Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!XEROX.COM!Pavel.pa
From: Pavel.pa@XEROX.COM
Newsgroups: comp.lang.scheme
Subject: Re: semantics of DEFINE
Message-ID: <890503-103409-9762@Xerox>
Date: 3 May 89 17:33:55 GMT
Sender: daemon@bloom-beacon.MIT.EDU
Organization: The Internet
Lines: 37

Too many people are referring to DEFINE as a form that ``side-effects the
top-level environment'' for me to keep out of the fray.  In the current
(unreleased) draft of the Scheme specification, R^(3.95)RS, the following
definitions appear (my wording):

-- A program is a mixed sequence of definitions and expressions.
	
-- The meaning of a program P is the same that that of the following
expression:
	((lambda (I*) P') <undefined> ...)
where I* is the set of variables defined in P (i.e., appearing as the CADR
of a DEFINE form), P' is the sequence of expressions obtained by replacing
each definition in P with the corresponding assignment, and <undefined> is
an expression producing some useless value.

More informally, to execute a program, you wrap it in a big LET binding all
of the defined variables to useless values, change the DEFINEs to SET!s,
and evaluate that expression.

Note, please: no mutation of any environment takes place.

I believe that too many people base their understanding of the semantics of
Scheme on a read-eval-print loop model or on the low-level details of
particular (or perhaps all) implementations.  It seems to me irrelevant
that I might use side-effects to resolve the separate compilation (or just
separate loading) of files of code with the semantics given above.  The
semantics of DEFINE, and therefore the most accurate way to conceptualize
its meaning, has no mention of mutation.

Of course, this little diatribe has no direct bearing on the meaning of
internal DEFINEs.  Personally, I'd rather they had the same meaning (as
given above) as top-level ones.  The meaning in terms of LETREC has always
struck me as gratuitously incompatible, and frequently inconvenient as
well.  If you're going to use the same name, you should give it the same
semantics...

	Pavel