Subject: psyntax: bug in bound-identifier=?

From: Mark H Weaver <[email-masked]>
Date: Sun, 15 Dec 2013 19:04:04 -0500

While reading psyntax.scm, I noticed that the definition of 'bound-id=?'
does not match the definition in "Syntax Abstraction in Scheme" by
Dybvig, Hieb, and Bruggeman.

The paper states "Two identifiers that are bound-identifier=? are also
free-identifier=?".  The following expression shows that this is not the
case in Guile 2.0:

  (let* ((x 1) (s1 #'x)
         (x 2) (s2 #'x))
    (list (bound-identifier=? s1 s2)
          (free-identifier=? s1 s2)))
  => (#t #f)

Racket reports (#f #f) for the same expression.

According to the paper, two identifiers are 'bound-id=?' if and only if
they resolve to the same binding name (gensym) and have the same marks
(i.e. they were both introduced by the same macro instantiation, or
neither were introduced by a macro).  However, the implementation in
'psyntax.scm' does not compare the binding names (gensyms); it instead
compares only the symbolic names.

      Mark