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

[David Kastrup <dak@gnu.org>]

Mark H Weaver <mhw@netris.org> writes:

> Hi Noah,
>
> Noah Lavine <noah.b.lavine@gmail.com> 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)
  (call-with-current-continuation
   (lambda (x)
     (env (list x form)))))

(define-macro (my-env)
  (call-with-current-continuation
   identity))
     

(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