[plt-scheme] define-syntax question

I see the problem now.  syntax-rules requires the pattern to start with the
keyword for the macro (here, "label"), and that won't work in this case.
I'm not sure how to achieve the effect I want.  I want to be able to say

((label fact (lambda (n) (if (= n 0) 1 (* n (fact (- n 1))))))
 10)

and get 10 factorial.

BTW it's possible that the common lisp term is "labels"; I'm not sure.
This may also be an archaic old lisp form.

Mike
        

> From: Eli Barzilay <eli at barzilay.org>
> Date: Thu, 26 Jun 2003 22:51:42 -0400
> 
> On Jun 26, Michael Vanier wrote:
> > I'm trying to come up with a scheme equivalent for a restricted
> > version of common lisp's "label" construct, but I'm getting an error
> > I don't understand.  Here is my code:
> > 
> >   (define-syntax label
> >     (syntax-rules ()
> >       (((_ f (lambda (p1 p2 ...) e)) a1 a2 ...)
> >        (letrec ((f (lambda (p1 p2 ...) e)))
> >          (f a1 a2 ...)))))
> 
> Looks like a case of bad parens, shouldn't that be
> 
>   (define-syntax label
>     (syntax-rules ()
>       ((_ f (lambda (p1 p2 ...) e) a1 a2 ...)
>        (letrec ((f (lambda (p1 p2 ...) e)))
>          (f a1 a2 ...)))))
> 
> ?
> 
> Even that is very likely not right -- do you really want to have a
> `lambda' as part of the pattern?  If so, you should add it to the
> first syntax-rules argument.  But that doesn't make much sense, so it
> is probably better to match on all values:
> 
>   (define-syntax label
>     (syntax-rules ()
>       ((_ f func a1 a2 ...)
>        (letrec ((f func))
>          (f a1 a2 ...)))))
> 
> but that doesn't look like something useful which gets back to the
> question of what exactly is CL's `label'?  AFAIK, it doesn't have one.
> 
> -- 
>           ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
>                   http://www.barzilay.org/                 Maze is Life!
>