Re: recursive-descent error recovery

In article <655@ima.ISC.COM> chuck@amdahl.amdahl.com (Charles Simmons) writes:
| I claim it is impossible to write a reasonable compiler
| using YACC. I base this claim on the fact that ... it is
| impossible to produce reasonable error messages using YACC.


Well, it's certainly difficult, but that's not because YACC is a
bottom-up parser. It's because you're trying to do the error recovery
by hand. Bernard Dion's thesis work at Wisconsin (with Charlie
Fischer) demonstrated how to perform locally least-cost *automatic*
error *correction* in bottom-up parsers, with modest space and
negligible time overhead. [Compiler writer gives the parser generator
a table of insertion and deletion costs for tokens; error corrector
makes the least cost set of insertions and deletions that allows one
more real token to be shifted.] Good quality corrections make pretty
good error messages. Dion's techniques are incorporated in the ECP
(error correcting parser) package distributed by Fischer's group.
--
Michael L. Scott
University of Rochester (716) 275-7745
scott@cs.rochester.edu scott%rochester@CSNET-RELAY
{decvax, allegra, seismo, cmcl2}!rochester!scott


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