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Re: [PATCH] Preserve keyword in 'syntax-rules' and 'define-syntax-rule'

From: Alex Shinn
Subject: Re: [PATCH] Preserve keyword in 'syntax-rules' and 'define-syntax-rule'
Date: Thu, 11 Oct 2012 11:02:57 +0900

On Thu, Oct 11, 2012 at 5:41 AM, Ludovic Courtès <address@hidden> wrote:
> Hi,
> Mark H Weaver <address@hidden> skribis:
>> Unfortunately, preserving the macro keyword breaks one of Oleg
>> Kiselyov's macros, namely 'ppat' in system/base/pmatch.scm:
> [...]
>> Oleg's macro uses '_' in the keyword position of the pattern, even
>> though '_' is in the literals list.  Therefore, it fails to match
>> because 'ppat' does not match that literal.
> I would call it a bug in ‘ppat’.  However, the real question is how
> frequent that “bug” is.  If people have come to rely on the current
> behavior, then it may be more reasonable to stick to it.

This is not a bug.  R5RS states:

     The keyword at the beginning of the pattern in a <syntax rule> is
     not involved in the matching and is not considered a pattern
     variable or literal identifier.

R6RS forbids _ as a literal.  R7RS retains the R5RS ignoring
of the initial keyword, adds _ as a wildcard, but allows it to be
used as a literal.  So this code would only break in R6RS.


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