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

[David Kastrup <dak@gnu.org>]

David Kastrup <dak@gnu.org> writes:

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

Actually, if the evaluator does macro expansion on the fly instead of en
bloc, it may even be a practical joke.

-- 
David Kastrup