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Re: Issue: LET-TOP-LEVEL (version 1)
- To: Jon L White <edsel!jonl%labrea.Stanford.EDU@multimax>
- Subject: Re: Issue: LET-TOP-LEVEL (version 1)
- From: Dan L. Pierson <pierson@mist>
- Date: Wed, 02 Mar 88 08:54:41 EST
- Cc: cl-cleanup%sail.Stanford.EDU@multimax
- In-reply-to: Your message of Tue, 01 Mar 88 17:06:53 -0800. <8803020106.AA07266@bhopal.lucid.com>
Date: Tue, 1 Mar 88 17:06:53 PST
From: Jon L White <edsel!jonl@labrea.Stanford.EDU>
Subject: Issue: LET-TOP-LEVEL (version 1)
As usual, please excuse regrinding.
re: Problem description:
The main defining forms in CLtL do not support defining (as opposed to
creating) closures in lexical environments.
. . .
Common Lisp tries to support lexical closures but this support is
really confined to anonymous lambdas within other function
definitions. This proposal attempts to move lexical closures closer
to first-class status in Common Lisp.
I don't believe this problem description. In particular, previous
discussions on these mailing lists suggested that DEFUN is merely
a macro that turns into something like:
(setf (symbol-function 'foo) #'(lambda (x y ...) ...))
so that anything said about "anonymous lambdas" should also be equally
applicable to "defun"'s.
The relevant sentence is on page 67: "Other implementation-dependent
bookkeeping actions may be taken as well by DEFUN."
re: Current practice:
The above example works in both Ibuki Lisp and Lucid, however only
Ibuki Lisp describes (SYMBOL-FUNCTION 'GET-THING) as a closure.
In Lucid Common Lisp, *all* functions are "closures". It doesn't
serve any purpose to distinguish one which has some captured
lexical environment "inside it" from another that doesn't.
[However, I recently adduced reasons why it would be profitable to
distinguish those that are merely "trampolines" back into the
interpreter from the real "compiled functions".]
I sit corrected, however there appears to be no way in Lucid Common
Lisp to inspect or debug a function which has some captured lexical
environment to see what the captured environment is so I couldn't find
re: Current practice:
. . . Both
Symbolics and TI are reported to leave GET-THING and FREE-THING as
named-lambdas instead of compiling them.
At least some persons from Symbolics have agreed that this is a flaw in
the particular compiler's techniques -- not a flaw in language design.
I agree philosophically, but see page 66, second paragraph on
-- JonL --