Reserved pattern symbols break when a binding of the same name exists

[Moritz Heidkamp]

When reserved pattern symbols like ? or _ are bound in the lexical environment of a match expression, they don't work anymore:

(import matchable)

(let ((? 1))
  (match '(x y)
    ((? odd?) (print ?))))

(match '(x y)
  ((_ _) (print 'ok)))

(let ((_ 1))
  (match '(x y)
    ((_ _) (print 'ok))))

Output:

$ csc matchbug.scm && ./matchbug
x
ok

Error: (match) no matching pattern

	Call history:

	matchbug.scm:5: print	  
	matchbug.scm:8: print	  
	matchbug.scm:10: failure391	  
	matchbug.scm:10: error	  	<--

[Moritz Heidkamp]

Just for the record: The same happens with Chibi 0.7.

[Alex Shinn]

This is basically just hygiene working as expected, and gives
that result for the same reason that

(let ((else #f))

(cond (else #t) (#t #f)))

returns #f.

Admittedly this is one of those cases where it's not useful to
match the operator position id against the current lexical scope
and one might be tempted to deliberately match unhygienically.
However, this may change in future versions. Moreover,
other macros which expand into match may depend on the
fact that inserted identifiers match hygienically.

[Moritz Heidkamp]

Indeed, we just discussed this issue on IRC and came to the same conclusion. I guess it's just a fact of life^{WScheme we have to live with, then. Perhaps it would be worthwhile to add a caveat section about this to the manual?}

Anyhow, thanks for the quick response!