[plt-scheme] Macro is misbehaving...

On 12/06/06, Ryan Culpepper <ryan_sml at yahoo.com> wrote:
> --- "Paulo J. Matos" <pocmatos at gmail.com> wrote:
>
> > Hi all,
> >
> > I'm having a macro that's giving me the creeps. I've oversimplified
> > it, but the error is still there so don't worry if it really
> > doesn't
> > make sense, what matters is why this is happening:
> > foo.scm
> > (module foo mzscheme
> >
> >   (require (prefix slide: (lib "slide.ss" "slideshow"))
> >            (lib "class.ss"))
> >
> >   (define obj%
> >     (class object%
> >       (define/public (get-pict s)
> >         (slide:inset 10 10))
> >       (super-new)))
> >   (define obj (make-object obj%))
> >
> >   (define-syntax slide/foo
> >     (syntax-rules ()
> >       ((_ obj s ...)
> >        (slide:slide
> >         (if (symbol? s)
> >             s
> >             (send* obj s))
> >         ...))))
> >
> >   (provide slide/foo)
> >   )
> >
> > bar.scm
> > (module bar (lib "slideshow.ss" "slideshow")
> >
> >   (require "foo.scm")
> >
> >   (slide/foo
> >    (t "Hello")
> >    'next
> >    (t "Bye"))
> > )
> >
> > I get:
> > expand: unbound variable in module in: next
>
> (slide/foo (t "Hello") (quote next) (t "Bye"))
> =>
> (slide:slide
>   (if (symbol? (quote next))
>       (quote next)
>       (send* (t "Hello")
>         (quote next))))
>
> You are producing a send* expression that tries to call the "quote"
> method with the single argument "next". That's why you're getting the
> error about "next" being unbound.
>

I'm sorry but I still cannot understant what the problem is. (quote
next) doesn't need to evaluate next. (symbol? (quote next)) => #t, so
it should return the symbol.

> You are also duplicating the evaluation of "s"---you shouldn't do
> that either.

I know, that's the part of the oversimplification to show here.

>
> > wierder is that if I change send* to send I get:
> > send: method name is not an identifier in: (quote next)
>
> Yes, because in that case you have
>
>   (send (t "Hello") (quote next))
>

This one was a stupid try because it would not work.
The fact is that the macro receives a list with a method and its
arguments, so that's why I call send* instead of send. I receive
(method args) so (send* obj (method args)) would work.

> so it's trying to interpret "(quote next)" as the name of the method
> to call.
>

Yes, I understand that, thanks.

> > Problem is, I need to send an the argument of slide/foo to an
> > object unless this argument is 'next, 'alt, generally speaking:
> > a symbol. I'm not being able to do this. Any ideas?
>
> Do you mean to always call the "get-pict" method if it isn't a
> symbol? That's missing from your macro.
>
> My hunch is you want the else branch of the if expression to be
>
>   (send obj get-pict s)
>

No, not really, again a problem with oversimplification. If I call,
for example,
(slide/foo (get-pict 10)) I would like it to call the method get-pict
with arg 10 on the object. If I call (slide/foo 'next) it should
return next.

Still, can't understand your explanation on why this doesn't work.
next should never be evaluated. Afterall (quote next) is exactly not
to evaluate next.

> Ryan
>
> > _________________________________________________
> >   For list-related administrative tasks:
> >   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> >
>
>


-- 
Paulo Jorge Matos - pocm at sat inesc-id pt
Web: http://sat.inesc-id.pt/~pocm
Computer and Software Engineering
INESC-ID - SAT Group