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

On Thu, Nov 12, 2009 at 1:46 PM, John Clements
<clements at brinckerhoff.org> wrote:
> Unlib.plt doesn't compile, as you (Sam) reported several weeks ago. I tried
> to fix the problem, and started wondering whether the underlying problem was
> whether 'define-match-expander' was working incorrectly.  For instance, what
> should this program produce?
>
> foo.ss:
>
> #lang scheme
>
> (define-match-expander my-cons
>  (error 'dontcare "aagh!")
>  cons)
>
> (provide (rename-out [my-cons cons]))
>
> bar.ss:
>
> #lang scheme
>
> (require "foo.ss")
>
> (cons 3 4)
>
> Reading the docs, it looks like it should produce the pair containing 3 and
> 4, but it actually signals the error. It appears to me that the untyped
> folks are (mis?)reading the docs in the same way I am.
>
> Am I wrong, or is define-match-expander broken, or is something else going
> on?
>
> Thanks,
>
> John

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