Re: Fwd: new function

[Maxime Devos <maximedevos@telenet.be>]

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Damien Mattei schreef op zo 19-09-2021 om 11:18 [+0200]:
> ---------- Forwarded message ---------
> From: Damien Mattei <damien.mattei@gmail.com>
> Date: Sun, Sep 19, 2021 at 9:54 AM
> Subject: new function
> To: <guile-devel@gnu.org>
> 
> 
> hello,
> i'm developing an extension to Scheme
> and i need a procedure or macro that define a variable only if it is not
> bind and if it is just set! it.

Could you give an example in which the hypothetical define-or-set! is used?
I wonder where such a thing would be useful.

> I can not do it in Guile or any Scheme,and i'm desperately searching a way
> to do that. I finally conclude that it can be done only by adding it in the
> language.
> 
> Can someone include a such function in Guile next release?
> i know guile have a predicate defined?

defined? only works on global variables, not on lexicals,
and depends on the (current-module), which is not necessarily
the module defined? is used in.

>  to test binfing of a vairable but
> writing a macro with it is not possible because define can be used in an
> expression context.

You can use 'syntax-local-binding'.

(use-modules (system syntax))
(define-syntax define-or-set!
  (lambda (s)
    (syntax-case s ()
      ((_ var value)
       (case (syntax-local-binding #'var)
         ((lexical displaced-lexical) #'(set! var value))
         ((global) #'(define var value))
         (else ???))))))

,expand (define-or-set! a 0) ; $_ = (define a 0)
,expand (let ((a 0)) (define-or-set! a 0) a) ; $_ = (let ((a 0)) (set! a 0) a)
(define-or-set! a 0)
,expand (define-or-set! a 0) ; $_ = (define a 0) (might or might not be acceptable for your use case
,expand (let () (define-or-set! a 0) (define-or-set! a 1) a) ; $_ = (let () (define a 0) (set! a 1))

Greetings,
Maxime.

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