Re: error Wrong type to apply: #<syntax-transformer

[Maxime Devos]

Op 09-08-2023 om 01:00 schreef Jean Abou Samra:
> Le mardi 08 août 2023 à 21:38 +0200, Maxime Devos a écrit :
> > As such, this not working on the top-level seems a bug to me -- after
> > all, a module definition is conceptually just a big let:
> >
> > <enable extra reader syntax> (if applicable)
> > (let ()
> >     <magic to make imports work>
> >     (define ...)
> >     (define-syntax-rule ...) ...
> >     ;; use a new macro using syntax-local-binding
> >     ;; to extract the syntax transformer (*).
> >     <insert stuff in hash tables>)
> >
> > (*) not sure if that precise approach actually works in this context
>
>
>
> This is very tempting to believe, and I wish it were true, but it's not true.
>
> At least in Guile, the <insert stuff in hash tables> part doesn't happen
> at the end of evaluating the module. Each module variable is created and
> inserted while evaluating the define form. Otherwise this would give
> an error:
>
> (define a 5)
> (define b (module-ref (current-module) 'a))
> (display b)

I don't see a problem here, only a little backwards-incompatibility (I 
mean, you could just ... not do that, and do (define b a) instead).

And furthermore that incompatibility could be resolved by doing the 
<insert stuff> after the corresponding (define ...) instead of at the end:

(define modules (make-hash-table))
(let ((this-module (make-hash-table)))
  (hash-set! modules '(this module) this-module)
  (define (foo)
    (bar 'quux))
  (hash-set! this-module 'foo foo)
  (define foo-via-module-ref (hash-ref this-module 'foo))
  (hash-set! this-module 'foo-via-module-ref foo-via-module-ref)
  (define bar-procedure
    (syntax-rules ()
      ((x) x)))
  (define-syntax-rule (bar x) x)
  ;;(hash-set! this-module 'bar <make a syntax transformer>)
  ;; output: quux
  (display (foo))
  (newline))
;; output: quuxquux
(display ((hash-ref (hash-ref modules '(this module)) 'foo)))
(display ((hash-ref (hash-ref modules '(this module)) 'foo-via-module-ref)))
(newline)

> The consequences are not innocent.
>
> A variable is associated to a binding which is a "place" in the terminology
> of some other languages, i.e., something you can set!. For toplevel variables,
> because you can use module-set!, a module variable is used as the place.
>
> And because module variables are bound to names which are just symbols,
> duplicate definitions stomp on each other. Consider this:
>
> (define a 5)
> (define (b) a)
> (define a 6)
> (display (b))
> [...]

That problem can be avoided by disallowing duplicate definitions and 
requiring using 'set!' for such things instead.

> This is valid in Guile and prints 6. The second definition for `a` reuses
> the same variable as the first one, effectively acting like a set!.
> Contrast this with
>
> (let ()
>    (define a 5)
>    (define (b) a)
>    (define a 6)
>    (display (b)))
>
> which raises an error due to the duplicate binding.
>
> As https://okmij.org/ftp/Scheme/macros.html#syntax-rule-dark-corner
> puts it, "the top level of Scheme is indeed under-specified and treacherous".

It can be made a bit more specified and less treacherous.

> Best,
>
> Jean

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