[plt-dev] define-match-expander <-> unlib.plt doesn't compile

From: Dave Gurnell (d.j.gurnell at gmail.com)
Date: Thu Nov 12 14:08:26 EST 2009

Carl Eastlund wrote:
> John,
> Looks right to me.  The define-match-expander form takes two
> expressions, both of which are evaluated immediately at phase 1
> (compile-time) to produce transformers.  It's basically define-syntax
> for a different kind of binding.  You are writing as if it were
> define-syntax-rule, where you just fill in a pattern.  Replace your
> use of it with:
> (define-match-expander my-cons
>  (lambda (stx) (error 'dontcare "aagh!"))
>  (syntax-rules () [(_ a b) (cons a b)]))
> Then you'll probably get the behavior you expected.
> --Carl

Gah! Apologies again for moving so slowly on this. I'll try to fix it  
now. Two things:

1. If I uninstall Unlib and try:

     planet install untyped unlib.plt 3 18

I get a heck of a lot of Scribble errors, but I don't get any failure  
to compile. I'm using an older PLT...

     $ mzscheme -v
     Welcome to MzScheme v4.2.1.5 [3m], Copyright (c) 2004-2009 PLT  
Scheme Inc.

2. If John's looking at define-match-expander, the culprit must be  
this bit in match.ss in Unlib:

; (_ expr pattern ...)
(define-match-expander match:eq?
   (syntax-rules ()
     [(_ expr pattern ...)
      (? (cut eq? <> expr) pattern ...)])

; (_ expr pattern ...)
(define-match-expander match:equal?
   (syntax-rules ()
     [(_ expr pattern ...)
      (? (cut equal? <> expr) pattern ...)])

I guess the second argument should be a syntax transformer procedure.  
The documentation seems to be ambiguous on this, although it's a  
pretty dumb misunderstanding on my part.

-- Dave

Posted on the dev mailing list.