Re: LR (k) vs. LALR

kamalp@acm.org (Kamal R. Prasad)
15 Aug 2004 22:18:30 -0400

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From: kamalp@acm.org (Kamal R. Prasad)
Newsgroups: comp.compilers
Date: 15 Aug 2004 22:18:30 -0400
Organization: http://groups.google.com
References: 04-08-037 04-08-055 04-08-073
Keywords: LALR, parse

Jean-Marc Bourguet <jm@bourguet.org> wrote in message news:04-08-073...
> "Tim Bauer" <tbauer@cadrc.calpoly.edu> writes:
>
> > > [Some grammars are easier to express with more than one token of lookahead.
> > > You can rewrite gramars to LR(1), but sometimes at the cost of huge and
> > > ugly bloat. -John]
> >
> > Didn't Knuth prove that any LR(k) grammar can be rewritten to LR(1), albeit
> > at a potential exponetial increase in the parse tables (number of distinct
> > parse items).
> > However, does this extend to an LALR(k) conversion to LALR(1)?
>
You reduce from LR(k) to LR(1), and then the LR(1) grammar is handled
by the LALR(1) parser generator.


> Every language for which a LR(1) grammar LR(1) exists has also an
> LALR(1) grammar. (Search for the archive of this group, I started a
> thread on the subject).
>
The BNF remains the same as in LR(1), but the number of parser states
is reduced in an LALR(1) parser. Either Im making a mistake or the
text above is misleading to indicate that the grammar needs to be
changed when moving from Lr(1) to LALR(1).
[The moderator may want to filter out erroneous statements].


> > > I have a grammar that requires more than one token of look ahead,
> > > is there any way that it could be solved using yacc or Bison?
> >


By definition -no. Yacc [or its gnu equivalent Bison] does not support
more than one lookahead. Feeding such a grammar to it will result in a
reduce-reduce conflict. You need to modify your grammar to reduce
those conflicts and yet achieve the purpose.


regards
-kamal


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