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Re: Anything better for delayed lexical evaluation than (lambda () ...)?

From: David Kastrup
Subject: Re: Anything better for delayed lexical evaluation than (lambda () ...)?
Date: Wed, 14 Dec 2011 23:12:39 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.92 (gnu/linux)

Mark H Weaver <address@hidden> writes:

> Hi Noah,
> Noah Lavine <address@hidden> writes:
>> Perhaps this is obvious to everyone else, but it just occurred to me
>> that (capture-local-environment) is just
>> (call-with-current-continuation), but running in the environment of
>> the evaluator instead of the program being evaluated. It's as though
>> the evaluator was going to look in a tree for more code, but hit a
>> special node and did a (call/cc). I hope other people find this
>> interesting.
> Ah yes, that's an excellent point!

(define (my-eval form env)
   (lambda (x)
     (env (list x form)))))

(define-macro (my-env)

(format #t "~a" (my-eval '(+ x 3) (let ((x 4)) (my-env))))

> In fact it makes me wonder whether `the-environment' and `local-eval'
> could actually be implemented this way.  I see some complications that
> might make this strategy impractical or fragile, most notably that we
> must be assured that the (call/cc) does not happen until
> (the-environment) would have been _evaluated_, whereas the
> expander/memoizer/evaluator will want to see what code is there _before_
> evaluation.  I'll have to think about this.  There might be an easy and
> robust way to do this, or maybe not.

Feel free to experiment with the above.  I have my doubt that it leads
to sane behavior.  In particular, it will refinish macro expansion (so
you don't want significant material behind it) and reevaluate the
_whole_ eval it is in up to the point of calling my-env (so you don't
want significant material before it).

So it is more a joke than anything of practical value.  But is _is_ good
for a few dropjaws.

David Kastrup

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