Re: Compiler warning for unused local top-level variables

[Ludovic Courtès]

Howdy,

Andy Wingo <address@hidden> writes:

> On Mon 11 Jan 2010 02:02, address@hidden (Ludovic Courtès) writes:

[...]

>>   - Local variables used only by a macro are incorrectly flagged as
>>     unused.  Example:
>>
>>           (define (bar) 'foo)
>>           (define-syntax baz
>>             (syntax-rules () ((_) (bar))))
>>
>>     Here ‘bar’ is incorrectly flagged as unused.  This is because the
>>     definition of ‘baz’ expands to ‘(define baz (make-syntax-transformer
>>     ...))’ and this code doesn’t contain any literal
>>     ‘(toplevel-ref bar)’.  Instead, the reference to ‘bar’ is buried in
>>     an opaque syntax object.
>>
>>     Andy: any idea how to work around this?
>
> No, I don't think it's possible to work around this. At least, nothing
> comes to mind. You could disable the check if a module exports macros.

You mean consider that no variables is unused when a module exports one
or more macros?

I was thinking about somehow “opening” syntax objects.

This expression ‘(define-syntax baz (syntax-rules () ((_) asinh)))’
compiles to:

--8<---------------cut here---------------start------------->8---
(define baz
  (apply (@@ (guile) make-syntax-transformer)
         (const baz)
         (const macro)

[...]

                                             (const #(syntax-object
                                                      asinh
                                                      ((top)
                                                       #(ribcage
                                                         #(dummy)
                                                         #((m69 top))
                                                         #("i74"))
--8<---------------cut here---------------end--------------->8---

If we had, say, ‘syntax-object-toplevel?’ to check whether a syntax
object captures a top-level binding, that’d be enough.  Then we could
use ‘syntax->datum’ to get the symbol, and voilà.

What am I missing?  :-)

Thanks,
Ludo’.