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From: death@ZERMATT.LCS.MIT.EDU (Mark A. Sheldon)
Newsgroups: comp.lang.scheme
Subject: Scheme Digest #106
Message-ID: <19890425161147.8.DEATH@MICKEY-MOUSE.LCS.MIT.EDU>
Date: 25 Apr 89 16:11:00 GMT
References: <Scheme-Request@mc.lcs.mit.edu>
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One crucial problem with DEFINEs placed willy-nilly in programs arises from
DEFINE's being an environment mutator.  Consider this modification to our
standing example:

(define (foo bool)
   (if bool
        (define (result) #t)
        'do-nothing
   (result)))

Allowing DEFINEs everywhere gives us a weird sort of dynamic scoping.
Restricting DEFINEs to beginnings of blocks allows us to think of a block
of internal DEFINEs as a LETREC (though CALL/CC may expose implementations
that don't implement these blocks as a LETREC).  If Scheme is a statically
scoped language, then this sort of DEFINE is anathema.  If Scheme is to
support dynamic scoping, then I think FLUID-LET is cleaner because I don't
have to think about environment mutation.

Top level DEFINEs don't bother me so much because I think of the top level
as a debugger.

-Mark A. Sheldon