Re: Fwd: new function

[Matt Wette <matt.wette@gmail.com>]

On 9/19/21 3:38 AM, Maxime Devos wrote:
> 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.

Does this work for you?

scheme@(guile-user)> (define a 1)
scheme@(guile-user)> (define a 2)
scheme@(guile-user)> a
$2 = 2
scheme@(guile-user)>