Re: YACC with infinite lookahead

Chris F Clark <>
20 May 1999 01:47:52 -0400

          From comp.compilers

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From: Chris F Clark <>
Newsgroups: comp.compilers
Date: 20 May 1999 01:47:52 -0400
Organization: The World Public Access UNIX, Brookline, MA
References: 99-05-072
Keywords: yacc, parse

Daniele Benegiamo asked:

> Someone know if there exists a version of YACC with infinite lookahead?
> I'm writing a compiler for a new language and I have found that LALR(1)
> implemented by Yacc needs too adjustments in the grammar that make it
> too unreadable.
> I have tried to use PCCTS but his syntax is too complex and the problem
> of readability is not resolved. Instead the syntax of Yacc is ok but
> have the lookahead problem.

As of 2.3 (or perhaps it was 2.2) Yacc++(R) supports syntactic
predicates for infinite lookahead (via controlled backtracking). The
rest of the notation is essentially yacc extended with regular
expressions. Note that it is a commercial product and not public
domain, free software, or shareware though.

Visual Parse++ has a "natural language" mode that appears to be
Tomita/Lang style parsing (simulating an Earley parser using LR tables
and generating "shared parse forests"). Again, it is a commercial

The tool Btyacc applies backtracking rules to yacc grammars to achieve
"infinite lookahead". I don't remember whether it is commercial or not.

Disclaimer, I helped write Yacc++.

Hope this helps,

Chris Clark Internet :
Compiler Resources, Inc. CompuServe : 74252,1375
3 Proctor Street voice : (508) 435-5016
Hopkinton, MA 01748 USA fax : (508) 435-4847 (24 hours)
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