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Re: Fwd: new function
From: |
Maxime Devos |
Subject: |
Re: Fwd: new function |
Date: |
Sun, 19 Sep 2021 16:56:13 +0200 |
User-agent: |
Evolution 3.34.2 |
Matt Wette schreef op zo 19-09-2021 om 07:45 [-0700]:
> On 9/19/21 3:38 AM, Maxime Devos wrote:
> > [...]
> > (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)>
If I use 'define', the following evaluates to zero:
(define a 0)
(let ()
(define a 1)
(values))
a
(the inner 'a' shadows the global 'a', thus the last a evaluates to 0 instead
of 1)
However, Damien Mattei asked for a variant of 'define' that only defines a
variable
if it is not yet bound (let's call this variant define-or-set!)
(define-or-set! a 0)
(let ()
(define-or-set! a 1)
(values))
a
As within the 'let' expression, 'a' is already bound (to the global variable),
due to the
first (define-or-set! a 0), the inner 'define-or-set!' should set! 'a' instead
of
defining a new variable 'a'. As such, the code above should expand to:
(define a 0)
(let ()
(set! a 1)
(values))
a ; evaluates to 1.
Possibly Damien Mattei wants something else though ...
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