Re: Closure?

[Kjetil S. Matheussen]

On Fri, 11 Jul 2008, Maciek Godek wrote:

> Kjetil S. Matheussen:
>
> > The function "local-eval" is probably what you want.
> >
> > I use local-eval a lot, it's one of the really really
> > nice features provided by Guile:
> >
> >
> > (define-macro (with env . code)
> >  `(local-eval (quote (begin ,@code)) ,env))
> >
> >
> > (define ++
> >  (let ((c 0))
> >    (c-display "env" (the-environment))
> >    (lambda()
> >      (set! c (1+ c)) c)))
>
> I don't actually get the line with c-display.
> Does it require any additional module?
> 'Cause it ain' workin' for me.
> (and besides what does it do?
> the remaining part works fine)

Just ignore that one. c-display is just a debugging
function I forgot to remove. :-)


> > (++)
> > => 1
> > (with (procedure-environment ++) (set! c 20))
> >
> > (++)
> > => 20
>
> I'm really impressed :) Great respect!
>
> I only wonder if there's a way to add a new
> variable to an existing closure, or to delete one.
> (not that I need it right now; but I imagine that
> it can be implemented quite easily and efficiently
> in C if the notion of closure is well-defined and
> explicit)

Adding is easy:

(define (add-var-to-environment env name val)
  (local-eval `(let ((,name ,val)) (the-environment))
              env))

(define env (add-var-to-environment (the-environment) 'a 50))

(local-eval 'a env)
=> 50


I don't know if there's possible to delete a variable
in a clean way.