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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.91Jun28140824@montreux.ai.mit.edu>
Date: 28 Jun 91 18:08:24 GMT
References: <1991Jun27.192117.8609@jato.jpl.nasa.gov>
	<MARKF.91Jun27182528@montreux.ai.mit.edu> <norman.678125326@crevenia>
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In-reply-to: norman@parc.xerox.com's message of 28 Jun 91 16:08:46 GMT

In article <norman.678125326@crevenia> norman@parc.xerox.com (Norman Adams) writes:

   Unless your interpreter is very smart, the environment of each
   instance will include a first class procedure for each method.

You're quite right. I was thinking of something like:

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

       (define ( <method-proc2 [args]> )
	 ; reference ivars as free vars )
       ...
       (define ( <class-constructor> [args])

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

	   (define (self msg [args])
	     (case msg
		   (msg1 <method-proc1> )
		   (msg2 <method-proc2> ) ...)) self))
       <class-constructor>))

which, of course, doesn't work.

   I think the right approach is to have the compiler support this idiom
   (or an equivalent one).  It is not very hard, just a small twist in
   your closure analysis.   The T compiler does this.  

But the original author wanted something which would work in a
standard Scheme.

Originally (before I came up my erroneous claim), I was going to
respond to the original author by suggesting a macro (e.g.
`WITH-INSTANCE-VARIABLES') which would take a class as an argument and
walk over his method code replacing variables references (and
settings) with appropriate access (and setting) procedures. In that
scheme instance variables would probably become slots in some data
structure held in the instance closure.  It's hairy, but not
unreasonable. Of course macros are not yet standard, but at least the
RnRS authors are committed to putting them in R4RS.

-Mark
--

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

markf@zurich.ai.mit.edu