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Middlesex University Research Repository Middlesex University Research Repository An open access repository of Middlesex University research http://eprints.mdx.ac.uk Clark, Tony (1997) Metaclasses and reflection in smalltalk. Technical Report. University of Bradford. Available from Middlesex University’s Research Repository at http://eprints.mdx.ac.uk/6181/ Copyright: Middlesex University Research Repository makes the University’s research available electronically. Copyright and moral rights to this thesis/research project are retained by the author and/or other copyright owners. The work is supplied on the understanding that any use for commercial gain is strictly forbidden. A copy may be downloaded for personal, non-commercial, research or study without prior permission and without charge. Any use of the thesis/research project for private study or research must be properly acknowledged with reference to the work’s full bibliographic details. This thesis/research project may not be reproduced in any format or medium, or extensive quotations taken from it, or its content changed in any way, without first obtaining permission in writing from the copyright holder(s). If you believe that any material held in the repository infringes copyright law, please contact the Repository Team at Middlesex University via the following email address: [email protected] The item will be removed from the repository while any claim is being investigated. Metaclasses and Reection in Smalltalk A N Clark Department of Computing University of Bradford Bradford West Yorkshire BD DP UK email ANClarkcompbradacuk tel Septemb er Keywords ob jectoriented metaob jects reection inheritance message passing Abstract Many Ob jectOriented Programming Languages provide reective features which may b e used to control the interpretive mechanism of the language Often these features are dened with resp ect to a golden braid consisting of ob jects classes and metaclasses This pap er describ es the Smalltalk golden braid and generalize it for multiple inheritance Multiple inheritance leads to choices b etween many dierent inheritance strategies The reective features of Smalltalk cannot aect the basic mechanisms of inheritance and so an arbitrary choice must b e made for multiple inheritance A language is describ ed in which the reective features of Smalltalk are extended so as to allow programmer dened inheritance strategies Intro duction The evaluation of a programming language expression e in a given context c may b e describ ed by the evaluation of a program p which takes a representation of e and c as input e is termed an ob jectlevel construct whilst p and the representations of e and c are termed metalevel constructs For illustration we use an op erator M which maps ob jectlevel constructs to metalevel constructs If the languages which are used for b oth the ob ject and metalevels are the same and causally connected then the language is reective Ob jectOriented Programming Languages OOPLs have interpretive mechanisms which are based up on classes ob ject creation message passing and inheritance Classes typically dene the lo cal state and op erations for ob jects which are their instances When a message is passed to an ob ject the op eration with the message name is invoked with resp ect to the lo cal state A class inherits from another class by including all the inherited storage and op eration denitions along with its own The metalevel of an OOPL describ es how to p erform inheritance message passing instance creation etc If the OOPL is reective then these mechanisms are describ ed in terms of messages which are sent to ob jects at the meta level Consider the ob jects at some base level B the ob jects and messages which describ e how to p erform creation of ob jects at B message passing at B etc are dened at level MB these ob jects are called classes A class at level MB is characterized by controlling the creation and subsequent b ehaviour of a collection of ob jects at level B The ob jects which are classes 2 at level MB are created and controlled by ob jects at level M B these ob jects are called 2 metaclasses A metaclass at level M B is characterized by controlling the creation and subsequent b ehaviour of classes at level MB coined the term golden braid to describ e the relationship b etween ob jects classes and metaclasses Of course since classes are ob jects then we can view the golden braid starting at 3 2 MB and ending at M B which reinterprets the metaclasses at level M B as classes n A feature of an OOPL is builtin at all levels when its b ehaviour is the same for M n Such features cannot b e extended and may b e said to b e intransigent An OOPL can b e reective whilst still having intransigent features an OOPL which has no intransigent features may b e said to b e ful ly reective Reective OOPLs include Smalltalk Lo ops CLOS KRS Ob jVLisp These languages dier in terms of the ways in which the golden braid is implemented and the extent to which it may b e used to aect the basic interpretive mechanisms of the resp ective languages This pap er describ es the reective p ower of Smalltalk identies an intransigent language feature send and prop oses a language extension which increases the reective p ower by including the feature into the metalevel x describ es a simple functional language which will b e used to implement three reective ob jectoriented systems which dier with resp ect to how expressive their reective features are x describ es a system called Abstract Smalltalk AS which is an implementation of the relevant features of the language Smalltalk AS exhibits single inheritance which is generalised to multiple inheritance in the system AS with Multiple Inheritance ASMI describ ed in x Both AS and ASMI have intransigent language features including ob ject representation and message passing which means that it is dicult to have multiple typ es of inheritance mechanism coexistsing in the same system This is particularly a drawback when the system exhibits multiple inheritance b ecause there are many orthogonal multiple inheritance schemes ASMI with Reective Send ASMIRS describ ed in x makes b oth ob ject representation and message passing a reective feature of the system This is shown to supp ort dierent typ es of multiple inheritance strategy within the same system Functional Representation The systems AS ASMI and ASMIRS are constructed using a simple functional language which following has b een enriched with data values op erators and evaluation rules which are characteristic of ob jectoriented programming languages The language has a call byvalue semantics and enriches the evaluation rules for the calculus with pattern matching currying rst class environments and up dateable lo cations The syntax of the language is divided into two the kernel syntax which is given a semantics using a state transition system based on the SECD machine and the sugared syntax which is given a semantics by translating into the kernel For more information ab out functional languages see and The sugared syntax is given b elow T let D D I E j F + + F I P E j meth I P E j F jF j P P j KP j N j S P I j + E I j N j S j P E j EE j EOE j if E then E else E j E E j E j E E + + E E j E where D j let D in E j case E of A end j op en E in E A P E j AA A program is a sequence of top level recursive denitions t T and expressions e E D is the syntax of declarations which may b e a simple value such as i or a functional declaration f F such as add x y x y Functional declarations may b e overloaded and may include metho ds b oth of which are describ ed in app endix A Patterns p P are used in binding p ositions to limit the domain of a function and to decomp ose the value which is supplied as an argument to the function by extracting subcomp onents and binding them to identiers A pattern may b e an identier i in which case the supplied value is b ound to i a wildcard in which case the supplied value is ignored a tuple p p in which case 1 n the supplied value must b e a tuple of the same length and the corresp onding subcomp onents must match against the subpatterns a constructor k K applied to a pattern p in which case the supplied value must b e constructed using k eg k v and v must match p or a constant numb er n N or string s S in which case the supplied value must b e the constant An expression e is an identier numb er or string a function p p p e which may 1 2 n b e curried and have patterns in the binding p ositions a prex application e e an inx 1 2 application e e where O denes a collection of inx op erators a conditional expression a 1 2 tuple a parenthesized expression a sequenced expression e e which is used to control side 1 2 eects a list expression e e where e constructs singleton lists and is the empty 1 n list a where or let expression b oth of which may have a sequence of declarations which are established in parallel a case expression case e in p e p e end where e 1 1 n n is evaluated and tested against the patterns in turn the rst pattern which matches will deconstruct the value of e p ossibly bind some identiers and evaluate the corresp onding expression an op en expression op en e in e where e pro duces an environment binding 1 2 1 identiers to values which is added to the current environment for the scop e of the evaluation of expression e 2 Environments are collections of asso ciations bindings b etween keys and values The op erational semantics of programming languages often
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