* bug#16158: psyntax: bug in bound-identifier=? @ 2013-12-16 0:04 Mark H Weaver 2013-12-16 0:11 ` Mark H Weaver 2013-12-16 7:49 ` Marco Maggi 0 siblings, 2 replies; 5+ messages in thread From: Mark H Weaver @ 2013-12-16 0:04 UTC (permalink / raw) To: 16158 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 ^ permalink raw reply [flat|nested] 5+ messages in thread
* bug#16158: psyntax: bug in bound-identifier=? 2013-12-16 0:04 bug#16158: psyntax: bug in bound-identifier=? Mark H Weaver @ 2013-12-16 0:11 ` Mark H Weaver 2013-12-16 7:49 ` Marco Maggi 1 sibling, 0 replies; 5+ messages in thread From: Mark H Weaver @ 2013-12-16 0:11 UTC (permalink / raw) To: 16158 Fixed in stable-2.0, commit 70c74b847680d3b239e591afa2e99c51a712980c Mark ^ permalink raw reply [flat|nested] 5+ messages in thread
* bug#16158: psyntax: bug in bound-identifier=? 2013-12-16 0:04 bug#16158: psyntax: bug in bound-identifier=? Mark H Weaver 2013-12-16 0:11 ` Mark H Weaver @ 2013-12-16 7:49 ` Marco Maggi 2013-12-16 16:38 ` Mark H Weaver 2013-12-17 4:03 ` Mark H Weaver 1 sibling, 2 replies; 5+ messages in thread From: Marco Maggi @ 2013-12-16 7:49 UTC (permalink / raw) To: Mark H Weaver; +Cc: 16158 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> -- "Now feel the funk blast!" Rage Against the Machine - "Calm like a bomb" ^ permalink raw reply [flat|nested] 5+ messages in thread
* bug#16158: psyntax: bug in bound-identifier=? 2013-12-16 7:49 ` Marco Maggi @ 2013-12-16 16:38 ` Mark H Weaver 2013-12-17 4:03 ` Mark H Weaver 1 sibling, 0 replies; 5+ messages in thread From: Mark H Weaver @ 2013-12-16 16:38 UTC (permalink / raw) To: marco.maggi-ipsu; +Cc: 16158 Hi, Marco Maggi <marco.maggi-ipsu@poste.it> writes: > 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. Yes. >> 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. I think that indicates a bug in Ikarus/Vicare. >> 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 don't see how that's relevant to this example. > 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) Whether #`(let ((#,s1 123)) #,s2) works is equivalent to asking whether s1 and s2 are 'bound-identifier=?', by definition. That's precisely what 'bound-identifier=?' is supposed to be used for: to determine whether a binding for one should capture the other. I don't see why you think #`(let ((#,s1 123)) #,s2) should work. Why would you use two identifiers with different binding names (s1 and s2) to construct that code? Can you construct a more realistic example? Thanks, Mark ^ permalink raw reply [flat|nested] 5+ messages in thread
* bug#16158: psyntax: bug in bound-identifier=? 2013-12-16 7:49 ` Marco Maggi 2013-12-16 16:38 ` Mark H Weaver @ 2013-12-17 4:03 ` Mark H Weaver 1 sibling, 0 replies; 5+ messages in thread From: Mark H Weaver @ 2013-12-17 4:03 UTC (permalink / raw) To: marco.maggi-ipsu; +Cc: 16158 Marco Maggi <marco.maggi-ipsu@poste.it> writes: > 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. Interesting. I looked closer, and found this in the R6RS definition of 'bound-identifier=?': Operationally, two identifiers are considered equivalent by bound-identifier=? if and only if they have the same name and same marks (section 12.1). I also found this in the R6RS errata: § 12.1 The remark "An algebra that defines how marks and substitutions work more precisely is given in section~2.4 of Oscar Waddell's PhD thesis." is somewhat misleading and should be qualified as follows: "Note, however, that Waddell's thesis describes slightly different semantics for bound-identifier=? - it specifies that for two identifiers to be equal in the sense of bound-identifier=?, they must have the same marks and be equal in the sense of free-identifier=?, whereas this report requires instead that they must have the same marks and have the same name." I guess that Kent Dybvig changed his mind about how 'bound-identifier=?' should behave. I don't fully understand the issues, so I'm inclined to go along with the R6RS definition. Therefore, I've reverted 70c74b847680d3b239e591afa2e99c51a712980c. Thanks, Mark ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2013-12-17 4:03 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2013-12-16 0:04 bug#16158: psyntax: bug in bound-identifier=? Mark H Weaver 2013-12-16 0:11 ` Mark H Weaver 2013-12-16 7:49 ` Marco Maggi 2013-12-16 16:38 ` Mark H Weaver 2013-12-17 4:03 ` Mark H Weaver
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).