Subject: Re: Dr. Scheme "local"?
From: (Rob Warnock)
Date: 2000/06/19
Newsgroups: comp.lang.scheme
Message-ID: <8iju42$88ld$>
Boris Schaefer  <> wrote:
| What about:
|   ((lambda ()
|      (begin
|        (define x 3)
|        (define y 2)
|        (define z (* x y))
|        z)))
| As R5RS says: 
|   The <expression>s are evaluated sequentially from left to right, and
|   the value(s) of the last <expression> is(are) returned.  This
|   expression type is used to sequence side effects such as input and
|   output.
| So, I would think the above is equivalent to (local ...), as the begin
| guarantees that the internal defines are evaluated sequentially.  Or
| am I still missing something?

Yes, you're still missing something, I'm afraid. Internal definitions
are not themselves "expressions", they're "internal definitions" --
and as such are *exactly* equivalent to a *single* "letrec" expression.
[See R5RS Section 5.2.2 "Internal definitions".] That is, your example
above is *defined* to mean this:

    ((lambda ()
       (begin			; now redundant
         (letrec ((x 3)
		  (y 2)
		  (z (* x y)))

The "begin" contains only one expression (the "letrec"), and thus there
is no "left to right" sequentiality involved at all.

Oh, and the "letrec" bindings violate the rules [R5RS 4.2.2 & 5.2.2]
requiring that "it must be possible to evaluate each <init> without
assigning or referring to the value of any <variable>"...


Rob Warnock, 41L-955
Applied Networking
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