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Re: Fwd: new function

From: Matt Wette
Subject: Re: Fwd: new function
Date: Sun, 19 Sep 2021 07:45:40 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0

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 <>
Date: Sun, Sep 19, 2021 at 9:54 AM
Subject: new function
To: <>

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

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))


Does this work for you?

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

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