Mark H Weaver wrote:
> 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=?".
I think you are referring to this paragraph from the paper[1] (page 12):
Two identifiers that are bound-identifier=? are also
free-identifier=?, but two identifiers that are free-identifier=?
may not be bound-identifier=?. An identifier introduced by a macro
transformer may refer to the same enclosing binding as an identifier
not introduced by the transformer, but an introduced binding for one
will not capture references to the other.
> 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)
The expander in Ikarus/Vicare also returns this value.
> Racket reports (#f #f) for the same expression.
Racket is different because its expander implements a variant of phase
separation; if the whole form is evaluated at phase N, the "x" in "#'x"
should be searched among the bindings at phase N-1 (if any) (I am not
authoritative in how Racket works, there is always something that
escapes me). Your code works, but when you actually try to use the
identifiers for something:
#!r6rs
(import (rnrs))
(define-syntax doit
(lambda (stx)
(let* ((x 1) (s1 #'x)
(x 2) (s2 #'x))
#`(let ((#,s1 123))
#,s2))))
(doit)
$ plt-r6rs ~/var/tmp/proof.sps
/home/marco/var/tmp/proof.sps:7:23: x: identifier used out of context
in: x
context...:
/opt/racket/5.3.5/lib/racket/collects/r6rs/run.rkt: [running body]
while the same program works fine in Ikarus, Vicare, Sagittarius and
Guile (Larceny's opinion would be interesting, but I do not have it
installed). IMHO this program should work for Racket, too, but maybe it
refuses to run code that "looks wrong" (indeed, usually, in a correct
program we do not define identifiers this way).
I dunno how Guile's evolution of psyntax works, but the two #'x must
be bound-identifier=? because the following result must stand:
(define-syntax doit
(lambda (stx)
(let* ((x 1) (s1 #'x)
(x 2) (s2 #'x))
#`(let ((#,s1 123))
#,s2))))
(doit) => 123
IMHO it is an error in the paper. Some paragraphs from the paper
preceding "the one" have been recycled in the R6RS document, but this
one paragraph has not; maybe this means something.
HTH
[1] <http://www.cs.indiana.edu/~dyb/pubs/LaSC-5-4-pp295-326.pdf>
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