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 ...