[plt-scheme] Macro Problem

From: Eli Barzilay (eli at barzilay.org)
Date: Thu Mar 27 16:36:17 EST 2003

On Mar 26, Bill Wood wrote:
>   ;;; (dgdo ((V1 E1)              (letrec
>   [...messy, especially with email wrapping...]
> The code I wrote is
>   (define-syntax dgdo (syntax-rules ()
>   		        ((dgdo ((var val) ...)
>                                (final ...)
>                            (test exp ...) ...)
>   		         (letrec
> 			   ((loop
> 			      (lambda (var ...)
> 			        (cond (test exp ... (loop var ...))
>                                       ...
> 				      (else (begin final ...))))))
> 			   (loop val ...)))))

IMO, this is *much* more readable than the above...

The problem is that in the recursive call `var ...' is being repeated
for every clause and it looks like mzscheme wants to treat it as a
doubly nested macro.  So it's not your understanding of syntax-rules,
but the general pattern matcher.  A more compact demonstration of this

  (define-syntax foo
    (syntax-rules ()
      ((x (a ...) ((b ...) ...))
       (+ (* a ... b ...) ...))))

FWIW, I've checked this with scsh which fails like mzscheme but bigloo
seems to be fine with it.  I don't know if this should be considered a
bug or not, but you can probably get over it easily by using

          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                  http://www.barzilay.org/                 Maze is Life!

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