Home

THE MELD PROGRAMMING LANGUAGE USER MANUAL

1. The syntax is statements where statements can be any number of statements including parallel sequential or atomic blocks 5 1 1 Partial Ordering of Data Dependency For every parallel block there is a partial ordering of statement inputs and outputs 1 e right hand and left hand side variables according to the rules of data dependency The execution order is determined by this partial ordering In figure 5 1 1 we have an example of a parallel block associated with the selector ADD of class Bank Because no data dependency exists between statements 1 and 2 they will be executed in parallel CLASS Bank Add x INTEGER gt counter counter 1 1 all all x 2 END CLASS Bank Figure 5 1 Parallel Block Program Fragment In the cases that data dependency does exists between statements the partial ordering is determined among statements For example CLASS Bank METHODS Add amount REAL gt tax REAL counter counter 1 1 total total tax 2 tax 0 0825 amount 3 END CLASS Bank A data dependency relation exists between statement 2 and statement 3 through local variable tax In other words a value for tax must be computed and assigned before statement 2 can be executed Therefore only the ordering requirement that statement 3 runs before statement 2 exists The execution ordering between statement 1 and statem
2. j INTERFACE IMPLEMENTATION OBJECT Main UserInterface UserInterface CREATE CLASS UserInterface val INTEGER 0 METHODS Initialization constraint send Help to SELF Help method Show user valid commands Help gt printf Welcome to the game of Hi Lo n printf The object of the game is for you to guess printf a number Commands are Mn printf help exit play lt number gt n n Internal method called when user wins Winner gt printf Congratulations you are a winner n printf Don t you want to play again say play Nn Integer guess method see if it is high or low Sd guess INTEGER gt if guess val then printf High n else if guess lt val then printf Low n else SSELF Winner play method Start a new game give up printf The val was d n val play val rand 10 END CLASS UserInterface END FEATURE HiLo Figure 3 1 Language Features Example MELD PROGRAMMING LANGUAGE 25 4 Classes Objects Methods and Messages 4 1 Overview All executable statements in MELD exist within classes A class is a definition of an abstract data type consisting of private storage and operations on that storage The storage is called instance variables and the operations are called methods Since the class is only a description of the storage and operations you instantiate the class t
3. 2 has x as an output variable left hand side variable there is no data dependency between 1 and 2 because both statements are in the same sequential block The statements will be executed sequentially i e 1 gt 2 gt 3 Another example shows the data dependency between the statements in a sequential block and the statements outside the block MELD PROGRAMMING LANGUAGE 42 CLASS Foo a b c d INTEGER METHODS Foo gt a vus 1 ess py 2 b a 1 3 d c 1 4 END CLASS Foo The execution order is 1 gt 3 gt 2 gt 4 5 4 5 Fifth Rule of Data Dependency Constraint Rule A constraint has no effect on the dependency of any statements either statements in a method or another constraint For example CLASS Foo a b c INTEGER METHODS a b 1 1 Foo gt C atl 2 b c 1 3 END CLASS Foo The execution order is 2 gt 3 gt 1 There is no circularity of data dependency since the constraint has no affect relation with statement 2 and statement 3 5 4 6 Sixth Rule of Data Dependency Dynamic Rule When two methods in one object are executing concurrently and the first five rules of data dependency have been applied to each method the execution order of statements among the two methods is further limited by the Dynamic Rule The Dynamic Rule says that when two methods in the same obj
4. Both methods will be activated when the message arrives and both statement and statement will be executed in parallel 4 4 5 Special Selectors A special form of selector is available that will match when no other symbolic method selector matches Il any selector Figure 4 4 5 shows how this is done Note There is no string form that matches only when no other selector matches The string will always match even if their are other matching selectors 4 5 External Methods and Variables As discussed in section 1 1 the INTERFACE clause is used when a number of features are separately developed and combined into one program The syntax for the features clause is MELD PROGRAMMING LANGUAGE 33 FEATURE Match Any INTERFACE IMPLEMENTATION OBJECT Intf UserInterface UserInterface CREATE CLASS UserInterface METHODS printf Received an unknown message s n S SSELECTOR do gt SSELF FUBar END CLASS UserInterface END FEATURE Match Any Figure 4 5 Any Example program FEATURE identifiers INTERFACE externals IMPLEMENTATION body END FEATURE identifiers pt externals EXPORTS port listopt IMPORTS port listopt port list IDENTIFIER IDENTIFIER identifier list port list port list The programmer specifies the external features to be used by means of the IMPORTS clause and those classes and globals that are visible to other features with the EXPORTS clau
5. block BEGIN statements END statements statements statements A block consists of zero or more statements enclosed by delimiters We have covered the different statement types in chapter 3 and 4 These are statement block opt assignment if statement object def function call send statement delay statement RETURN expression Statements Statement Statements statement The runtime execution of the statements is determined by the delimiters surrounding the block as follows BEGIN parallel END Equivalent to statements parallel The statements are potentially executed in parallel according to the partial ordering of data dependencies sequential The statements enclosed in the block are executed sequentially atomic The statements enclosed in the block are executed without interleaving with any statements outside the block By default the statements are executed in data flow order but a sequential block may be nested inside an atomic block to provide sequential execution without interleaving with other statements An atomic block executes without interruption from statements outside the block Normally a method may have more than one thread of execution running at one time the atomic block allows entry of only one thread of execution at a time MELD PROGRAMMING LANGUAGE 38 5 1 Parallel Block A parallel block is a set of statements enclosed by
6. is sent to an object it is matched against all of the object s string selectors Any that match are invoked in parallel The syntax of the string selector is str selector regular exp regular parm exp param list where regular exp is a regular expression that defines a set of strings regular parm exp is a regular expression with some notations to specify parameter occurrences For a description of regular expressions refer to grep 1 in the Unix M 4 3BSD programmer s and user s manuals The param list is a list of typed parameters param list identifier list type specifier param list param list Some examples of string selectors are hello gt When hello is received then hello a zA Z gt when hello concatenated with a name is received 0 9 gt when an integer is received then In regular parm exp the notations for specifying parameter occurrences is similar to the scanf function The current Meld implementation supports 6d f and 96s which indicate the occurrences of variables of types INTEGER REAL and STRING respectively Examples are X is d x INTEGER gt Withdraw f cash REAL gt My name is s Doe name STRING gt When multiple parameters are specified they must occur in the same order in the input string as in the parameter list The first example below is incorrect the second shows the correct parameter list for t
7. 32 32 35 37 38 38 39 39 MELD PROGRAMMING LANGUAGE 5 4 The Rules of Data Dependency 5 4 1 First Rule of Data Dependency Assignment Rule 5 4 2 Second Rule of Data Dependency If Then Else Rule 5 4 3 Third Rule of Data Dependency Change Type Rule 5 4 4 Fourth Rule of Data Dependency sequential block rule 5 4 5 Fifth Rule of Data Dependency Constraint Rule 5 4 6 Sixth Rule of Data Dependency Dynamic Rule 5 4 7 Data Dependency Deadlock 5 4 8 Seventh Rule of Data Dependency Recursion Rule 5 4 9 Race Condition 6 Constraints 6 1 Equation Constraints 6 2 Change Type Constraints 6 2 1 C procedure Constraints 6 2 2 Asynchronous Send Constraints 6 3 Conditional Constraints 6 4 Limitations 6 5 A Small Example II Reference Manual 7 MELD Reference Manual 7 1 Introduction 7 2 Syntax Notation 7 3 Lexical Conventions 7 3 1 Comments 7 3 2 Identifiers 7 3 3 Keywords 7 3 4 Constants 7 3 4 1 System Constants 7 3 4 2 Integer Constants 7 3 4 3 Floating Point Constants 7 3 4 4 Character Constants 7 3 5 Strings 7 3 6 Embedded C Code 7 4 Meld Programs 7 4 1 Interface 7 4 2 Implementation 7 4 3 Object Declaration 7 4 4 Merges 7 4 5 Class Declarations 7 4 6 Methods 7 4 6 1 Selectors 7 4 7 Constraints 7 4 7 1 Equation Constraints 7 4 7 2 Change Type Constraints 7 4 7 3 Conditional Constraints 7 4 8 Type Specifiers 7 4 9 Statements 7 4 9 1 Compound Statement or Block 7 4
8. 9 2 Assignment Statement 7 4 9 3 Conditional Statement 7 4 9 4 Procedure Call 7 4 9 5 Send Statement 7 4 9 6 System Variables Sender Self and Selector 7 4 9 7 Delay Statement 7 4 9 8 Return Statement MELD PROGRAMMING LANGUAGE 7 4 10 Constant 8 Complete Grammar 9 Unimplemented Features and Bugs 9 1 C Loops 9 2 Type Specifier 9 3 Views 9 4 Union 9 5 Data Dependency Bug 9 6 FEATURE name 9 7 Comments 9 8 Object Class 9 9 Feature Names 9 10 Type checking 9 11 Bug in string selectors Appendix I Running Meld Appendix II Bibliography Index iii
9. Integer gt MELD PROGRAMMING LANGUAGE 58 The any selector is a special form that matches only when no other symbolic selector matches any selector um 7 4 7 Constraints Constraints are statements in the METHODS part of a class definition that have no associated selector they are not invoked directly by a message Constraints define a relation between variables and actions When the value of a variable changes some predefined action is triggered Constraints are declarative They are executed whenever the associated input variables are changed and not by some explicit command There are three types of constraint equations constraint equation constraint change constraint condition constraint 7 4 7 1 Equation Constraints An equation constraint defines a relation between a set of input instance variables and one output instance variable The syntax is that of an assignment statement equation constraint variable expression where IDENTIFIER is the output instance variable defined by the class and expression is a C expression When any input i e right hand side variable changes value the expression is reevaluated and assigned to the output variable 7 4 7 2 Change Type Constraints A change type constraint does not have an output variable This constraint defines an action that will be evaluated when the class object is created and whenever the input instance variables are modified The action can
10. and C The second two statements are assignments to the constants 5 and 7 so they have no reliance on values of variables The data flow dependencies force A B C to be executed after both B 5 and C 7 but do not specify which of those two must be executed first On a multiprocessor system they might conceivably be executed in parallel although is this particular example there is little merit If on the other hand B and C were each being set to the return value of some complicated function with no side effects executing them in parallel might be very worthwhile Let s consider the following example program which implements the part of a simple savings account that handles deposits We will add to this example at various points in this manual In this example if you were to change the order of the two action equations in the Deposit method you would find that due to data dependency balance balance cash printf the balance is d balance is equivalent to printf the balance is d balance balance balance cash By examining Fig 1 5 you can also find that A B C Bo v 5 Co oem is equivalent to MELD PROGRAMMING LANGUAGE 10 FEATURE Bank INTERFACE IMPLEMENTATION OBJECT CitiSaver Savings Account Savings Account CREATE CLASS Savings Account balance INTEGER 0 METHODS Deposit d cash INTEGER gt balance balance cash printf the
11. be a C procedure call a synchronous send or an asynchronous send The parameter list of variables are the input instance variables in a change type constraint change constraint send statement function call 7 4 7 3 Conditional Constraints Constraints in the form of an if statement are called conditional constraints A conditional if is recomputed in response to changes in the arguments to the conditional expression condition constraint df statement See section 7 4 9 3 for the if statement syntax 7 4 8 Type Specifiers There are six basic types in MELD MELD PROGRAMMING LANGUAGE 59 basic type INTEGER BOOLEAN CHAR STRING REAL DOUBLE The type specifier is type specifier basic type ARRAY INTEGER INTEGER OF basic type ARRAY INTEGER INTEGER OF IDENTIFIER IDENTIFIER 7 4 9 Statements statement block opt assignment if statement object def function call send statement delay statement RETURN expression Statements Statement Statements statement 7 4 9 1 Compound Statement or Block Several statements can be used anywhere one is expected by surrounding the sequence of statements by the delimiters described below The delimiters define the type of concurrency parallel or sequential and whether the block is executed atomically block BEGIN statements END statements statements statements The statement block consists of zero or more
12. by the variable For example if you have an object balance of type PositiveInteger as defined in the previous example you can send synchronous messages using the syntax balance Add 6 10 The synchronous message is also called the MELD function call since along with SEND see below it is the principal mechanism for calling other methods Note also that MELD allows calling external C functions using the normal C function call syntax MELD is a concurrent programming language Two or more methods of any object may be active at the same time One way to achieve concurrency is to use the asynchronous form of the SEND statement With SEND execution continues immediately after sending the message and in effect you create a new thread of control You can use SEND with symbolic or string messages For example you can use SEND to output on your terminal by sending a string message to the system defined object stdout SEND Hello World n to stdout Everything sent to stdout will be displayed on the standard output usually your terminal Figure 1 4 is an example of a method that interacts with the user for input and then uses MELD functions CitiSaver Withdraw cash is a MELD function call that sends the message Withdraw cash to object CitiSaver and then waits for the result sent back from CitiSaver Since the return value will be assigned to balance and the following statement 2 depends on the value of balance by the rules of data
13. change and not directly upon receipt of a message Constraints are further discussed later on in this chapter and in chapter 6 It suffices to say that the constraint statement in our example is executed when the object World is created Let s continue with another program that does some input as well as output see 1 1 In this example we have a method with a string selector Hello The basic syntax for a method is selector gt statement The statement as you ll see later can be a block of statements and the selector can be a string or symbolic form A string selector method has simple semantics when a string message sent to an object matches a string selector the statement will be executed The string selector may contain a regular expression or scanf like arguments so matching and extracting arguments is very The grammar here is simplified for readability you can actually have objects and classes appear in any order See the full grammar for details MELD PROGRAMMING LANGUAGE 7 program FEATURE identifier list INTERFACE externals IMPLEMENTATION OBJECT object def CLASS LABEL object def METHODS method END CLASS identifier listy y END FEATURE identifier listy y Figure 1 2 Basic Syntax in MELD FEATURE Animal World INTERFACE IMPLEMENTATION OBJECT Polly Parrot Parrot CREATE CLASS Parrot METHODS Hello gt printf How are you n END CLASS Parrot END FEATURE Animal
14. dependency statement 2 will not be executed until the function call returns and balance is assigned its new value Next we show an example of using asynchronous sending to initiate another thread of control see figure 1 4 MELD PROGRAMMING LANGUAGE 12 FEATURE Bank INTERFACE IMPLEMENTATION OBJECT teller Teller Teller CREATE CitiSaver Savings Account Savings Account CREATE CLASS Teller METHODS withdraw d cash INTEGER gt balance INTEGER balance CitiSaver Withdraw cash 1 printf the balance is d n balance 2 ww END CLASS Teller CLASS Savings Account balance INTEGER 1000 METHODS Withdraw cash INTEGER gt balance balance cash RETURN balance END CLASS Savings Account END FEATURE Bank Figure 1 7 Interactive Bank Program In figure 1 4 statement 1 sends the message Withdraw cash to object CitiSaver Statement 2 may then run concurrently with statements 3 and 4 and the output could be either ordering of statements 2 and 4 Note that the teller does not know the new balance and in fact cannot tell when the withdrawal has been completed This situation could be avoided by having withdraw explicitly SEND a notification message back to its caller but in most cases where information must be returned to the caller synchronous message passing function call should be used as in the previous exampl
15. identifiers opt identifiers IDENTIFIER identifiers IDENTIFIER 7 4 1 Interface The interface clause consists of an optional declaration specifying the imported features exported classes and objects externals EXPORTS port listo pt IMPORTS port listopt externals externals The EXPORTS clause lists those classes defined in the implementation that may be used externally The IMPORTS clause lists those features whose exported classes may be used internally Import supports reuse through data abstraction and information hiding in the same fashion as Ada packages The imported classes may be used in the implementation body as 1 the types of instance variables 2 the types of global objects and 3 within a merges clause In the EXPORTS clause the port list IDENTIFIER denotes an exported class or global object When a class is exported the names within braces denote instance variables and methods that will be visible outside the defining feature In the IMPORTS clause the IDENTIFIER denotes a feature and names within braces denote classes that will be used inside the current feature port list z IDENTIFIER IDENTIFIER identifier listyy port list port list identifier list IDENTIFIER identifier list IDENTIFIER 7 4 2 Implementation The body of the IMPLEMENTATION part consists of global object definitions class declarations and merge clauses These components may be used in any order and repeated as many t
16. in MELD similar to a C subroutine but invoked with a message instead of a procedure call A method consists of a selector and statements to be executed when a message matching the selector is received selector gt statement selector result type gt statement A method without a selector is called a constraint and is defined in section 7 4 7 method z method attr constraint method attr sym selector selector type opt gt statement method attr yy str selector statement method attr y any selector statement method attr OVERRIDE DEFAULT INSIST OVERRIDE DEFAULT OVERRIDE INSIST selector type type specifier The method attr field defines the behavior of the method when used in a MERGES statement OVERRIDE On a method of the new class means you use only the methods described by the new class and not any of the other methods of the same name DEFAULT On a method of the merged classes means this method is the default action to be taken if no other non default method of the same name occurs in any of the other merged classes If two default methods occur they get merged MELD PROGRAMMING LANGUAGE 57 Not that DEFAULT on the new class only applies to classes that later MERGES new class INSIST On a method of the merged classes means the subclass cannot override it Because MELD is strongly typed you must specify a selector type that is a result datatype if the method returns
17. input parameters The input parameters should be the instance variables declared in the class Some examples are described below e Change type constraints might be used to define the relation between the internal data and the screen display Assume a point on the screen that is represented as X and Y in a class Whenever X or Y is changed the point will be moved to the proper location A C procedure constraint can be defined to simplify the program CLASS TerminalGraphics X y INTEGER METHODS Move x y Move X d INTEGER gt x x d Move Y d INTEGER gt y y d END CLASS TerminalGraphics where Move X Y is a C procedure constraint Move X and Move Y are methods defined in class Coor Move x y will move the point in the screen to thelocation x y e C procedure constraints might be used to display messages associated with some instance variables For example CLASS Bank money INTEGER METHODS printf the money is d n money Deposit x INTEGER gt money money x END CLASS Bank In sequential programming in order to trace the history of money printf statements would need to be inserted wherever the instance variable money is changed Here only one printf statement is needed 6 2 2 Asynchronous Send Constraints An asynchronous send constraint is a send statement that will send a message to an object and activate a method The syntax is the same as that de
18. necessary if this method uses the RETURN statement to return a value The possibilities for statement have been covered in chapter 3 and the class specific statements are described further along in this chapter The symbol gt is used to associate selector with statement When selector is received the statement is executed 4 3 1 Send Statement Meld provides an easy way to create another thread of control asynchronous sending Unlike the procedure call or synchronous message an asynchronous send does not delay the caller The message is sent and execution continues normally Any return value from the method called asynchronously is lost The syntax of asynchronous sending is SEND symbolic message TO object or SEND string message TO object where symbolic message is an identifier with a sequence of typed parameters object is an object variable string message is a string with a sequence of typed parameters A method will be activated when receiving a matched message The receiving method and the sending method will run in parallel as far as is possible due to data flow and other execution ordering constraints For example The statement X 1 and Y 2 will be executed in parallel or in either order on a uniprocessor 4 3 2 Synchronous Send The synchronous send is also known as the MELD function call Synchronous sending causes the sender process to stop and wait for the result from the receiver process The syntax of syn
19. resolved by associating the ELSE with the closest else less IF MELD PROGRAMMING LANGUAGE 23 3 3 3 Return Statement The return statement is used for a method to return a value The format is return statement RETURN expression 3 3 4 C Interface MELD provides an interface to the C language MELD programs can simply use C function calls or procedures by using the ordinary C function call syntax A C function call is legal wherever a MELD function call synchronous send is legal The parameter types in MELD programs will be converted as follows INTEGER int REAL float DOUBLE double STRING char BOOLEAN int CHARACTER char The current implementation of MELD does not do any checking on the types or number of arguments to C functions Our programs using printf throughout this manual are examples of an external C function call 3 4 Comments The format for a comment in MELD is comment 3 5 A Small Example The following example in figure 3 5 uses some of the language features covered in this chapter The example also uses a construct that will be explained fully in chapter 5 namely a group of statements enclosed in square brackets statements These statements will be executed sequentially one right after another the same as using BEGIN and END in Pascal or in C MELD PROGRAMMING LANGUAGE FEATURE HiLo HiLo Play the game of High Low Try to guess the computer s secret number
20. 21 3 Operators Expressions and Statements This chapter covers the expression and statement syntax of MELD This syntax is conveniently similar to C except that the assignment operator is instead of 3 1 Operator Summary MELD supports arithmetic relational and boolean operators 3 1 1 Arithmetic Operators In MELD there are three classes of arithmetic operators binary arithmetic operators and modulus operator which produce the remainder when the first operand is divided by the second operand unary operators the and signs for example 3 The precedence of these operators in MELD is the same as in the C language 3 1 2 Relational Operators The relational operators are gt Greater than gt Greater than or equal to lt Less than lt Less than or equal to or Equal to lt gt or Not equal to 3 1 3 Logical Operators OR or OR operator AND or amp AND operator XOR or Exclusive OR operator NOT or NOT operator 3 2 Expressions Expressions in MELD are identical to expressions in C 3 3 Statement Summary The statement in MELD is the unit of execution and concurrency Chapter 5 describes how to combine statements into blocks that execute their statements sequentially or in parallel Section 7 4 7 describes statements used as constraints within a class and chapter 4 describes those statements used for communicating among objects MELD PROGRAMM
21. D BEGIN objdecl part END SET OF type specifier KEY IDENTIFIER ORDERED SET OF type specifier KEY IDENTIFIER ORDERED direction BY IDENTIFIER 9 3 Views Views are not present assignment var VIEWS IS PROD IDENTIFIER var VIEWS IS PROD IDENTIFIER BOOLOP express 9 4 Union The notion of union Kaiser 87a Kaiser 89a is not present body unit union union GETS IDENTIFER IDENTIFER IDENTIFER 9 5 Data Dependency Bug BALANCE BALANCE CASH SEND the balance is d BALANCE TO STDOUT There is a bug in the data dependency analysis This example will output the old balance The proper behavior may be obtained by using printf rather than SEND to do the output i e printf the balance is d BALANCE 9 6 FEATURE name The FEATURE name may be a list of words but if you include a reserved word in that list such as AND you get a very obtuse error message Try naming a feature Savings and Loan and see what happens MELD PROGRAMMING LANGUAGE 68 9 7 Comments The comment characters are comment comment should also be allowed 9 8 Object Class There is no system defined type hierarchy and no Object class 9 9 Feature Names Feature names consist of an arbitrary string of identifiers but you cannot include a MELD reserved word in that list 9 10 Type checking Since there is no type checking on procedures lint should be called as part of the MELD script 9 11 Bug in string
22. E INSIST type specifier sym selector param list str selector regular exp IDENTIFIER param list MELD PROGRAMMING LANGUAGE 64 opt identifier list type specifier param list param list regular exp regular parm exp param list u STRING regular parm exp STRING any selector constraint equation constraint IDENTIFIER expression change constraint condition constraint df statement signed constant basic type type specifier block zm equation constraint change constraint condition constraint send statement function call constant constant constant INTEGER BOOLEAN CHAR STRING REAL DOUBLE basic type ARRAY INTEGER INTEGER OF basic type ARRAY INTEGER INTEGER OF IDENTIFIER IDENTIFIER BEGIN statements END statements statements statements MELD PROGRAMMING LANGUAGE 65 statement Statements H if statement c assignment asgn operator des function call delay statement return statement send statement symbolic message string message block opt assignment if statement object def function call send statement delay statement return statement statement statements statement IF expression THEN statement IF expression THEN statement ELSE statement variable asgn opera
23. END SET OF STRING THEN TO TRUE UNION USES VIEWS XOR Note because MELD generates C all C reserved words are also illegal as identifier names and will cause C compiler errors 7 3 4 Constants 7 3 4 1 System Constants There are several constants predefined by the MELD compiler TRUE FALSE NIL NULL OOoOH 7 3 4 2 Integer Constants An integer constant consists of a sequence of digits O through 9 and is always interpreted in base 10 INTEGER 1 9 0 9 0 Note this format is different from C which allows hexadecimal and octal integer constants MELD PROGRAMMING LANGUAGE 53 7 3 4 3 Floating Point Constants Floating point constants consist of an integer the integer part a decimal point and an integer the fraction part Both the integer part and the fraction part must be present FLOATP 0 9 0 9 Note this format differs from C which allows an exponent and either the integer or the fraction part but not both to be missing 7 3 4 4 Character Constants A character constant is a printable character surrounded by single quotation marks as in g CHARACTER n IIMO A176 This format only allows a single printable character not a backslash followed by a number the backslash form above is only used to describe the valid range of character inputs Note There is currently no facility for entering characters outside the printable range of 40 space to 176 tilde ascii This defici
24. ERSIST keyword remain in existence even when the MELD program terminates The next time the program or another is invoked all data from persistent objects may be restored Objects are stored and retrieved from the file MELD db in the currently connected directory This file should not be removed unless you want to forget class CREATE may also be used as a function call to create objects MELD PROGRAMMING LANGUAGE 26 the persistent objects To create and access persistent objects you first declare the class with the PERSIST attribute and then use class CREATE name to create and identify the name of the object MELD will look for name in the database and restore all class variables to the new object otherwise MELD creates a new object and initializes the class variables as determined by the class declaration Figure 4 2 1 is an example of using PERSIST in this way Any instance variables changed during the execution of a method of a persistent object will be written back to the database at the time the method finishes executing Note The current MELD compiler will not allow arguments to CREATE if CREATE is part of the object declaration Instead you need to use class CREATE in an assignment statement as part of a method or constraint Note There is no way to remove persistent objects aside from deleting the MELD db file DESTROY does not exits in the current implementation Note The name can be any ascii str
25. ING LANGUAGE 22 This section will provide you with an description of the basic statements in MELD assignment condition return and the interface to external C procedures 3 3 1 Assignment Statement The syntax of the assignment statement is assignment variable asgn operator expression where variable can be a global variable an instance variable or a local variable declared in a method The expression is the same as a C language expression and the asgn operator is defined as asgn operator ess aves l s amp gt l gt gt lt lt The assignment operators are identical to their C language counterparts except they use instead of Expressions such as i i 41 in which the left hand side is repeated can be written as i 1 where is an asgn operator The possible assignment operators are Simple assignment Addition Subtraction Multiplication f Division o Remainder gt gt Right shift lt lt Left shift amp Bitwise AND operator Bitwise XOR operator Bitwise OR operator 3 3 2 Conditional Statement The syntax of the IF THEN ELSE statement is if statement IF expression THEN statement IF expression THEN statement ELSE statement Where expression is a boolean expression and statement is a statement described in this chapter or a block statement described in chapter 5 As is normally the case any ambiguity is
26. Random Int Class Random Int Class CREATE RandomReal Random Real Class Random Real Class CREATE CLASS Random Int Class METHODS Get Random gt Random Count Random Count 1 return rand 0 END CLASS Random Int Class CLASS Random Real Class METHODS Get Random Random Count Random Count 1 return rand 0 1 0 END CLASS Random Real Class END FEATURE Random Feature Figure 4 6 Random External Example Note external features do not work with the current compiler as of 9 28 89 MELD PROGRAMMING LANGUAGE 35 FEATURE Do Random Things INTERFACE IMPORTS Random Feature Random Count RandomInt IMPLEMENTATION OBJECT Main Be Random Be Random CREATE CLASS Be Random METHODS random send The d random is d n RandomInt Get Random Random Count to stdout END CLASS Be Random END FEATURE Do Random Things Figure 4 7 Use Random External Example 4 6 Merges Statement The MERGES statement is used to create subclass of existing classes The syntax is merge decl MERGES identifier list AS IDENTIFIER Where identifier list is a list of existing classes and IDENTIFIER is a new class name Normally MERGES merges together the methods of all classes mentioned in the statement and produces a new class If two methods contain the same name the statements are merged into one method and executed in dataflow order It is possible however to s
27. S The MERGES clause is separate from any class declarations so that it is simple to define a class that is solely the combination of two or more other classes If an instance variable or a method with the same name is inherited from more than one MELD superclass MELD automatically merges the code in each method using dataflow dependencies 7 4 5 Class Declarations The class declaration is used to specify the methods procedures and instance variables private object storage of a class The declaration includes the optional keyword PERSIST described below the class name which may be any valid IDENTIFIER the object definitions for instance variables and the methods themselves MELD PROGRAMMING LANGUAGE 56 class decl PERSIST CLASS IDENTIFIER object def methods part END CLASS identifiers pt methods part METHODS method The PERSIST keyword is used to preserve any class objects across executions This is implemented with the aid of a file stored in the users connected directory see section 4 2 1 The object def defines types and optional initial values for instance variables Instance variables are accessible only by the methods of the class they are defined in Each object an instance of the class manages a private copy of these variables The methods part starts with the keyword METHODS and is followed by any number of method declarations and constraints 7 4 6 Methods Methods are the basic program unit
28. THE MELD PROGRAMMING LANGUAGE USER MANUAL Bill N Schilit Wen Wey Hseush Shyhtsun Felix Wu Steven S Popovich Technical Report CUCS 461 89 Columbia University Department of Computer Science New York NY 10027 28 September 1989 Copyright 1989 Bill Schilit Wen wey Hseush Shyhtsun Felix Wu and Steven S Popovich Research in Programming Systems is supported by National Science Foundation grants CCR 8858029 and CCR 8802741 by grants from AT amp T Citicorp IBM Siemens Sun and Xerox by the Center for Advanced Technology and by the Center for Telecommunications Research Schilit is supported in part by the Center for Advanced Technology Hseush Wu and Popovich are supported in part by the Center for Telecommunications Research MELD PROGRAMMING LANGUAGE 1 Notes to the Reader The Structure of this Manual This manual is divided into two parts a tutorial introduction to the MELD programming language and a language reference manual The index spans both parts A bibliography of MELD publications and technical reports is available in Appendix II If you are interested in the decisions leading to the current design you should refer to this literature the intent of this manual is to provide a working description of the current language and its implementation and not an exposition of the language issues Chapter 9 describes the language features which are not yet implemented A Note on Object Oriented Programming The pri
29. World Figure 1 3 Parrot Program flexible For example Deposit d cash INTEGER gt printf Deposited d n cash will extract the integer 10 from the string message Deposit 10 For further information on string selectors see section 4 4 2 To run this program you should type meld o hello 2 hello 2 m hello 2 Hello How are you This program acts a little like a parrot Whenever it hears Hello it responds with How are you MELD PROGRAMMING LANGUAGE 8 The program operates as follows The global object Polly is created during initialization Since no other action is possible the runtime system waits for user input When the user types a string followed by a return the runtime system matches the input line against all string selectors in all objects The runtime finds a match with a selector in object Polly and sends it the line of input If other string selectors matched then they too would get messages 1 2 Variables and Types You can define variables with basic types constructor types or user defined types that is classes Basic types include integer boolean char string real and double Types are described in Chapter 2 Here we only give a simple example of how to define instance variables using the basic type INTEGER FEATURE Money World INTERFACE IMPLEMENTATION OBJECT Ruckhouser Person Person CREATE CLASS Person Salary INTEGER 40000 METHODS you ge
30. a value The selector may be a symbolic string or a special form These are described below Note A string selector can only be called asynchronously with the SEND statement and a string selector method cannot return a value 7 4 6 1 Selectors When a message is received by an object the run time system searches for a matching selector and invokes the appropriate method The symbolic selector is designed for internal use by programs and resembles a procedure header sym selector IDENTIFIER param list Here IDENTIFIER is the selector identifier and the param list specifies the names and types of the formal parameters param list identifier list type specifier param list param list The string selector is intended for external use i e input from users of MELD programs When a string message is sent to an object it is matched against all of the object s string selectors and all matching selectors are invoked The string selector format is str selector regular exp regular parm exp param list A regular exp and regular parm exp are quoted strings containing a regular expression as described in the manual entry for ed 1 STRING regular exp regular parm exp 2 STRING A regular parm exp allows parameters through the use of x placed within the regular expression indicating the occurrence of parameters The implementation supports sd Integer Sf Real s String For example x is d x
31. aflow programming however is somewhat different from normal programming because the approach is declarative that is you specify the statements to be executed but not the order of their execution MELD employs the dataflow paradigm and MELD programmers may take full advantage of dataflow programming at the statement level or not if they choose MELD s dataflow does not presume a particular underlying machine architecture For further information on dataflow and dataflow programming languages see Wadge 85 Broy 85 Sharp 85 A Note on the Examples The examples in this manual show all MELD reserved words in CAPITALS and all other program components in Mixed case This is merely a convention case is not significant for MELD reserved words Case is however significant for variable and selector names The output examples in this manual show user typein in italics and the program output in bold The example programs are tested MELD programs available in the proj meld man examples directory Those examples marked Program Fragment are not complete programs MELD PROGRAMMING LANGUAGE I Tutorial MELD PROGRAMMING LANGUAGE MELD PROGRAMMING LANGUAGE 9 1 A Tutorial Introduction This chapter will give you a quick introduction to the MELD programming language Our aim is to show enough of the basic elements of MELD to write a few small programs 1 1 Getting Started How hard is it to write a Hello World prog
32. ait for a message that matches the selector name and then continues executing That is when a message is received by an object any methods in the current object that were using DELAYUNTIL on that method will continue their execution The DELAYUNTIL statement may be used for synchronization of two methods by for example the program fragment in figure 4 3 3 shows how to do this The selector COMPLETION does not need to be defined as a method in this example a method would be extraneous FOO gt I SEND COMPLETION TO SENDER BAR gt SEND FOO DELAYUNTIL COMPLETION Figure 4 4 DELAYUNTIL Example Fragment 4 4 Selectors There are two kinds of selectors that can be employed The symbolic selector is designed for internal use for messages between program components a symbolic selector looks much like a Pascal procedure header As we will see methods with symbolic selectors are used similarly to procedures in other languages 4 4 1 Symbolic Selectors The symbolic selector receives symbolic messages The format is sym selector z IDENTIFIER param list y Here IDENTIFIER is a name for the method and param list consists of the typed parameters MELD PROGRAMMING LANGUAGE 31 for example Withdraw cash INTEGER INTEGER gt Sort a ARRAY 1 20 OF INTEGER gt 4 4 2 String Selectors The string selector is intended for external uses of programs i e input When a string message
33. balance is d n balance END CLASS Saving Account END FEATURE Bank Figure 1 6 Bank Program and in fact all orderings of these three action equations are equivalent Regardless of ordering A will be equal to 12 after these three action equations are executed 1 4 Messages to Objects Our examples so far have used string messages and string selectors in the methods The runtime system as we saw sends lines of user input to objects with matching string selectors Another form of selector is called the symbolic selector which matches symbolic messages sent to an object The symbolic message and symbolic selector look very similar to a procedure call and a procedure header in Pascal Whereas string selectors are used for messages that interact with the user symbolic selectors are used for messages internal to the program Here is an example of a method using symbolic selectors Note that selectors look like procedure headers and you must have a type for each selector argument MELD PROGRAMMING LANGUAGE 11 CLASS PositiveInteger METHODS Add i j INTEGER gt return i j Sub i j INTEGER gt if i j gt 0 then return i j else return 0 END CLASS PositiveInteger You can invoke methods using a synchronous or asynchronous message The synchronous message looks like a normal C function call except it is preceded by a variable name and a dot The message is sent to the object represented
34. chronous sending is object message where object is an object identifier message can be only be a symbolic message For example figure 4 3 2 shows how to use the return statement and synchronous sends using SSELF Other examples are MELD PROGRAMMING LANGUAGE 29 Foo Obj Foo Foo CREATE Bar Obj Bar Bar CREATE CLASS Foo y INTEGER METHODS Foo SEND Bar TO Bar Obj y 2 END CLASS Foo CLASS Bar x INTEGER METHODS Bar gt x 1 END CLASS Bar Figure 4 2 Send Program Fragment FEATURE Bank INTERFACE IMPLEMENTATION OBJECT CitiSaver Savings Account Savings Account CREATE CLASS Savings Account balance INTEGER 0 METHODS Deposit d m INTEGER gt printf Your balance is now dWMn S SELF Deposit m Deposit a INTEGER INTEGER gt balance balance a RETURN balance END CLASS Saving Account END FEATURE Bank Figure 4 3 Return Statement MELD PROGRAMMING LANGUAGE 30 Bar Obj Bar Foo Obj Foo x y z The arguments of a function call can be any arbitrary expression 4 3 3 DelayUntil Statement The DELAYUNTIL statement is used to wait for a message to be received by the current method The syntax is DELAYUNTIL selector name where selector name is a symbolic selector that does not need to be defined in the current method see below example The statement causes the current thread of execution to w
35. e MELD PROGRAMMING LANGUAGE 13 FEATURE Bank INTERFACE IMPLEMENTATION OBJECT teller Teller Teller CREATE CitiSaver Savings Account Savings Account CREATE CLASS Teller METHODS withdraw d cash INTEGER gt SEND Withdraw cash TO CitiSaver 1 printf your transaction is being processed n 2 END CLASS TELLER CLASS Savings Account balance INTEGER 0 METHODS Withdraw cash INTEGER gt balance balance cash 3 printf the balance is d n balance 4 END CLASS Saving Account END FEATURE Bank Figure 1 8 Asynchronous Bank Program 1 5 Constraints We have already seen a simple example of a constraint statement in figure 1 1 A constraint statement differs from a method in that it has no selector Once a constraint statement is defined for an object it will be executed whenever the right hand side variables in the statement change and when the object is created Figure 1 5 shows an example of using a constraint statement in a class We have a constraint specifying the equivalent temperature in degrees Fahrenheit F for one given in degrees Celsius C This constraint causes F to be set to a new value immediately following any change to C keeping the Fahrenheit temperature consistent with the Celsius temperature It does not however cause C to respond to changes in F In general a constraint defining a relation between i
36. ect are running concurrently the total effect of execution order is determined by interleaving the two partial orderings according to the rules of data dependency 5 4 7 Data Dependency Deadlock Dynamic data dependency sometimes causes a deadlock situation due to the circularity of the effect of statements That is a circular chain of depends and effects exists and no statement is ready to run This situation is considered a fatal error and the MELD runtime will detect and print an error message when a deadlock occurs MELD PROGRAMMING LANGUAGE 43 Figure 5 4 7 is an example of a deadlock situation If the messages FOO and BAR arrive at the object at the same time the execution order of statements cannot be determined because of the circularity 1 gt 2 gt 3 gt 4 gt 1 CLASS Foo a b c d INTEGER METHODS FOO gt a b 1 1 Goss a aa 2 BAR gt d c 2 3 b d 4 4 END CLASS FOO Figure 5 4 Deadlock Example Fragment 5 4 8 Seventh Rule of Data Dependency Recursion Rule There is no data dependency between two activations of the same method because of the possibility of deadlock the methods are treated separately 5 4 9 Race Condition A race condition is the possibility for some unpredictable outcome to occur as the result of two or more interacting concurrent processes The example in figure 5 4 7 would have caused a deadlock if both
37. ency exists in the tokenizer meld 1 that only accepts 40 176 i e one character in quote marks as a character constant However it should be possible to allow the fuller C syntax for character constants since MELD generates C code 7 3 5 Strings A STRING is a sequence of characters starting and ending with quotation marks and possibly including backslash quoted quote marks as in C 7 3 6 Embedded C Code The characters start a block that must be ended with All text between and is sent directly to the generated C file and therefore you may include arbitrary C code between the delimiters 7 4 Meld Programs A MELD program is a named feature i e modular unit or package consisting of name interface and implementation A feature usually consists of related classes bundled together to provide a coherent functionality The feature name may be any arbitrary string of IDENTIFIERS for example Double Pane Window Manager The feature may also end with some arbitrary string of IDENTIFIERS but not necessarily the same string Note You cannot use a MELD reserved word such as AND in the IDENTIFIERS string This is a bug The INTERFACE part is used to declare any imported or exported classes and variables the MELD PROGRAMMING LANGUAGE 54 IMPLEMENTATION part consists of the storage and class declarations program FEATURE identifiers INTERFACE externals pt IMPLEMENTATION body END FEATURE
38. ent 2 and between statement 1 and statement 3 is not restricted and hence nondeterministic from the MELD PROGRAMMING LANGUAGE 39 programmers point of view That is 1 and 2 are concurrent and so are 1 and 3 The partial ordering below indicates how two threads of control might be used to execute the three statements 3x Ses 2 HS sez Bou A 5 2 Sequential Block A sequential block is a set of statements enclosed by The syntax is statements where statements can be any number of statements including parallel sequential or atomic blocks Sequential blocks are appropriate when ordering of the statements must be one after another ReadFile gt open x read x close Here the programmer requires the file to be opened first then read then closed By enclosing these statements in the block will be executed sequentially Note ReadFile open close and read x are not parts of the MELD language or runtime they are made up just for this example 5 3 Atomic Block An atomic block is a set of statements enclosed by The syntax is statements where statements can be any number of statements including parallel sequential or atomic blocks The statements are run in dataflow order when the block is activated and the whole block is treated as a unit The statements enclosed by the atomic block are executed without interleaving with the statements outside the bloc
39. ent ELSE statement The expression is evaluated and if it is non zero then statement is executed If ELSE is used then statement is executed only when the expression evaluates to FALSE 0 As is normally the case any ambiguity is resolved by associating the ELSE with the closest else less IF 7 4 9 4 Procedure Call The procedure call is a synchronous message to a MELD object when the dotted form is used or a subroutine call to an external C subroutine In the dotted form object is an instance global or local variable or system defined object name such as SENDER containing the object that will receive the message and symbolic message consists of the method name and parameters procedure z object symbolic message symbolic message Note a string message is not allowed in this form use the SEND statement When calling C procedures the parameter types are converted as follows MELD C INTEGER int REAL float STRING char BOOLEAN int CHAR char DOUBLE double MELD PROGRAMMING LANGUAGE 61 No type checking is performed on the arguments for external procedure calls 7 4 9 5 Send Statement The SEND statement sends a message to an object and immediately continues execution i e the message is sent asynchronously send statement SEND symbolic message TO variable SEND string selector TO variable The symbolic message is a selector identifier with a sequence of parameters symbolic message IDENTIFIER ex
40. executed only once when debug changes to FALSE Only one of these two constraints is active at any time The value for balance will be printed out when MELD PROGRAMMING LANGUAGE 48 debug is set to TRUE and from then one when balance is changed or the message debug disabled will be printed out when debug changes from TRUE to FALSE 6 4 Limitations Some limitations for using constraints 1 There is no block structure Except for the blocks associated with if then else constraints no block parallel block sequential block etc is allowed in constraints 2 Only parallel blocks can be used to enclose if set statements and else set statements in conditional constraints 3 The implementation disallows synchronous sends in constraints 6 5 A Small Example Figure 6 5 shows an example program using the three types of constraints discussed in this chapter FEATURE Bank INTERFACE IMPLEMENTATION OBJECT CitiSaver Savings Account Savings Account CREATE CLASS Savings Account cash flow INTEGER 0 balance INTEGER 0 debug BOOLEAN FALSE METHODS if debug then printf balance is now d n balance else printf debugging is disabled n Dd ebug s s STRING gt debug strcmp s on Deposit d cash INTEGER gt balance balance cash END CLASS Saving Account END FEATURE Bank Figure 6 2 Constraint Example MELD PROGRAMMING LANGUAGE II Re
41. ference Manual 49 MELD PROGRAMMING LANGUAGE 50 MELD PROGRAMMING LANGUAGE 51 7 MELD Reference Manual 7 1 Introduction The MELD language is an active research project at Columbia This manual tries to provide a reference point for questions and issues arising from the use of the language however the final arbiter is the source code and your own test programs In particular you can look at the compiler and preprocessor source code in the project directories proj meld baseline compiler and proj meld baseline preproc The implementation of MELD generates C code which is then compiled into an executable program Restrictions of the C language and compiler may therefore affect your program For example using the C reserved word void as a MELD identifier will produce a C compilation error but not a MELD compilation error This manual does not itemize all such restrictions so the MELD programmer must remain aware of the inherent limitations of the compilation process This manual describes the MELD programming language as of June 1989 7 2 Syntax Notation The syntactic notation used in this manual is a modified BNF form Syntactic categories nonterminals are presented in italics for example expression The subscript opt means the symbol is optional For example remote list remote list opt remote object decl denotes a repetition of one or more remote object decl Zero or more repetitions of a symbol is represented with a s
42. fined in a method SEND message TO object Here message can be either a symbolic message or a string message and object is an object identifier One example is to use a send constraint to self initialize an object MELD PROGRAMMING LANGUAGE 47 CLASS sort METHODS SEND Randomize to SSELF Randomize srand 0 END CLASS Sort As we can see the message Radomize in the constraint has no input parameter so that the constraint will be evaluated exactly once when the object is created In some cases an object wants to notify other objects when some instance variables are changed CLASS Bank money INTEGER METHODS SEND Display money TO Display Object Deposit x INTEGER gt money money x END CLASS Bank CLASS Display Display x INTEGER printf d n x END CLASS Display 6 3 Conditional Constraints The syntax is the same as the conditional statement used in methods see section 3 3 2 There are two sets of constraints associated with a conditional constraint if set and else set Only one set of statements is considered active at any time For example CLASS Foo balance INTEGER debug BOOLEAN METHODS IF debug THEN printf Balance is now d n balance ELSE printf debug disabled n The change type constraint printf Balance is now d n balance is active when debug is TRUE The constraint printf debug disabled n is active when debug is FALSE and is
43. fines the interface of a feature See section 4 5 The IMPLEMENTATION part of a MELD program contains the storage and code declarations You define and initialize global object variables in the OBJECT section and specify the behavior for each class of objects in the CLASS section The basic syntax of MELD is illustrated in Figure 1 2 the full grammar is presented in 8 Note that there are two types of object declarations Declarations directly after the keyword OBJECT denote global variables and declarations inside the CLASS part define variables local to the object i e instance variables In this example the global variable Main is initialized to a new instance of the World class using CREATE This is a very useful sort of initialization and most often is the one you want to use If no initialization is done on a variable its value will start out as the null object nil and messages sent to it will not do what you expect they will not do anything This common bug can be avoided by making sure that every variable is initialized in its declaration The executable code in a MELD program is contained in the METHODS part of a class definition In this case we have a special type of method called a constraint that can be identified by the fact that it has no selector Selectors are like templates for matching incoming messages A constraint statement has no selector because it executes when an object is created or when an object s variables
44. ger MD but until then remember this example 1 6 Method Interleaving MELD is a concurrent programming language calls to two or more methods of an object may be active simultaneously Here we show an example of what can happen when two methods are activated concurrently MELD PROGRAMMING LANGUAGE 15 FEATURE Bank INTERFACE IMPLEMENTATION OBJECT teller Teller Teller CREATE SuperSaver Savings Account Savings Account CREATE CLASS Teller METHODS withdraw d cash INTEGER gt SEND Withdraw cash TO SuperSaver printf your transaction is being processed n END CLASS TELLER CLASS Savings Account balance INTEGER 0 METHODS constraint printf the balance is d n balance Withdraw cash INTEGER gt balance balance cash 3 printf the balance is d n balance 4 END CLASS Saving Account END FEATURE Bank Figure 1 10 Bank Program with Debug Constraint MELD PROGRAMMING LANGUAGE 16 printf a d A 1 printf b d B 2 j printf c d C 3 printf d d D 4 FOO X INTEGER gt A X 1 5 C B 1 6 BAR X INTEGER gt B X A 7 D C 1 8 If method FOO and method BAR are called at the same time the statements in the methods will interleave according to the rules of data dependency In this example the
45. gramming 2 Deadlock 42 Debugging 14 Declarations 17 Declarations class 55 Declations object 55 Delay statement 30 61 Equation constraints 45 58 Examples a note on 2 Expressions 21 Floating point constants 53 Function call 11 28 Identifier 52 Inheritance 35 Instances 25 74 MELD PROGRAMMING LANGUAGE Integers 52 Interface clause 54 Keywords 52 Lexical conventions 51 Literals 17 18 Looping statements 67 Merge decl 55 Merges statement 35 Message 11 Messages 10 Method interleaving 14 Methods 28 56 Named objects 26 Naming Service 26 NOT 21 Object declation 55 Object Oriented Programming Languages 1 OOPLS 1 Operators 21 OR 21 Parallel block 8 37 38 59 Persistent classes 25 Procedure call 60 Race condition 43 Race conditions 16 Remote Objects 26 Return statement 23 Selectors 30 56 57 Selectors special 32 Selectors string 31 Selectors symbolic 30 Send statement 28 61 Send synchronous 28 Separate compilation 32 Sequential block 37 39 59 Statements 21 String selectors 31 Strings 53 Symbolic selectors 30 Synchronous message 11 Synchronous send 28 Syntax 51 System constants 52 System variables 61 Type specifier 58 Types 8 17 Types compound 18 Unimplemented features 67 Variables 8 17 19 Variables system 61 Variables system defined 20 Views 67 comment 23 21 75 MELD PROGRAMMING LANGUAGE 76 MELD PROGRAMMING LANGUAGE Table of Conte
46. hat is create an instance as you would create a new record structure in Pascal given its template The method we have seen for instantiating a class is by using CREATE in a variable declaration OBJECT mazda Car Car CREATE This creates a new Car object called mazda We say that mazda is an instance of the class Car An object has its own private storage known as instance variables Once an object has been created you can invoke the methods defined by the class Using the SEND statement or the MELD function call you send messages to an object and invoke one of its methods The methods themselves may invoke other methods using SEND and the MELD function call or they may use external C procedures or the simple MELD statements described in chapter 3 4 2 Classes and Instances A MELD class definition is class decl PERSIST object def methods part yy END CLASS identifiers pt CLASS IDENTIFIER The data of class objects may persist across program executions by using the PERSIST keyword see the next section for details The object def section is used to declare instance variables and the methods part is used to declare functions and constraints A single IDENTIFIER is used to name the class and an optional list of identifiers is allowed for the END CLASS these identifiers do not need to match For information on how to declare objects see section 2 1 4 2 1 Persistent Classes Objects of a class declared with the P
47. he same input string Withdraw f from account d account INTEGER amount REAL gt Withdraw f from account d amount REAL account INTEGER gt MELD PROGRAMMING LANGUAGE 32 4 4 3 Line Oriented Matching After an object receives a message it checks whether the message matches any string selectors The matching is line by line where a line ends with newline n So each line of the message will be compared with the string selectors For example incoming message my name is Ishmael string selector my name is s name STRING The incoming message will activate the method and the variable will be bound to Ishmael Another example shows a non matching case coming message They call me Ishmael string selector my name is s name STRING The input doesn t match the string selector even though part of the input does match the string selector In order for input to match a string selector it must match from the beginning of the line 4 4 4 More Than One Matching Since two string selectors can describe two overlapping sets of strings incoming messages may match more than one selector That is more than one method might be activated by one message The activated methods are not ordered in any way and may be executed in parallel For example coming message Open the cargo doors HAL string selector Open the s name STRING gt statement string selector Open gt statement
48. iables in C Of course you can also create instances during the execution of a method Methods are invoked by sending messages to objects instead of the normal notion of a subroutine call Messages are employed because the binding that is the decision of what code gets executed occurs at runtime for many OOPLS It is this runtime matching of message to method that gives OOPLS their power because if a message does not match any method selector also called protocol then the message is forwarded to the parent class superclass In our example the message turn right is understood by Pinto even though we never wrote the turn method because turn would be defined by the superclass of Pinto namely car In this way an object inherits the actions of the more general superclass Because of inheritance the programmer only needs to specify how MELD determines what code to run at compile time MELD PROGRAMMING LANGUAGE 2 some desired action differs from the superclass action thereby making software reuse a natural part of the paradigm A Note on Dataflow Programming Data flow programming is a paradigm useful for achieving a high degree of parallel execution The concept behind dataflow is simple statements instructions subroutines etc may be executed as soon as their inputs become available Dataflow can take advantage of architectural parallelism since at any given time a number of statements may be available for execution Dat
49. imes as desired body class decl merge decl object decl body body MELD PROGRAMMING LANGUAGE 55 7 4 3 Object Declaration The object decl is used to define and initialize global object variables These objects are accessible to all methods in the feature but in order to be used by other features they must be included in the EXPORTS statement Objects may be initialized to a constant value or via a method such as CREATE If no initializer is specified then the value will be set to the nil object The object declaration portion begins with the keyword OBJECT and then any number of object definitions of the form object type initializer A list of objects of the same type may be declared in one statement object decl OBJECT object def object def identifier list type specifier identifier list type specifier signed constant identifier list type specifier identifier CREATE A signed constant is defined in section 7 4 10 The identifier in identifer CREATE must be a class name 7 4 4 Merges The merges decl clause provides the inheritance mechanism for MELD Merging permits objects to inherit instance variables and methods defined in the imported features The syntax for the merges clause is merge decl MERGES identifier list AS IDENTIFIER The merges clause combines the classes and instance variables of any number of features into a single new class the class named after the A
50. ing however don t use in the name 4 2 2 Remote Objects Remote objects are named objects that are registered through a network wide Naming Service When you send a message to a remote object the message is routed to the machine containing the object then the object executes the message and if a synchronous MELD function call with a return value was used the result is sent back When you use class CREATE name the following steps take place 1 The local database is searched for a persistent object of the same name If one is found it is returned 2 The Naming Service is asked to locate remote objects of the same class and name on the network The Naming Service will not locate the object unless it or a member of the same class is being used in a running MELD program If an object is found it is returned 3 If steps 1 and 2 are not able to provide an object a new object is created and given the specified name The name is also registered with the Naming Service for future accesses If you require that the object already exist you can use class GET name This is similar to CREATE described above except for the last step GET will never create a new object Instead when GET is unable to locate a named object it returns a place holder object that will re lookup and try to locate the named object at the time of use If lookup is still unsuccessful at time of use the message is discarded A similar result occu
51. k For example The execution order is 3 gt 1 gt 2 gt 4 Since statement 1 and statement 2 are in a atomic block no other statements can be executed between them In the next chapter we will show the use of constraints in MELD programming It is sometimes very important to use atomic blocks to avoid premature activation of constraints For example a constraint is used to automatically move the point X Y on the screen when the internal data X or Y are changed MELD PROGRAMMING LANGUAGE 40 CLASS Foo a b c d INTEGER METHODS Foo gt a ij box 1 1 Ge deb ts 2 qe 3 SEND A is d n a TO stdout 4 END CLASS Foo Figure 5 2 Atomic Block Program Fragment CLASS Move X y INTEGER METHODS constraint to move the point whenever x or y changes Move Point x y MOVE dx xy INTEGER gt X X dx 1 y i y dy d 2 gt END CLASS Move Without using a atomic block the move point constraint would be activated twice The first time is when x is changed in statement 1 and the second is when y is changed in statement 2 The point on the screen will be moved horizontally and then be moved vertically Using a atomic block the constraint is activated only once 5 4 The Rules of Data Dependency 5 4 1 First Rule of Data Dependency Assignment Rule For two different assignment statements if the lef
52. lared class name The initializer assigns the value to each element in the var list so for example Bob Carol Ted Alice People People CREATE will create four objects and BobAge CarolAge INTEGER 24 will assign 24 to both BobAge and CarolAge You would use two declaration statements in order to initialize these variables to different values Depending on its placement in the MELD program the declaration statement can be used to define global variables instance variables or local variables Global variables are accessible by all classes in the feature and if exported they are accessible to other features as well Instance variables are private to each object When an object is created it gets a private copy of the instance storage And local variables are private to each method like local variables in a subroutine their value is not preserved across calls The three types of variables are shown in the example of figure 2 1 This example shows the use of a global variable FDIC Limit that defines the limit of insurance per savings account offered by the Federal Deposit Insurance Corporation Clearly we want this constant to be the same for all Account objects so it is defined in the OBJECT section and made global The MELD message CREATE is useful for initializing variables see section 4 1 for more information 2 2 Types There are six basic types in MELD Integer an integer typically reflecting the natural size of intege
53. mary programming paradigm used in MELD is object oriented We assume the reader is familiar with the concept of object oriented programming and if not urge them to refer to the Smalltalk 80 Goldberg 85 and C books Stroustrup 86 or the articles on Flavors Keene 85 Moon 86 which are common examples of the paradigm A brief review of the terminology used in OOPLS Object Oriented Programming Languages is in order Object oriented programming is an approach to software development involving the use of objects a notion similar to abstract data types Both are ways of encapsulating actions however objects usually refer to the notions of messages class hierarchy and inheritance as well An object has private procedures called methods private storage called instance variables and a public interface to the procedures sometimes called the protocol and in MELD this is called the selector and parameters Objects tie together procedures and data to form a functionality that is accessible only through an interface When programming in an OOPL you must create a description called a class for the object usually by specializing i e adding special case code to a more general class For example you would define class Pinto to be a specialization of Car with the difference that the gas tank of a Pinto explodes on a rear end collisions of more than 45 M P H You then declare instantiations instances of the object as you would declare var
54. messages arrived at exactly the same time However if message FOO arrives slightly earlier than message BAR then statement 1 might have gotten executed before dynamic data dependency is applied and if BAR arrives slightly earlier statement 3 will be executed first These cases lead to three different results solely because of the timing of messages therefore a race condition exists Race conditions are sometimes difficult to avoid you may need to resort to atomic or sequential blocks to control the interaction of concurrent methods MELD PROGRAMMING LANGUAGE 44 MELD PROGRAMMING LANGUAGE 45 6 Constraints As we have seen already statements defined in a class can be associated with a selector as part of a method or they can be constraints without a selector In this chapter we will further explore the constraint statements and their uses A constraint defines a relation between instance variables in the class The relation will hold true through the life time of the object Technically speaking constraints are declarative with the highest priority of execution so the constraints will be evaluated whenever the associated input instance variables are changed There are three types of constraints equation constraints change type constraints and conditional constraints 6 1 Equation Constraints An equation constraint defines an equation relation between a set of input instance variables and one output instance variable During
55. nstance variables in a class will be executed whenever a variable on the right hand side changes but not when a variable on the left hand side MELD PROGRAMMING LANGUAGE 14 FEATURE Weather World INTERFACE IMPLEMENTATION OBJECT thermometer FConverter FConverter CREATE CLASS FConverter F C REAL 0 METHODS F 32 C 9 0 5 0 constraint Set S x REAL gt C x F printf F fWMn F END CLASS FConverter END FEATURE Weather World Figure 1 9 Weather Program changes A two way constraint may easily be specified by giving two constraints one for each direction For example the two constraints F 32 C 9 0 5 0 constraint C F 32 5 0 9 0 inverse constraint together specify that whenever either the Fahrenheit or Celsius temperature changes the other one should be updated in a consistent manner Our next example see figure 1 5 shows a useful debugging feature how a constraint can be used to display all changes to a variable as they happen It shows a modification of our banking example to display the balance every time it changes Constraints are declarative In order to display the balance whenever and wherever changed the programmer need only declare one constraint In the absence of any effective sort of debugger for MELD constraints are the only way available to trace instance variables when debugging Eventually MELD will have a debug
56. nts Notes to the Reader I Tutorial 1 A Tutorial Introduction 1 1 Getting Started 1 2 Variables and Types 1 3 Action Equations and Data Dependency 1 4 Messages to Objects 1 5 Constraints 1 6 Method Interleaving 2 Declarations Variables Literals and Types 2 1 Declarations 2 2 Types 2 2 1 Compound Types 2 3 Constants Literals 2 4 Variables 2 4 1 System Variables 3 Operators Expressions and Statements 3 1 Operator Summary 3 1 1 Arithmetic Operators 3 1 2 Relational Operators 3 1 3 Logical Operators 3 2 Expressions 3 3 Statement Summary 3 3 1 Assignment Statement 3 3 2 Conditional Statement 3 3 3 Return Statement 3 3 4 C Interface 3 4 Comments 3 5 A Small Example 4 Classes Objects Methods and Messages 4 1 Overview 4 2 Classes and Instances 4 2 1 Persistent Classes 4 2 2 Remote Objects 4 3 Methods 4 3 1 Send Statement 4 3 2 Synchronous Send 4 3 3 DelayUntil Statement 4 4 Selectors 4 4 1 Symbolic Selectors 4 4 2 String Selectors 4 4 3 Line Oriented Matching 4 4 4 More Than One Matching 4 4 5 Special Selectors 4 5 External Methods and Variables 4 6 Merges Statement 5 Block Structure 5 1 Parallel Block 5 1 1 Partial Ordering of Data Dependency 52 Sequential Block 5 3 Atomic Block coc uU Un W E 10 14 17 17 17 18 18 19 20 21 21 21 21 21 21 21 22 22 23 23 23 23 25 25 25 25 26 28 28 28 30 30 30 31 32 32
57. ors Object oriented Programming on Symbolics Computers In Common Lisp Conference December 1985 David A Moon Object Oriented Programming with Flavors In Object Oriented Systems Languages and Applications Conference pages 1 8 Portland OR September 1986 Special issue of S GPlan Notices 21 11 November 1986 Steven S Popovich and Gail E Kaiser MELDing Parallel and Distributed Programming Technical Report CUCS 402 88 Columbia University Department of Computer Science October 1988 J A Sharp Data flow computing Halsted Press New York 1985 Bjarne Stroustrup The C programming Language Addison Wesley Reading Mass 1986 William W Wadge and Edward A Ashcroft Lucid the dataflow programming language Academic Press New York 1985 gt 21 gt 21 CLASS 56 EXPORTS 32 54 FEATURE 54 any selector 32 58 asgn operator 22 60 assignment 60 basic type 59 block 37 59 body 54 change constraint 58 class decl 56 condition constraint 58 constant 62 constraint 58 equation constraint 58 exp list 61 externals 54 IDENTIFIER 52 identifier list 54 identifiers 54 if statement 60 method 56 methods part 56 object decl 55 object def 55 param list 57 port list 54 procedure 60 program 7 54 regular exp 57 regular parm exp 57 return statement 61 selector 6 selector type 56 send statement 61 signed constant 62 statement 37 59 statements 37 59 str selector 57 string message 61 sym selecto
58. ount balance INTEGER Il eo METHODS printf Your balance is d n balance deposit d cash INTEGER gt balance balance cash if balance FDIC Limit then printf careful d is uninsured Mn balance FDIC Limit END CLASS Account END FEATURE SavingsAndLoan Figure 2 2 Global Variables MELD language defines a number of system constants for your use TRUE 1 FALSE 0 NIL 0 NULL 0 2 4 Variables A variable name starts with a character of the alphabet and continues with any number of alphanumeric or underscore characters Case is significant in variable names You may not use a variable name that conflicts with one of the MELD reserved words see 7 3 3 or one of the C language reserved words refer to your local C compiler reference manual PThese constants may not be redefined MELD PROGRAMMING LANGUAGE 20 2 4 1 System Variables System variables are distinguished by a dollar sign as the first character in their names These variables are generally read only unless specified otherwise and include SSELF Refers to the object that is executing the current statement SSENDER Refers to the object that sent the current message SSELECTOR The entire string message that invoked this method For symbolic selectors SSELECTOR holds the selector name as a string When used with the selector you may inspect SSELECTOR to see what the message received was MELD PROGRAMMING LANGUAGE
59. p list opt exp list expression exp list exp list The string message is a string with a sequence of optional parameters string message STRING STRING exp list 7 4 9 6 System Variables Sender Self and Selector Two system defined variables are provided for referencing the object itself SSELF and the message sender SSENDER These variables cannot be used on the left hand side of assignment statements SSELECTOR The entire string message that invoked this method For symbolic selectors SELECTOR holds the selector name as a string When used with the selector you may inspect SSELECTOR to see what the message received was 7 4 9 7 Delay Statement The DELAYUNTIL statement is used to wait for a message to be received by the current method The syntax is DELAYUNTIL selector name where selector name is a symbolic selector that does not need to be defined in the current method The statement causes the current thread of execution to wait for a message that matches the selector name and then continues executing That is when a message is received by an object any methods in the current object that were using DELAYUNTIL on that method will continue their execution 7 4 9 8 Return Statement A method returns a result by means of the return statement return statement RETURN expression MELD PROGRAMMING LANGUAGE 62 7 4 10 Constant A constant in MELD consists of a user specified number string or charac
60. pecify a combination of three attributes in a class definition to customize the MERGES behavior These attributes are OVERRIDE On a method of the new class means MELD uses only the methods described by the new class and not any of the other methods of the same name DEFAULT On a method of the merged classes means this method is the default action to be taken if no other non default method of the same name occurs in any of the other merged classes If two default methods occur they get merged Note that DEFAULT on the new class only applies to classes that later MERGES new class INSIST On a method of the merged classes means the subclass cannot override it MELD PROGRAMMING LANGUAGE 36 MELD PROGRAMMING LANGUAGE 37 5 Block Structure There are three types of blocks in MELD parallel sequential and atomic The block type defines the runtime behavior of the statements within the block Statements within a parallel block will be potentially executed in parallel according to the partial ordering of data dependency A sequential block will execute statements sequentially according to their textual ordering Statements within an atomic block are executed without interleaving with statements outside the block by default they are executed in parallel or in data flow order but atomic and sequential blocks may be combined to give sequential execution without interleaving with other statements The MELD grammar defines a block as
61. r 57 symbolic message 61 type specifier 59 IMPLEMENTATION 54 IMPORTS 32 54 INTERFACE 54 MELD program 53 MELD how to execute 5 MERGES 55 OBJECT 55 PERSIST 56 SELECTOR 20 61 SSELF 20 61 SSENDER 20 61 MELD PROGRAMMING LANGUAGE 73 MELD PROGRAMMING LANGUAGE CREATE and named objects 26 GET and named objects 26 IMPLEMENTATION 54 Action equations 8 AND 21 Array 18 Assignment statement 22 60 Asynchronous send constraints 46 Atomic block 37 39 59 Block 59 Block atomic 37 39 Block parallel 37 38 Block sequential 37 39 C code embedded 53 Cinterface 23 C procedure constraints 46 Change type constraints 58 Change type constraints 45 Character constant 53 Classes 25 Comments 23 51 Compilation separate 32 Compound types 18 Conditional constraints 47 58 Conditional statement 22 60 Constant 62 Constant character 53 Constant integer 52 Constants 18 52 Constants floating point 53 Constants string 53 Constants system 52 Constraint 6 Constraints 13 45 58 Constraints asynchronous send type 46 Constraints C procedure type 46 Constraints change type 58 Constraints change type 45 Constraints conditional 58 Constraints conditional type 47 Constraints equation 58 Constraints equation type 45 Constraints limitations 48 Create 25 Data dependency 8 38 40 41 Data dependency and constraints 42 Data dependency and deadlock 42 Data dependency and if then else 41 Dataflow pro
62. ram in MELD Not very Here is an example FEATURE World INTERFACE IMPLEMENTATION OBJECT world Main Main CREATE CLASS Main METHODS printf Hello World Nn END CLASS Main END FEATURE World Figure 1 1 Hello World Program First put this program into the file hello m in your working directory or copy the file hello m from the examples directory The compiler is run from a shell script meld that is stored in the proj meld bin directory You will either need to use the full proj meld bin meld name when invoking the compiler or you may prefer to add the MELD bin directory to your shell PATH variable To compile and then run with input hello m and executable output specified as hello you would type meld o hello hello m hello The program would then output Hello World 2Examples are in proj meld man examples 5We use italics to denote user typein and bold to denote program output MELD PROGRAMMING LANGUAGE 6 From this example you can see that a MELD program begins with a FEATURE name and consists of an INTERFACE and IMPLEMENTATION part MELD FEATURE is a unit of reusability similar to an Ada package that is a collection of private storage and class types with a well defined interface The INTERFACE portion of a MELD program is also a mechanism for reusability It is employed when a number of files actually FEATURE are being used together and as you might expect it de
63. re is only one valid execution ordering 5 gt 1 gt 7 gt 2 gt 6 gt 3 gt 8 gt 4 This ordering occurs because data dependencies force 5 to be executed before 7 7 before 6 and 6 before 8 giving us a partial execution path before constraints are considered of 5 gt 7 gt 6 gt 8 Then since constraints are executed as soon as possible after their right hand side variables change the complete execution path will be as given above Race condition may occur because of the timing of message passing For example if the message for FOO arrives earlier than the message for BAR then FOO might start executing and the interleaving of the statements may cause adverse effects MELD PROGRAMMING LANGUAGE 17 2 Declarations Variables Literals and Types MELD is a strongly typed language you must declare all of your variables and the compiler will enforce certain rules in their use Some basic variable types are offered by MELD and of course using classes MELD programmers can create new types for themselves In this chapter we talk about what types are offered by MELD how to declare them and how to use them 2 1 Declarations A declaration in MELD consists of one or more variable names a colon a type specifier and an optional initializer object def var list type specifier var list type specifier signed constant var list type specifier LABEL CREATE where LABEL is a dec
64. rs on the host machine MELD PROGRAMMING LANGUAGE FEATURE VarExample INTERFACE IMPLEMENTATION OBJECT Main V Test V Test CREATE global INTEGER 1 CLASS V Test instance INTEGER 2 METHODS show gt local INTEGER 3 printf d d d n global instance local END CLASS V Test END FEATURE VarExample Real a single precision floating point number Double a double precision floating point number Boolean True 1 or False 0 Character one byte machine dependent but usually ASCII String a one dimensional array of characters Figure 2 1 Variables in a Program The declarations Real and Double will reflect their types natural size on the host machine 2 2 1 Compound Types In addition you can define a type using the following constructors ARRAY INTEGER INTEGER OF basic type ARRAY INTEGER INTEGER OF LABEL Here basic type is one of the types defined above Note initialization does not work for arrays and there is no range checking on the indices 2 3 Constants Literals String character integer and floating point constants may be used in your MELD program The format is similar to the C language format and is described in section 7 3 4 In addition the MELD PROGRAMMING LANGUAGE 19 FEATURE SavingsAndLoan INTERFACE IMPLEMENTATION OBJECT FDIC Limit INTEGER 200000 Savings Account Account CREATE CLASS Acc
65. rs when the server system containing MELD PROGRAMMING LANGUAGE 27 FEATURE Linger Linger Example of how class storage can persist INTERFACE IMPLEMENTATION OBJECT Main UserInterface UserInterface CREATE PERSIST CLASS LingerClass val INTEGER 1 METHODS set n INTEGER val n show SEND The value is d n val TO stdout END CLASS LingerClass CLASS UserInterface LingerOne LingerClass METHODS send init to SELF init gt LingerOne LingerClass create One Show LingerOne show Set d n INTEGER gt LingerOne set n END CLASS UserInterface END FEATURE Linger Figure 4 1 Persistent Class a named object crashes or the MELD program containing a named object exits One technique for discerning whether a named object was actually located by GET is to call a simple method created just for this purpose that returns a non nil value something like you there Note The name can be any ascii string however don t use in the name MELD PROGRAMMING LANGUAGE 28 4 3 Methods Methods are the basic program units defined in classes The format for a method is selector result type statement or selector statement depending on whether the method returns a result or not Here selector is a description of or template for a message sent to the object The result type is a declaration of the datatype of the method and is only
66. s In 6th Annual International Phoenix Conference on Computers and Communications pages 280 285 Scottsdale AZ February 1987 Gail E Kaiser and David Garlan Composing Software Systems from Reusable Building Blocks In 20th Annual Hawaii International Conference on System Sciences pages 536 545 Kona HI January 1987 Gail E Kaiser and David Garlan Melding Software Systems from Reusable Building Blocks IEEE Software 17 24 July 1987 Gail E Kaiser Concurrent MELD September 1988 Presented at the Workshop on Object Based Concurrent Programming Available from the author Gail E Kaiser and David Garlan Synthesizing Programming Environments from Reusable Features Software Reusability Addison Wesley Reading MA 1989 pages 35 55 Chapter 2 Kaiser 89b Kaiser 89c Keene 85 Moon 86 Popovich 88 Sharp 85 Stroustrup 86 Wadge 85 MELD PROGRAMMING LANGUAGE 72 Gail E Kaiser Object Oriented Programming Language Facilities for Concurrency Control Technical Report CUCS 439 89 Columbia University Department of Computer Science April 1989 Submitted for publication Gail E Kaiser Steven S Popovich Wenwey Hseush and Shyhtsun Felix Wu Melding Multiple Granularities of Parallelism In Stephen Cook editor 3rd European Conference on Object Oriented Programming pages 147 166 Cambridge University Press Nottingham UK July 1989 Sonya E Keene and David A Moon Flav
67. se The IMPORTS clause IMPORTS Feature Random Random Int Class Random Count declares that Feature Random is an external feature and what follows inside the brackets is a list of methods and global variables to be used by the importing feature If no list is supplied then all methods and global variables declared in the EXPORTS list for Feature Random are available The EXPORTS clause EXPORTS Random Int Class Get Random Int Random Real Class Random Count declares that Random Int Class is a class available to external features and if followed by a bracketed list of method names only those methods are visible If no methods are specified MELD PROGRAMMING LANGUAGE 34 after the class name then all methods are accessible to external features The EXPORTS list may also contain global variables accessible to external features When you import a class you are able to declare variables of the external types to create instances of the classes and to send messages to instances of the class perhaps through global objects However you cannot use the class name in a MERGES statement described below that is you cannot create subclasses of the external class Figures 4 5 and 4 5 are programs that show how to use external features FEATURE Random Feature INTERFACE EXPORTS Random Int Class Get Random Random Count Random Real Class RandomInt RandomReal IMPLEMENTATION OBJECT Random Count INTEGER 0 RandomInt
68. selectors The selector Open the s doors HAL what STRING gt printf match 1 s n what does not match the string Open the cargo doors HAL This seems like a bug MELD PROGRAMMING LANGUAGE 69 Appendix I Running Meld The procedure for compiling and linking a Meld program is simple The command proj meld bin meld foo m will compile the single file foo m and link it with the runtime library producing an a out file The command proj meld bin meld o foo foo m will work the same way but will produce an executable file named oo rather than one named a out The command proj meld bin meld foo m bar m will compile both files and link them into a single a out file A switch may be added as above to change the name of the executable file For true separate compilation use the command proj meld bin meld c foo m This compiles foo m but without producing an a out file The switch is similar to the one in the C compiler that performs the analogous function Compilation produces three files foo o foo mo used by ml to build its tables and FooFeat fo if the feature contained in foo m is named FooFeat this file contains a listing of the feature s exports If you have previously compiled oo m and bar min this way you should then use the command proj meld bin ml foo o bar o which will generate some tables necessary for linking separately compiled Meld files and link everything together producing an a ou
69. statements enclosed by delimiters The runtime execution of the statements is determined by the delimiters as follows BEGIN parallel END Equivalent to statements parallel The statements are potentially executed in parallel according to the partial ordering of data dependencies sequential The statements enclosed in the block are executed sequentially atomic The statements enclosed in the block are executed without interleaving with any statements outside the block By default the statements are executed in data flow order but a sequential block may be nested inside an atomic block to provide sequential execution without interleaving with other statements MELD PROGRAMMING LANGUAGE 60 An atomic block executes without interruption from statements outside the block Normally a method may have more than one thread of execution running at one time the atomic block allows entry of only one thread of execution at a time 7 4 9 2 Assignment Statement The assignment statement sets variable to be the value of expression The variable may be a global instance or local variable assignment variable asgn operator expression asgn operator Seer wes ser ES amp gt o9 V V l Il The operators have the same meaning as in C the C form is the same but without the colon 7 4 9 3 Conditional Statement if statement IF expression THEN statement IF expression THEN statem
70. t a raise gt Salary Salary 4000 what is your salary gt printf My current salary is d n Salary END CLASS Person END FEATURE World Figure 1 4 Money World Program The what is your salary method in this example like the earlier Hello method is a string selector and is executed whenever the string what is your salary is typed at the terminal 1 3 Action Equations and Data Dependency One significant difference between MELD and other object oriented languages is the notion of action equations An action equation is a statement that is executed by data dependency rather than sequentially Normally statements within a begin end block are executed one after another The execution of action equations however is determined by when their inputs variables on the right hand sides become available MELD PROGRAMMING LANGUAGE 9 A parallel block in MELD is enclosed by curly braces action block statements METHODS M1 gt A B C B 5 Quarc mas 4 t gt B 5 No gt Method A B C gt gt C 7 Figure 1 5 Data Dependency Relations Among Action Equations in MELD Consider the method in figure 1 5 In this example there are three action equations A B C B 5 and C 7 Itis clear that the first statement relies on variables B
71. t file The name of the executable file may be changed as in the compiler examples above by using the o switch There are other switches The switch v prints out messages as each compiler phase runs and t times each phase MELD PROGRAMMING LANGUAGE 70 Broy 85 Goldberg 85 Hseush 88a Hseush 88b Kaiser 87a Kaiser 87b Kaiser 87c Kaiser 87d Kaiser 88 Kaiser 89a MELD PROGRAMMING LANGUAGE 71 Appendix II Bibliography Manfred Broy editor Control flow and data flow concepts of distributed programming Springer Verlag New York 1985 Adele Goldberg Smalltalk 80 Addison Wesley Reading Mass 1985 Wenwey Hseush and Gail E Kaiser Concurrent Breakpointing Technical Report CUCS 402 88 Columbia University Department of Computer Science October 1988 Wenwey Hseush and Gail E Kaiser Data Path Debugging Data Oriented Debugging for a Concurrent Programming Language In ACM SIGPlan SIGOps Workshop on Parallel and Distributed Debugging pages 236 246 Madison WI May 1988 Special issue of SIGPlan Notices 24 1 January 1989 Gail E Kaiser and David Garlan MELDing Data Flow and Object Oriented Programming In Object Oriented Programming Systems Languages and Applications Conference pages 254 267 Orlando FL October 1987 Special issue of S GPlan Notices 22 12 December 1987 Gail E Kaiser and David Garlan MELD A Declarative Notation for Writing Method
72. t hand side variable of one assignment also appears in the right hand side of another assignment then there is a data dependence relation between these two assignments We say the left hand side affects the right hand side and the right hand side depends on the left hand side MELD PROGRAMMING LANGUAGE 41 A single assignment statement cannot depend on itself Even if a left hand side variable also appears on the right hand side of the assignment there is no data dependency 5 4 2 Second Rule of Data Dependency If Then Else Rule There is no data dependency between the statements in the if branch and the statements in the else branch The reason is that only one branch is active at any time in an If Then Else statement 5 4 3 Third Rule of Data Dependency Change Type Rule All Statements have a depend on relation to the variables they use that is read 5 4 4 Fourth Rule of Data Dependency sequential block rule There is no data dependency between the statements in a sequential block But data dependency can however exist between a statement in the sequential block and a statement outside the sequential block CLASS Foo X y INTEGER METHODS Exchange gt temp INTEGER temp x 1 x y 2 se y temp 3 END CLASS Foo Figure 5 3 Fourth Rule Example Fragment Even though statement 1 has x as an input variable right hand side variable and statement
73. tar superscript as in externals Literal words terminals are written in a typewriter style and capital letters e g BEGIN END Lexical categories such as integer string etc are presented in uppercase italics for example INTEGER All lexical categories are defined using regular expressions based on their lex counterpart definitions 7 3 Lexical Conventions There are six lexical categories identifiers keywords constants strings operators and punctuation Each of these lexical categories is described in detail below White space consisting of space tab and newline characters as well as comments is ignored by the lexical analyzer and serves only to delimit input tokens as necessary 7 3 1 Comments The characters start a comment that must be ended with MELD PROGRAMMING LANGUAGE 52 7 3 2 Identifiers A identifier is any number of letters digits and underscore characters _ starting with a letter or underscore Uppercase and lowercase letters are distinct IDENTIFIER a zA Z a zA Z 0 9 7 3 3 Keywords The following identifiers are reserved for use as keywords SSELECTOR SSELF SSENDER ALL AND ARRAY AS AT BEGIN BOOLEAN BY CHAR CLASS DEFAULT DELAYUNTIL DOUBLE ELSE END END EXPORTS FALSE FEATURE HIGH IF IMPLEMENTATION IMPORTS IN INSIST INTEGER INTERFACE IS PROD KEY LOW MERGES METHODS NAME NIL NOT OBJECT OF OR ORDERED ORDERED SET OVERRIDE PERSISTENT REAL RECORD REMOTES RETURN S
74. ter or one of the system constants constant TRUE FALSE NIL NULL FLOATP INTEGER STRING CHARACTER See section 7 3 for the lexical definitions of these constant types FLOATP INTEGER STRING and CHARACTER are lexical classes of constants A signed constant is a constant with an optional unary sign signed constant constant constant constant MELD PROGRAMMING LANGUAGE 8 Complete Grammar program externals port list body merge decl global object decl class decl methods part c object def method method attr selector type 63 FEATURE identifiers INTERFACE externals IMPLEMENTATION body END FEATURE identifiers pt EXPORTS port list opt IMPORTS port list opt IDENTIFIER IDENTIFIER identifier list port list port list class decl merge decl global object decl MERGES identifier list A8 IDENTIFIER OBJECT object def PERSIST CLASS IDENTIFIER object def methods part END CLASS identifiers opt METHODS method identifier list type specifier identifier list type specifier identifier list type specifier signed constant variable CREATE method attribute constraint method attr sym selector selector type opt gt statement method attr str selector gt statement method attr opt any selector statement OVERRIDE DEFAULT INSIST OVERRIDE DEFAULT OVERRID
75. the lifetime of an object whenever the values of input variables change the value of the output variable will be updated so that the relation is maintained The syntax is output expression inputs where output is an instance variable declared in the class expression inputs is an expression with input instance variables The expression may be any valid MELD expression See figure 6 1 for example CLASS Temp C f REAL METHODS c f 32 0 5 0 9 0 INC x REAL gt f f x END CLASS Temp Figure 6 1 Constraint Program Fragment 6 2 Change Type Constraints Instead of defining a relation between the output instance variable and the input instance variables a change type constraint defines a relation between a fact or action and the input instance variable Change type constraints do not have an output instance variable defined in the statement A change type constraint defines an action that will be evaluated at the time of the object creation and whenever the input instance variables are changed The action can be a regular C procedure call an asynchronous send statement or a synchronous send statement A special case of a change type constraint is a constraint with no inputs This constraint is evaluated only at object creation and is useful for initializations MELD PROGRAMMING LANGUAGE 46 6 2 1 C procedure Constraints A C procedure constraint is a C procedure call that has one or more than one
76. tor expression gt gt amp gt o0 variable symbolic message symbolic message DELAYUNTIL IDENTIFIER RETURN expression SEND symbolic message TO variable SEND string message TO variable IDENTIFIER exp listopt STRING STRING exp list variable object primary expression operator constant exp list identifier list identifiers MELD PROGRAMMING LANGUAGE 66 IDENTIFIER IDENTIFIER expression variable variable IDENTIFIER IDENTIFIER expression variable function call constant expression primary expression expression basic type expression expression operator expression l gt A A m m V Il l oe A Il gt Il v NA V TRUE FALSE nil NULL FLOATP INTEGER STRING CHARACTER expression exp list exp list IDENTIFIER identifier list IDENTIFIER IDENTIFIER identifiers IDENTIFIER MELD PROGRAMMING LANGUAGE 67 9 Unimplemented Features and Bugs The MELD implementation is incomplete A number of features have been discussed in the literature but are not present in the current implementation This section outlines the missing features 9 1 C Loops All of the C loop statements should be added to MELD 9 2 Type Specifier A number of the sophisticated types are missing from fype specifier including type specifier basic type RECOR

THE MELD PROGRAMMING LANGUAGE USER MANUAL

Contents

Download Pdf Manuals

Related Search

Related Contents