yes it can reuse an upper and precedent definition but a nested definition
can not be used later at an upper level.

anyway it would not be portable , for all those reason i will use to
separate operator ,basically <- for set! and <+ for define ,so i can even
modify toplevel bindings , i did not want to have all the python behavior
which is not safe and did not allow local nested variables.

even using an exception will not solve the problem:
scheme@(guile-user)> (call-with-current-continuation
   (lambda (exit)
     (with-exception-handler
      (lambda (e)
        (exit "undefined ,you need to define somewhere")) (lambda () (set!
notdefined 7)))))
;;; <stdin>:97:69: warning: possibly unbound variable `notdefined'
$28 = "undefined ,you need to define somewhere"

??? i do not see any solution with a macro with exception

Damien


On Thu, Sep 23, 2021 at 10:27 PM Taylan Kammer <taylan.kammer@gmail.com>
wrote:

> On 23.09.2021 19:27, Damien Mattei wrote:
> > yes i know parsing the whole code is the only portable solution, but it
> is slow,even on a few dozen of lines the slowing is visible ,so i can even
> think of that on one thousand lines...
> >
> > I finally succeed in Guile with simple piece of code to make my example
> run with a single assignment operator <-  , here i define for variable the
> assignment operator <$ , <- is working with arrays too:
> >
> > *Preview:*
> >
> > (define-syntax <$
> >
> >   (lambda (s)
> >
> >     (syntax-case s ()
> >
> >       ((_ var value)
> >
> >        (case (syntax-local-binding #'var)
> >
> >          ((lexical) #'(begin
> >                       (display "<$ : lexical scope : ")
> >                       (display (quote var))
> >                       (newline)
> >                       (set! var value)))
> >
> >        ((displaced-lexical) #'(begin
> >                                 (display "<$ : displaced-lexical scope :
> ")
> >                                 (display (quote var))
> >                                 (newline)
> >                                 (set! var value)))
> >
> >          ((global) #'(begin
> >                      (display "<$ : global scope : ")
> >                      (display (quote var))
> >                      (newline)
> >                      (define var value)))
> >
> >          (else #'(begin
> >                  (display "<$ : unknow variable scope :")
> >                  (display (quote var))
> >                  (error "<$ : unknow variable scope : "))))))))
> >
>
> I can't seem to find syntax-local-binding in Guile 2.2 or 3.0.  Did you
> have to import some special module, or are you using another version?
>
> Either way, I suspect that the following will not work with your macro:
>
>   (let ()
>     (let ()
>       (<$ x 1))
>     (display x)
>     (newline))
>
> If I understand correctly, it will expand to:
>
>   (let ()
>     (let ()
>       (define x 1))
>     (display x)
>     (newline))
>
> And that won't work because 'x' is only defined in the inner 'let'.
>
> This is where we see the crucial difference between Scheme and Python:
> in Python there is nothing similar to an inner 'let'.  There is only
> one function-level scope.  In Scheme, there can be as many nested
> scopes as you want, and an inner scope can't affect an outer one.
>
> --
> Taylan
>