else So end where B is a variable that should be bound to a boolean value if B true Sj is executed otherwise if B false S is executed This is equivalent to if B true then S else S end Our example using if statement could be now written as shown in Figure 2 Since
62. ures Therefore declare Aptl Apt2 Aptl New AptC init Aptl New AptC init Browse Aptl streetName Browse Apt2 streetName will display york twice We may leave a feature uninitialized as in Al declare class MyAptCl from ApartmentC feat streetName end In this case whenever an instance is created the field of the feature is assigned a new fresh variable Therefore declare Apt3 Apt4 Apt3 New MyAptCl init Apt4 New MyAptCl init Apt3 streetName kungsgatan Apt4 streetName sturegatan will assign the feature st reetName of object Apt3 the value kungsgatan and the corresponding feature of Apt 4 the value sturegatan One more form of initialization is available A feature is initialized at the class declaration to a variable or an Oz value that has a variable In the following declare class MyAptCl from ApartmentC feat streetName f _ end the attribute is initialized to a tuple with an anonymous variable In this case all instances of the class will share the same variable So the following declare Aptl Apt2 Aptl New MyAptCl init Apt2 New MyAptCl init Browse Aptl streetName Browse Apt2 streetName Aptl streetName f york if entered incremently will show that the statement Apt1 streetName f york binds the corresponding feature of Apt2 to the same value as that of Apt1 10 5 Parameterized Classes There are many ways to get your classes more generic that can later be spe
63. ve that 1 2 3 is a single tuple of three elements 1 2 3 and not 1 2 3 With the operator there is no empty tuple notation and single element tuple is written as X 3 2 4 Lists As in many other symbolic programming languages e g Scheme and Prolog lists form an important class of data structures in Oz Lists do not belong to a single data type in Oz They are rather a conceptual structures A list is either the atom nil representing the empty list or is a tuple using the infix operator and two arguments which are respectively the head and the tail of the list Thus a list of the first three natural numbers is represented as 1 2 3 nil Another convenient special notation for a closed list i e list with determined number of elements is 1 2 3 The above notation is used only for closed list so a list whose first two elements are a and b but whose tail is the variable xX looks like 1 2 xX One can also use the standard notation for lists E2 Further notational variant is allowed for lists whose elements correspond to char acter codes Lists written in this notation are called strings e g TOZ 2 0 is the list 79 90 32 50 46 48 3 2 5 Virtual Strings Virtual strings are special compound tuples which represents strings with virtual concatenation i e the concatenation is performed when really needed Virtual strings are used for I O with files sockets and windows All atoms except nil and can b
64. y starting from Number and Record in Figure 1 defines the data types of Oz whose members values are equal only if they are structurally similar For example two numbers are equal if they have the same type or one is a subtype of the other and have the same value e g both are integers and are the same number or both are lists and their head elements are equal as well as their respective tail lists Structural equality allows values to be equivalent even if they are replicas occupying different physical memory location 3 2 1 Numbers The following program introduces three variables I F C and assigns I an integer F a float C the character t in this order then it displays the list consisting of I F and C local I F C in I 5 F 5 5 C amp t Browse I F C end Oz support binary octal decimal and hexadecimal notation for integers which can be arbitrary large An octal starts with a leading 0 and a hexadecimal starts with a leading 0x or 0X Floats are different from integers and must have a decimal points Other examples of floats are 3 141 4 5E3 712 0672 In Oz there is no automatic type conversion so 5 0 5 will raise an exception Of course there are primitive procedures for explicit type conversion These and many others can be found in 2 Character are subtype of integers in the range of 0 255 The standard ISO 8859 1 coding is used Printable characters have external representation e g amp 0 is

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