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From: markf@zurich.ai.mit.edu (Mark Friedman)
Newsgroups: comp.lang.scheme
Subject: Re: Simulating Environments with Continuations ?
Message-ID: <MARKF.91Jun27182528@montreux.ai.mit.edu>
Date: 27 Jun 91 22:25:28 GMT
References: <1991Jun27.192117.8609@jato.jpl.nasa.gov>
Sender: news@ai.mit.edu
Reply-To: markf@zurich.ai.mit.edu
Organization: M.I.T. Artificial Intelligence Lab.
Lines: 45
In-reply-to: brian@granite.jpl.nasa.gov's message of 27 Jun 91 19:21:17 GMT

In article <1991Jun27.192117.8609@jato.jpl.nasa.gov>
brian@granite.jpl.nasa.gov (Brian of ASTD-CP) writes:

   (define ( <class-constructor> [args])

     (let (( <ivar1> <init-val1> )
	   ( <ivar2> <init-val2> ) ...)

       (define ( <method-proc1 [args]> )
	 ; reference ivars as free vars )

       (define ( <method-proc2 [args]> )
	 ; reference ivars as free vars )
       ...
       (define (self msg [args])
	 (case msg
	   (msg1 <method-proc1> )
	   (msg2 <method-proc2> ) ...)) self))

   To instantiate an object (instance of a class), call the cons-
   tructor.  The instance is represented by the message dispatch
   procedure ("self") returned.  All the methods have access to the
   instance vars _via_ lexical scoping.  

   Ok, the only problem is that each instance of a class will have
   its own copy of the method procedures.  This is wasteful of space,
   since all the copies of each method proc are identical, having 
   been "define"d by the same source code.  What we want to do is 
   solve the problem through method sharing.  

Unless your interpreter is going out of its way to be wasteful and
silly (and unless it was careful it would be incorrect for the most
general case) the above approach will not copy the method-proc's in
each instance. Each instance (a closure) will have a pointer to the
environment which contains the method-procs.

-Mark
--

Mark Friedman
MIT Artificial Intelligence Lab
545 Technology Sq.
Cambridge, Ma. 02139

markf@zurich.ai.mit.edu