[plt-scheme] another macro+module anomaly

At Tue, 26 Nov 2002 23:06:45 EST, Benderjg2 at aol.com wrote:
> (module ex mzscheme
>   (provide test)
>   
>   (define-syntax (test stx)
>     (syntax-case stx (-)
>       ((_ (- a b))
>        (syntax '(subtract a b)))
>       ((_ (op a b))
>        (syntax '(op a b)))))
>   )
> 
> When I test this macro (at the top level),
>  (test (+ 3 4))   ==> (+ 3 4)   ;; this is "correct"
> and
>  (test (- 3 4))    ==> (- 3 4)    ;; this is wrong!

This is an annoying problem with the top level that I don't know how to
avoid while still conforming to R5RS.

One solution is to `(require mzscheme)' in the top level, if you have
control over it.

The more general solution is to use `module-or-top-identifier=?' for
comparing literals:

(module ex mzscheme
  (provide test)
  (require-for-syntax (lib "stx.ss" "syntax"))
  
  (define-syntax (test stx)
    (syntax-case* stx (-) module-or-top-identifier=?
      ((_ (- a b))
       (syntax '(subtract a b)))
      ((_ (op a b))
       (syntax '(op a b)))))
  )

Matthew