Re: Closure?

["Kjetil S. Matheussen" <k.s.matheussen@notam02.no>]

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.