The Grace Programming Language Draft Specification Version 0.132
Contents
1. gt Number 0 self 7 Objects and Classes Grace object constructor expressions and declarations produce individual objects Grace provides class declarations to create classes of objects all of which have the same structure Grace s class and inheritance design is complete but tentative We need experience before confirming the design 7 1 Objects Objects are created by object literals The body of an object literal consists of a sequence of declarations object def colour Colour Colour tabby def name String Unnamed var miceEaten 0 Object literals are lexically scoped inside their containing method or block In particular any initializer expressions on fields or constants are executed in that lexical context Whether methods are also in that scope is the nesting question see 1 The current design is that initializers are nested but not methods Each time an object literal is executed a new object is created Id spec tex 195 2011 10 21 19 24 33Z kjx 11 A constant can be defined by an object literal such as def unnamedCat object def colour Colour Colour tabby def name String Unnamed var miceEaten 0 to bind a name to an object Repeated invocations of the reader method unnamedCat will return the same object 7 2 Classes Objects literals have no provision for initializing the constant and variable attributes of the created object other than via2. 9 2 Case under discussion Grace will support a match case construct Match will take only one argu ment and match against a series of blocks introduced by case Pattern matching will support destructuring Examples match x match against a literal constant case 0 gt Zero typematch binding a variable looks like a block with parameter case s String gt print s match against the value in an existing variable requiring parenthesis like Scala case pi gt print Pi pi destructuring match binding variables case Some v gt print v match against placeholder matches anythin case _ gt print did not match 9 3 Exceptions under discussion Grace supports basic unchecked exceptions Exceptions will be generated by the raise keyword with an argument of some subtype of Exception Id spec tex 195 2011 10 21 19 24 33Z kjx 19 raise UserException new Oops Exceptions are caught by a catch _ case _ construct that syntactically parallels match _ case _ catch def f File open data store case e NoSuchFile gt print No Such File return case e PermissionError gt print No Such File return case Exception gt print Unidentified Error System exit finally f close Exceptions can t be restarted However the stack frames that are termi nated when an excepti
3. A 27 To Be Done As well as the large list in Section 1 of features we haven t started to design this section lists details of the language that remain to be done 1 2 10 11 12 13 14 15 16 specify full numeric types Block apply 5 How should we spell apply run confirm method lookup algorithm in particular relation between lex ical scope and inheritance 8 Out then Up Is that enough Does the no shadowing rule work If it does is this a problem confirm super or other mechaism for requesting overridden methods 8 6 confirm rules on named method argument parenthesization 8 1 how are mutually recursive names initialised make the def keyword optional or remove it or return to const 6 1 post 10 02 2011 support multiple constructors for classes 7 2 where should we draw the lines between object constructor expres sions named object declarations class declarations and hand built classes 7 3 what s the difference between class FOO and def FOO class for various values of class how do factories etc relate to uninitialized 6 2 decide what to do about equality operators 10 Support for identifying static type decltype and dynamic type typeid foo getType Support for type test like instanceof and static casts What is the namespace of types What is the syntax of types 11
4. or imperative first Is it necessary to support procedures first b The courses may be taught using dynamic types static types or both in combination in either order c We aim to offer some but not necessarily complete support for functional first curricula primarily for courses that proceed rapidly to imperative and object oriented programming University students taking second year classes in programming algo rithms and data structures concurrent programming software craft and software design Faculty and teaching assistants developing libraries frameworks ex amples problems and solutions for first and second year programming classes Programming language researchers needing a contemporary object oriented programming language as a research vehicle Designers of other programming or scripting languages in search of a good example of contemporary OO language design Id spec tex 195 2011 10 21 19 24 33Z kjx 3 3 Syntax Much of the following text assumes the reader has a minimal grasp of computer terminology and a feeling for the structure of a program Kathleen Jensen and Niklaus Wirth Pascal User Manual and Report Grace programs are written in Unicode Reserved words are written in the ASCII subset of Unicode As a matter of policy the names of methods defined in the required libraries are also restricted to the ASCII subset of Unicode 3 1 Layout Grace uses curly brack
5. More to the point what is the type system Multiple Assignment 6 2 f lt T gt 28 Id spec tex 195 2011 10 21 19 24 33Z kjx 17 Type assertions should they just be normal assertions between types so e g lt could be a normal operator between types 18 Grace needs subclass compatibility rules 19 BRANDS Brand Brand Brand Id spec tex 195 2011 10 21 19 24 33Z kjx 29 References 1 Eric Allen David Chase Joe Hallett Victor Luchangco Jan Willem 12 Maessen Sukyoung Ryu Guy L Steele Jr and Sam Tobin Hochstadt The Fortress language specification version 1 08 Technical report Sun Microsystems Inc March 2007 Henry G Baker Equal rights for functional objects or the more things change the more they are the same OOPS Messenger 4 4 October 1993 Gavin M Bierman Erik Meijer and Mads Torgersen Lost in transla tion formalizing proposed extensions to Cf In OOPSLA 2007 Gavin M Bierman Erik Meijer and Mads Torgersen Adding dynamic types to Cf In ECOOP 2010 Andrew P Black Eric Jul Norman Hutchinson and Henry M Levy The development of the Emerald programming language In History of Programming Languages ILI ACM Press 2007 Gilad Bracha Newspeak programming language draft specification ver sion 0 0 Technical report Ministry of Truth 2009 Gilad Bracha and David Griswold Stongtalk Typechecking Smalltalk in a production environment In OOPSLA ACM
6. Option lt T gt Some lt T gt null 11 11 Nested Types Option Types may be nested inside types written T1 T2 In this way a type may be used as a specification module 24 Id spec tex 195 2011 10 21 19 24 33Z kjx 11 12 Additional Types of Types option Grace may support nullable types written Type defined as Type null and exact types written Type option Grace probably will support Tuple types probably written Tuple lt T1 T2 Tn gt We re not yet sure how 11 13 Syntax for Types This is very basic but hopefully better than nothing Type GroundType GroundType amp GroundType GroundType BasicType BasicType lt Type gt Selftype BasicType TypelD TypelD TypelD TypelD 11 14 Reified Type Information Metaobjects and Type Lit erals option Types are represented by objects of type Type Hmm should be Type lt T gt Since Grace has a single namespace so types can be accessed by requesting their names To support anonymous type literals types may be written in expressions type Type This expression returns the type metaobject repesenting the literal type 11 15 Type Assertions option Type assertions can be used to check conformance and equality of types assert B lt A B conforms to A B is a subtype of A assert B lt foo _ C gt D B had better have a foo method from C ret
7. Press 1993 Gilad Bracha Peter von der Ah Vassili Bykov Yaron Kashai William Maddox and Eliot Miranda6 Modules as objects in Newspeak In ECOOP 2010 Tim Budd A Little Smalltalk Addison Wesley 1987 Luca Cardelli Type systems In Allen B Tucker editor Computer Science Handbook chapter 97 CRC Press 2nd edition 2004 Luca Cardelli James Donahue Lucille Glassman Mick Jordan Bill Kalsow and Greg Nelson Modula 3 reference manual Technical Re port Research Report 53 DEC Systems Research Center SRC 1995 Carlton Egremont III Mr Bunny s Big Cup o Java Addison Wesley 1999 13 Erik Ernst Family polymorphism In ECOOP 2001 30 14 15 16 17 18 19 20 21 22 23 24 25 26 Id spec tex 195 2011 10 21 19 24 33Z kjx Matthias Felleisen Robert Bruce Findler Matthew Flatt and Shriram Krishnamurthi How To Design Programs MIT Press 2001 Richard P Gabriel LISP Good news bad news how to win big AI Expert 6 6 30 39 1991 Joseph Gil and Itay Maman Whiteoak Introducing structural typing into Java In OOPSLA 2008 Brian Goetz Time Peierls Joshua Block Joesph Bowbeer David Holmes and Doug Lea Java Concurrency in Practice Addison Wesley Professional 2006 Adele Goldberg and David Robson Smalltalk 80 The Language and its Implementation Addison Wesley 1983 C A R Hoare Hints on programming language design Technical
8. Re port AIM 224 Stanford Artificial Intelligence Laboratory 1973 Ric Holt and Tom West OBJECT ORIENTED TURING REFER ENCE MANUAL seventh edition version 1 0 Technical report Holt Software Associates Inc 1999 Paul Hudak John Hughes Simon Peyton Jones and Philip Wadler A history of Haskell being lazy with class In History of Programming Languages III pages 12 1 12 55 ACM Press 2007 A Igarashi B C Pierce and P Wadler Featherweight Java A minimal core calculus for Java and GJ ACM Transactions on Programming Languages and Systems 23 3 396 450 2001 Atsushi Igarashi and Hideshi Nagira Union types for object oriented programming Journal of Object Technology 6 2 31 45 February 2007 http www jot fm issues issues 2007 02 article3 Daniel H H Ingalls Design principles behind Smalltalk BYTE Mag azine August 1981 Kathleen Jensen and Niklaus Wirth Pascal User Manual and Report Springer 1975 Brian W Kernighan and Dennis M Ritchie The C Programming Language Addison Wesley 2nd edition 1993 Id spec tex 195 2011 10 21 19 24 33Z kjx 31 27 28 29 30 31 32 33 34 35 36 37 38 39 Gregor Kiczales Erik Hilsdale Jim Hugunin Mik Kersten Jeffrey Palm and William G Griswold An overview of AspectJ In ECOOP 2001 Michael K lling Bett Koch and John Rosenberg Requirements for a first year object oriented teaching language In ACM Co
9. lexical scope Class declarations combine the definition of an object with the definition of a factory object where the factory object has a method named new that creates instances of the class A class declaration is similar to an object literal except that it may have parameters like a block Examples class CatFactory aColour aName gt def colour Colour aColour def name String aName var miceEaten 0 The new method takes as many arguments as the class has parameters The object that is returned by an execution of new has the fields and methods listed in the body of the constructor that follows the class keyword If there are formal parameters to the class body they are initialized to the arguments to new and are also in scope within the class So in the above example the constants colour and name are initialized from the parameters aColour and aName which are in turn initialized from the first and second arguments to new def fergus CatFactory new tortoiseshell Fergus Trouble If the programmer wants a factory object with more methods or method names other than new she is free to build such an object using nested object constructors The above declaration for class Cat is equivalent modulo types and modules to the following nested object declarations 12 Id spec tex 195 2011 10 21 19 24 33Z kjx def CatFactory object the cat factory method new aColour Colour aName Str
10. of the object is to execute the method When speaking of Grace we distinguish the act of requesting a method which is exactly what Smalltalkers call sending a message and involves only a method name and some arguments and executing that method which involves the code of the method which is always local to the receiver of the request 8 1 Named Methods A named method request is a receiver followed by a dot then a method name an identifier then any arguments in parentheses Parentheses are not used if there are no arguments To improve readability a long argument list may be interpolated between the words that makes up the method name This is determined by the declaration of the method If the receiver is self it may be left implicit i e the self and the dot may both be omitted canvas drawLineFrom source to destination canvas movePenToXY x y canvas movePenToPoint p print Hello world pt x Grace does not allow overloading on argument type Parenthesis may be omitted where they would enclose a single argument provided that argument is a block literal option a string literal or option a square bracket literal if we allow square bracket collection literals 8 2 Assignment Methods A assignment method is an explicit receiver followed by a dot then a method name an identifier followed by and then a single argument If the receiver is self it may be left implicit i e t
11. 1 The Grace Programming Language Draft Specification Version 0 132 Andrew P Black Kim B Bruce James Noble October 22 2011 Introduction This is a specification of the Grace Programming Language This spec ification is notably incomplete and everything is subject to change In particular this version does not address whether to support object nesting Beta Scala Newspeak or not Smalltalk Python collection syntax and collection literals tuples vs multiple values vs multiple returns nested static type system although we ve made a start encapsulation system module system metadata Java s annotations C attributes final abstract etc purity and non nulls reflection assertions data structure invariants pre amp post conditions contracts regexps test support libraries including more Numeric types For discussion and rationale see http gracelang org Where this document gives options we outline choices in the lan guage design that have yet to be made Id spec tex 195 2011 10 21 19 24 33Z kjx User Model All designers in fact have user and use models consciously or subconsciously in mind as they work Team design requires explicit models and assumptions Frederick P Brooks The Design of Design 2010 First year university students learning programming in CS1 and CS2 classes that are based on object oriented programming a The courses may be structured objects first
12. 4 33Z kjx 21 colour gt Colour name gt String miceEaten gt Number miceEaten _ Number gt None For commonality with method declarations method arguments may be given both names and types within type declarations A single identifier is interpreted as a formal parameter name with type Dynamic 11 3 Type Declarations Types and generic types may be named in type declarations type MyCatType color gt Colour name gt String care only about names and colours type MyGenericlType lt A B gt where A lt Hashable where B lt disposable hashStore _ A _ B gt Boolean pity not just A B cleanup _ B Grace has a single namespace types live in the same namespace as methods and variables 11 4 Relationships between Types Conformance Rules The key relation between types is conformance We write B lt A to mean B conforms to A that is that B is a subtype of A A is a supertype of B This section draws heavily on the wording of the Modula 3 report 11 with apologies to Luca Cardelli et al If B lt A then every object of type B is also an object of type A The converse does not apply If A and B are ground object types then B lt A iff e B contains every method in A e Every B method must have the same number of arguments as A with the same distribution in multi part method names 22 Id spec tex 195 2011 10 21 19 24 33Z kjx e Every method
13. 45 46 47 48 Id spec tex 195 2011 10 21 19 24 33Z kjx Martin Richards and Colin Whitby Stevens BCPL the language and its compiler Cambridge University Press 1980 Bjarne Stroustrup Why C is not just an object oriented program ming language In OOPSLA Companion ACM Press 1995 Gerald Sussman and Guy Steele SCHEME An interpreter for extended lambda calculus Technical Report AI Memo 349 MIT Artificial Intel ligence Laboratory December 1975 Don Syme The FY draft language specification Technical report Mi crosoft 2009 Antero Taivalsaari Delegation versus concatenation or cloning is in heritance too OOPS Messenger 6 3 1995 David Ungar and Randall B Smith SELF the Power of Simplicity Lisp and Symbolic Computation 4 3 June 1991 Larry Wall Perl the first postmodern computer language http www wall org larry pm html Spring 1999 Allen Wirfs Brock and Brian Wilkerson Modular Smalltalk In OOP SLA 1998 Niklaus Wirth Modula 2 and Oberon In HOPL 2007
14. Grace s encapsulation system has not yet begun in earnest Grace will use metadata annotations support lt lt private gt gt methods that can be requested only from self or super 8 8 Generic Method Requests Methods may optionally be requested with actual generic type arguments given explicitly Where a method declared with formal generic type parame ters is requested in a statically typed context without explicit actual generic type arguments the actual types arguments will be inferred Examples sumSq lt Integer64 gt 10 i64 20 164 sumSq 10 i64 20 164 9 Control Flow Control flow statements in Grace are syntactically method calls While the design of the module system is not complete in fact hardly yet begun we expect that instructors will need to define domain specific control flow constructs in libraries and these constructs should look the same as the rest of Grace 9 1 Basic Control Flow If statements if test then block if test then block else block While statement while test do block 18 Id spec tex 195 2011 10 21 19 24 33Z kjx For statement for collection do item gt block body for course students do s Student gt print s for 0 n do i gt print i To allow for conventional syntax with a leading keyword if while for these methods are treated as if they were implicitly sent to self which implies that all objects must inherit the corresponding method
15. aining method or block A naked block literal that is neither the target of a method request nor passed as an argument is a syntax error The body of a block consists of a sequence of declarations and expressions option and also statements if we have them 6 Declarations Def and var declarations may occur anywhere within a method or block their scope is the whole of their defining block or method It is an error to declare an identifier that shadows a lexically enclosing identifier 8 Id spec tex 195 2011 10 21 19 24 33Z kjx 6 1 Constants Constant definitions bind an identifier to the value of an initializer expres sion optionally at a precise type Examples def x 3 x 100 0 01 def x Number 3 means the same as the above def x Number Syntax Error x must be initialised Grace has a single namespace for methods and constants and variables A constant declaration of x can be seen as creating a nullary reader method x 6 2 Variables Grace supports variable declarations using the var keyword Uninitialized variables of any type are given a special uninitialized value accessing this value is an error caught either at run time or at compile time depending on the cleverness of your implementor Examples var x 3 type of x is inferred var x Rational 3 explicit type Instance variables are reassigned using assignment methods see 8 2 A variable declaration of x can b
16. ar where 3 5 Tabs and Control Characters Newline can be represented either by carriage return or by line feed however a line feed that immediately follows a carriage return is ignored Tabs and all other non printing control characters except carriage and line feed are syntax errors even in a string literal There are escape sequences for including special characters in string literals 4 Built in Objects 4 1 Numbers Grace will support a single type Number Number will maintain rational computations in arbitrary precision and inexact irrational computations approximated to at least 64bit precision Implementations may support other numeric types a full specification of numeric types is yet to be completed Grace has three syntactic forms for numerals literals that denote Numbers 1 decimal numerals written as strings of digits optionally preceded by a minus 2 explicit radix numerals written as a decimal number between 2 and 35 representing the radix a leading x and a string of digits where the digits from 10 to 35 are represented by the letters A to Z in either Id spec tex 195 2011 10 21 19 24 33Z kjx 5 upper or lower case As a special case a radix of 0 is taken to mean a radix of 16 Explicit radix numerals may not be preceded by a minus 3 base exponent numerals always in decimal which use e as the expo nent indicator Base exponent numerals may be preceded by a minus All literals evaluate to e
17. breaking space Table 1 Grace string escapes A platform dependent newline is either a line feed If or a carriage return cr or a cr lf pair depending on the platform Examples Hello World LEI t The End of the Line n LEI A 4 4 option String interpolation We are considering syntax so that strings or expressions returning objects that support the asString method can be directly interpolated into strings Examples Adding a to b gives atb 5 Blocks Grace blocks are lambda expressions they may or may not have parameters If a parameter list is present the parameters are separated by commas and terminated by the gt symbol Id spec tex 195 2011 10 21 19 24 33Z kjx 7 do something i gt i 1 sum next gt sum next Blocks construct objects with a single method named apply or apply n if the block has parameters The block is evaluated by requesting the apply method with the same number of arguments as the block has parameters It s an error to provide fewer or more parameters for 1 10 do i gt print i might be implemented as method for collection do block block apply collection at i Here is another example var sum 0 def summingBlock Block lt Number Number gt i Number gt sum sum summingBlock apply 4 sum is now 4 summingBlock apply 32 sum in now 36 Blocks are lexically scoped inside their cont
18. e seen as creating a reader method x and an assignment method x _ Grace s encapsulation system will control the accessibility of each of these methods You can think of the real instance variable as having a unique secret name which is known only to the accessor methods Block and method temporary variables really exist and can be the tar gets of real assignment statements It s a deliberate design decision that assignment to a local variable and requesting an assignment method on an object look identical It is an error to declare a block or method temporary variable that shadows an enclosing method or assignment method see 6 Assignments return Nothing Void None etc Id spec tex 195 2011 10 21 19 24 33Z kjx 9 6 3 Methods Methods are declared with the method keyword a name optionally an argument list potentially repeated optionally a return type declaration and a method body Methods may not be nested Methods may contain one or more return e statements If a return state ment is executed the method terminates with the value of the expression e If the method returns None then no expression may follow the return If ex ecution reaches the end of the method body without executing a return the method terminates and returns the value of the last expression evaluated Assignment methods are named by an identifier suffixed with Prefix operator methods will be named prefix followed by the ope
19. ets for grouping and semicolons as statement termi nators and infers semicolons at the end of lines Code layout cannot be inconsistent with grouping code with punctuation while stream hasNext do print stream read a code without punctuation while stream hasNext do print stream read A line break followed by an increase in the indent level implies a line continuation whereas line break followed by the next line at the same or lesser indentation implies a semicolon if one is permitted syntatically 3 2 Comments Grace s comments delimiters follow C and Java s line comments Comments are not treated as white space each comment is conceptually attached to the smallest immediately preceding syntactic unit comments following a blank line are attached to the largest immediately following syntactic unit comment to end of line 4 Id spec tex 195 2011 10 21 19 24 33Z kjx 3 3 Identifiers Identifiers in Grace must begin with a letter and consist of letters and digits thereafter Prime characters may be used after the first character of an identifier u D An underscore _ acts as a placeholder identifier it is treated as a fresh identifier everywhere it is used 3 4 Reserved Words The indicates words related to design options not yet chosen assert case catch class const def extends false finally match method object outer prefix raise return self super true type v
20. he self and the dot may both be omitted Examples XIS y 2 widget active true Id spec tex 195 2011 10 21 19 24 33Z kjx 15 Assignment methods must return Nothing 8 3 Binary Operator Methods Grace allows operator symbols sequences of operator characters for binary methods methods with an explicit receiver and one argument A binary operator method is one or more operator characters and may not match a reserved symbol for example is reserved but is not Most Grace operators have the same precedence it is a syntax error for two different operator symbols to appear in an expression without parenthe sis to indicate order of evaluation The same operator symbol can be sent more than once without parenthesis and is evaluated left to right Four simple arithmetic operators do have precedence and over and 09 Examples 1 2 3 evaluates to 6 1 2 3 evaluates to 7 1 2 x 3 evaluates to 9 1 2 3 evalutes to 7 1 4 x 4 syntax error Named method requests without arguments bind more tightly than op erator method requests The following examples show first the Grace ex pressions as they would be written followed by the parse Examples 1 2 1 2 1 b b sqrt 1 a x a b x b sqrt axa a x a b x b sqrt a x a b b sqrt axa bxb a x a b b a b c a b c a b c a b c 8 4 Unary Prefix Operator Method Grace supports
21. ibution medium for Grace programs objects and libraries is Grace source code Grace source files should have the file extension grace If for any bizzare reason a trigraph extension is required it should be grc Grace files may start with one or more lines beginning with these lines are ignored 26 Id spec tex 195 2011 10 21 19 24 33Z kjx 13 Libraries 13 1 Collections Grace will support some collection classes Collections will be indexed 1 size by default bounds should be able to be chosen when explicitly instantiating collection classes Acknowledgements The Scala language specification 2 8 37 and the Newspeak language speci fication 0 05 6 were used as references for early versions of this document The design of Grace so far has been influenced by Algol 39 36 As pectJ 27 BCPL 40 Beta 31 Blue 28 29 30 C 26 C 41 Ct A 3 Eiffel 33 34 Emerald 5 F 10 F 43 FGJ 22 FJV 23 FORTRESS 1 gBeta 13 Haskell 21 Java 12 17 Kevo 44 Lisp 15 ML 35 Modula 2 48 Modula 3 11 Modular Smalltalk 47 Newspeak 8 6 Pascal 25 Perl 46 Racket 14 Scala 38 37 Scheme 42 Self 45 Smalltalk 18 24 9 7 Object Oriented Turing 20 Noney 32 Whiteoak 16 at least we apologise if we ve missed any languages out All the good ideas come from these languages the bad ideas are all our fault 19 Id spec tex 195 2011 10 21 19 24 33Z kjx
22. ing gt Cat object the cat herself def colour Colour aColour def name String aName var miceEaten 0 Notice that the type Cat describes the object returned from Cat new not the factory object CatFactory 7 3 Inheritance Grace class declarations supports inheritance with single subclassing mul tiple subtyping like Java by way of an inherits C clause in a class dec laration or object literal A new declaration of a method can override an existing declaration but overriding declarations must be annotated with lt override gt Overridden methods can be accessed via super calls 8 6 It is a static error for a field to override another field or a method This example shows how a subclass can override accessor methods for a variable defined in a superclass in this case to always return 0 and to ignore assignments class PedigreeCatFactory aColour aName gt inherits Cat new aColour aName var prizes 0 lt override gt method miceEaten 0 lt override gt method miceEaten n Number return Just ignore The right hand side of an inherits clause is restricted to be a class name followed by a correct request for that class s new method 7 4 Understanding Inheritance under discussion Grace s class declarations can be understood in terms of a flattening trans lation to object constructor expressions that build the factory object Un derstanding this translation lets ex
23. nference on Computer Science Education SIGCSE 1995 Michael K lling and John Rosenberg Blue a language for teaching object oriented programming In ACM Conference on Computer Sci ence Education SIGCSE 1996 Michael K lling and John Rosenberg Blue a language for teach ing object oriented programming language specification Technical Re port TR97 13 Monash University Department of Computer Science and Software Engineering 1997 Ole Lehrmann Madsen Birger M ller Pedersen and Kristen Nygaard Object Oriented Programming in the BETA Programming Language Addison Wesley 1993 Donna Malayeri and Jonathan Aldrich Integrating nominal and struc tural subtyping In ECOOP 2008 Bertrand Meyer Object oriented Software Construction Prentice Hall 1988 Bertrand Meyer Eiffel The Language Prentice Hall 1992 Robin Milner Mads Tofte and Robert Harper The Definition of Stan dard ML MIT Press 1990 Peter Naur The European side of the development of ALGOL In History of Programming Languages I pages 92 139 ACM Press 1981 Martin Odersky The Scala language specification version 2 8 Technical report Programming Methods Laboratory EFPL July 2010 Martin Odersky and Matthias Zenger Scalable component abstrac tions In OOPSLA 2005 Alan J Perlis The American side of the development of ALGOL In History of Programming Languages I pages 75 91 ACM Press 1981 32 40 41 42 43 44
24. on is raised should be pickled so that they can be used in the error reporting machinery debugger stack trace catch _ case _ finally _ construct and a do _ finally _ construct support finalization even through exceptions Following Scala a using _ do _ construct supports resource allocation and deallocation using Closable new do stranger gt bound to the new Closable stranger doSomething the close method is automatically requested of the Closable when the block terminates 10 Equality and Value Objects All objects will automatically implement the following non overridable meth ods option Library programmers are able to override these methods 1 and operators implemented as per Henry Baker s egal pred icate 2 That is immutable objects are egal if they are of the same shape and if their fields contents are egal while mutable objects are only ever egal to themselves 2 hashcode compatible with the egal As a consequence immutable objects objects with no var fields which capture only other immutable objects will act as pure value objects with out identity This means that a Grace implementation can support value objects using whatever implementation is most efficient either passing by reference always by passing some times by value or even by inlining fields 20 Id spec tex 195 2011 10 21 19 24 33Z kjx into their containing objects and
25. pert programmers build more flexible factories Id spec tex 195 2011 10 21 19 24 33Z kjx 13 The above declaration for class PedigreeCat is broadly equivalent to the following nested object declarations not considering types modules and renaming superclass methods so that an object s method names must actually be unique def PedigreeCatFactory object the cat factory method new aColour Colour aName String gt PedigreeCat object the cat herself def colour Colour aColour def name String aName lt lt private gt gt var Cat_miceEaten 0 ugly super ugly var prizes 0 method miceEaten 0 method miceEaten n Number return Just ignore object method new object 7 5 Generic Classes Classes may optionally be declared or instantiated with generic type pa rameters Formal generic type parametrs may be constriained with where clauses Examples class VectorFactory lt T gt size gt var contents Array size size method at index Number gt T return contents at method at index Number put elem T class SortedVectorFactory lt T gt where T lt Comparable lt T gt 8 Method Requests Grace is a pure object oriented language Everything in the language is an object and all computation proceeds by requesting an object to execute 14 Id spec tex 195 2011 10 21 19 24 33Z kjx a method with a particular name The response
26. rator character s Methods may have a repeated parameter to provide variable arity aka varargs A repeated parameter if present must be the last parameter to a method and must be the only parameter after the last part of a multi part method name Repeated parameters are designated by a prefix star asterix x before the name of the parameter Inside the method the repeated parameter has the type of an immutable collection of the declared type a parameter declared foo args String will have a type such as args ImmutableCollection lt String gt Methods may optionally be declared or requested with generic type pa rameters Formal generic type parametrs may be constriained with where clauses Examples method pi 3 141592634 method greetUser print Hello World method Hother Point gt Point x other x y other y method Hother x other x y other y method Hother return x other x y other y method foo n Number gt None 10 Id spec tex 195 2011 10 21 19 24 33Z kjx print Foo currently foo now assigned n super foo n method choseBetween a Block lt None gt and b Block lt None gt gt None if Random nextBoolean then a apply else b apply method print xargs Printable gt None method sumSq lt T gt a T b T gt T where T lt Numeric a xa Hb b class NumberFactory method prefix
27. unary prefix operator methods since Grace does not sup port binary operator methods with implicit receivers there is no syntactic ambiguity 16 Id spec tex 195 2011 10 21 19 24 33Z kjx Prefix operators bind with the same precedence as method requests with no arguments and therefore need parenthesis to disambiguate Examples b 4 x a sqrt b squared illegal b squared b squared status ok engine isOnFire amp wings areAttached amp isOnCourse 8 5 Accessing Operator Method Grace supports an accessing operator option Grace supports a two argument accessing operator Using these operators print a 3 calls method on a with argument 3 a 3 Hello calls method on a with arguments 3 and Hello Note Somewhere we need to have a list of reserved operators that cannot be used normally 8 6 Super Requests The reserved word super may be used only as an explicit receiver In over riding methods method requests with the pseudo receiver super request the prior overridden method with the given name from self Note that no search is involved super requests can be resolved statically unlike other method requests Examples super foo super bar 1 2 6 super doThis 3 timesTo foo super 1 super Id spec tex 195 2011 10 21 19 24 33Z kjx 17 foo super syntax error 1 super syntax error 8 7 Encapsulation The design of
28. updating the field if the containing object assigns a new value 11 Types Grace uses structural typing 11 32 16 Types primarily describe the re quests objects can answer Fields do not directly influence types except in so far as a field with publicly visible accessor methods will cause those methods to be part of the type and in general to be visible to unconstrained clients Unlike in other parts of Grace Type declarations are always statically typed and their semantics may depend on the static types The main case for this is determining between identifiers that refer to types and those that refer to constant name definitions introduced by def which are interpreted as Singleton types 11 1 Basic Types Grace s standard prelude defines the following basic types e Object the common interface of most objects e Boolean methods for true and false Number numbers e String strings and individual characters e Pattern pattern used in match case statements e Dynamic dynamically typed expressions If no types are provided on method formal parameters the types are taken as dynamic by default There is also a top type which can be written as an empty object type 11 2 Object Types Object types give the type of objects methods The various Cat object and class descriptions see 7 would produce objects that conform to an object type such as the following Id spec tex 195 2011 10 21 19 2
29. urning D 11 16 Notes 1 Option Classes define a type of the same name currently this is NOT part of Grace 2 Sanity Check these rules Id spec tex 195 2011 10 21 19 24 33Z kjx 25 oN Oo 10 11 12 13 14 15 16 17 18 12 To be done add in path types types in objects What s the relationship between type members across inheritance and subtyping Classes are not types are we sure about this Types are patterns need to be to be matched against Reified Generics formals are also patterns see above On matching How does destructuring match works What s the pro tocol Who defines the extractor method not sure why this is here Somehow do classes need to define a type that describes the objects that are created by their factory methods Note that Generic Types use angle brackets viz ImmutableCollection lt Figure gt can a type extend another type where do where clauses go method return types Structural typing means we neither need nor want any variance anno tations Becaue Grace is structural programmers can always write an anonymous structural type that gives just the interface they need or such types could be storted in a library Should ObjectTypes permit formal parameter names or not 11 2 What actually gets returned from None 86 3 11 5 Tuples 11 12 Syntax as a type Literal Tuple Syntax Nesting Pragmatics The distr
30. with parameters P P gt R in A must have a corresponding method in B Q1 Qn gt S Argument types may be contravariant P lt Q Results types may be covariant S lt R If a class or object B inherits from another class A then B s type should conform to A s type If A and B are generic classes then similar instantions of their types should conform The conformance relationship is used in where clauses to constrain for mal generic type parameters of classes and methods 11 5 Any and None The type Any is the supertype of all types and may also be written as The type None is the subtype of all types There are no instances of None In particular neither undefined and nor any kind of nil are instances of None What happens if a method requested via Dynamic returns None but the caller attempts to use that None value 11 6 Variant Types Untagged retained variant types written T1 T2 Tn may refer to an object of any one of their component types Instances of variant are not themselves tagged as members of the variant however instances can be distinguised via their reified type information The only methods that may be requested via a variant type are methods with exactly the same declaration across all members of the variant Op tion methods with different signatures may be requested at the most most specific argument types and least specific return type Variant t
31. xact rational Numbers explicit conversions such as f64 must be used to convert rationals to other types Examples 1 1 42 3 14159265 13 343e 12 414 45e3 16xF00F00 2x10110100 Oxdeadbeef Radix zero treated as 16 4 2 Booleans The keywords true and false denote the only two values of Grace s Boolean type Boolean operators will generally be written using single characters amp for and for or and prefix for not Examples P amp Q toBe toBe not Short circuit a k a non commutative boolean operators take blocks as their second argument Examples P amp amp Q toBe toBe 4 3 Strings and Characters String literals in Grace are written between double quotes as in C Java and Python Strings literals support a range of escape characters such as 6 Id spec tex 195 2011 10 21 19 24 33Z kjx t b and also escapes for Unicode these are listed in Table 1 Individual characters are represented by Strings of length 1 Strings are immutable Grace values see 10 and so may be interned Strings conform to the protocol of an immutable IndexableCollection and Grace s standard library will include mechanisms to support efficient incremental string construction Escape Meaning Escape Meaning KY backslash y single quote i double quote b backspace n line feed r carriage return t tab l unicode newline f page down e escape left bracket right bracket space non
32. ypes are retained as variants they are not equivalent to the ob ject type which describes all common methods This is so that the exhaus tiveness of match case statements can be determined statically In detail SSI T TLS T S lt S amp T lt T gt S T lt S T Id spec tex 195 2011 10 21 19 24 33Z kjx 23 11 7 Intersection Types option Intersection types written T1 amp T2 amp amp Tn may refer to an object that conforms to all of the component types The main use of inter section types is as bounds on where clauses class Happy lt T gt where T lt Comparable lt T gt amp Printable amp Happyable foo gt 11 8 Union Types option Structural union types sums written T1 T2 Tn may refer to an object that conforms to any of the component types Unions are mostly included for completeness variant types subsume most uses 11 9 Type subtraction option A type written T1 T2 has the interface of T1 without any of the methods in T2 11 10 Singleton Types The names of singleton objects typically declared in object declarations may be used as types Singleton types match only their singleton object Singleton typs can be distinguised from regular types because Grace type declarations are statically typed def null object method isNull gt Boolean return true class Some lt T gt thing T gt method isNull gt Boolean return false type
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