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Re: Erroneous YYBACKUP skeleton

From: David Kastrup
Subject: Re: Erroneous YYBACKUP skeleton
Date: Tue, 11 Oct 2011 21:07:50 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.50 (gnu/linux)

David Kastrup <address@hidden> writes:

> "Joel E. Denny" <address@hidden> writes:


>>> I have not checked that the enhancement for reducing rules with more
>>> than one token does actually work.  It seems like an obvious change,
>>> but I don't really understand Bison enough to be sure.
>> Does this mean you haven't found need for that enhancement then?
> Up to now, I have been just doing basic tests and trying to figure out
> why I could not get stuff to work at all.  It seems one should always
> be able to program around the missing enhancement by creating
> one-element rule aliases to all multi-element rules one wants to back
> up.

Update: no, that does not work, since backing up only moves backwards
one reduction, so putting an alias in between will not help.  It would
seem, however, that YYPOPSTACK (yylen) is the wrong thing to do: when
the action is executed, apparently the stack has already been reduced to
have one lefthand value left.  So YYPOPSTACK (1) seems correct
regardless of the value of yylen (possibly excepting yylen==0 though I
don't think this can ever occur).

David Kastrup

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