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.