Re: References/locations

[Kjetil S. Matheussen]

"Maciek Godek":
> Subject: References/locations
> To: address@hidden
> Message-ID:
>         <address@hidden>
> Content-Type: text/plain; charset=ISO-8859-1
>
> Hi,
> it's me again, asking silly questions.

Not at all, your questions are very good. :-)



> This time I would like to know if there's
> a simple way to store a reference to
> a variable in another variable -- say,
> an element of vector or hash table.
>
> I imagine this could look like this:
> (define v #(1 2 3))
> (define v1 (vector-location v 1))
> v1
> : 2
> (set! v1 10)
> v
> : #(1 10 3)
>
> I've tried to do it using a "procedure with
> setter", but the problem is that set! doesn't
> evaluate its first argument (as long as it's a
> symbol), so I'd have to wrap everything
> up in macros to obtain:
> (set! (vector-location v 1) 10)
>
> Besides I think that the names "hash-ref"
> and "vector-ref" are confusing, since they
> don't return references, but values (therefore
> the names like "vector-get" or "hash-get" would
> be more apropreate)

Never thought about that, but it sounds
correct. vector-get and hash-get would
probably be mroe apropreate names.



> I also wonder if there's any point for allowing
> locations to any sorts of variables (similar to
> pointers in C or pointers to C++), that is,
> (define x 10)
> (define y (location x))
> (set! y 20)
> x
> : 20

Well, here's a relatively clean way, I think:


(define-macro (location name)
   (define new-val (gensym))
   `(lambda (,new-val)
       (set! ,name ,new-val)))

(define old-set! set!)

(define-macro (set! a b)
  `(if (procedure? ,a)  ;; Needs a better check.
       (,a ,b)
       (old-set! ,a ,b)))


guile> (define x 10)
guile> (define y (location x))
guile> (set! y 20)
guile> x
20
guile>