Re: Fwd: new function

[Damien Mattei]

i do not understand well what you want to mean with those example.
For me  define-once does not seems to be a solution, it act as define too
much ,does not set! variable if already exist and can not use to set it
again because, as define it is forbidden twice in the same block for the
same variable:

scheme@(guile-user)> (define (foo3)
  (define-once  x 1)
  (if #t
      (let ()
        (define-once x 2)
        ;;(set! x 2)
        (display "x=")
        (display x)
        (newline)
        (define-once x 3)
        ;;(set! x 2)
        (display "x=")
        (display x)
        (newline)

        )
      'never)
  (display x)
  (newline))
While compiling expression:
Syntax error:
unknown file:51:6: invalid or duplicate identifier in definition in form
(let () (define-once x 2) (display "x=") (display x) (newline) (define-once
x 3) (display "x=") (display x) (newline))

i will try (use-modules (system syntax)) that yesterday seems to give
better results

On Tue, Sep 21, 2021 at 9:03 PM Maxime Devos <maximedevos@telenet.be> wrote:

> Damien Mattei schreef op di 21-09-2021 om 15:04 [+0200]:
> > i have tested define-once
> > http://www.gnu.org/software/guile/docs/master/guile.html/Top-Level.html
> > (the defvar of Lisp)and idea are:
> > -unfortunately it is considered by scheme as a define,so there is some
> > context where it is not allowed in my code
> > -seems to work fine at toplevel (as mentioned in doc) but strange
> behavior
> > in a function, i did not understand really what happened but i got some
> > #unspecified value.
> >
> > here are my test code:
> > cheme@(guile-user)> (define (foo2)
> >   (define-once  x 1)
> >   (if #t
> >       (let ()
> >         (define-once x 2)
> >         ;;(set! x 2)
> >         (display "x=")
> >         (display x)
> >         (newline))
> >       'never)
> >   (display x)
> >   (newline))
>
> Possibly you want (added a set? argument for demonstration):
>
> (define (foo2 set?)
>   (define x) ; define an (undefined or unbound, not sure about
> terminology) variable
>   (if set?
>       (let ()
>         (set! x 2) ; change the value of x
>         (display "x=")
>         (display x)
>         (newline))
>       'never)
>   (display x)
>   (newline))
>
> That should be portable and avoids global state.
>
> scheme@(guile-user)> x
> ;;; <stdin>:20:0: warning: possibly unbound variable `x'
> ice-9/boot-9.scm:1685:16: In procedure raise-exception:
> Unbound variable: x
> scheme@(guile-user) [1]> ,q
>
> scheme@(guile-user) [1]> (foo2 #f)
> #<unspecified> ; I expected an error as would result from ...
>
> ;; ... this ...
> scheme@(guile-user)> (variable-ref (make-undefined-variable))
> ice-9/boot-9.scm:1685:16: In procedure raise-exception:
> In procedure variable-ref: Unbound variable: #<variable 7f6de46cde80
> value: #<undefined>>
>
> Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
> scheme@(guile-user) [2]>
>
> ;; instead  of #<unpecified> but whatever ...
>
> scheme@(guile-user) [1]> (foo2 #t)
> x=2
> 2
> scheme@(guile-user) [1]> (foo2 #t)
> x=2
> 2
>
>
> scheme@(guile-user) [1]> (define x 3)
> scheme@(guile-user) [1]> (foo2 #t)
> x=2 ; foo2 doesn't use the global variable 'x'
> 2
>
> scheme@(guile-user) [1]> x
> $1 = 3
>
> Does this seem reasonable to you?
>