Yacc shift/reduce conflicts

Martyn Weiss <anumber@compuserve.com>
19 Dec 2004 23:53:14 -0500

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From: Martyn Weiss <anumber@compuserve.com>
Newsgroups: comp.compilers
Date: 19 Dec 2004 23:53:14 -0500
Organization: CompuServe Interactive Services
Keywords: yacc, question
Posted-Date: 19 Dec 2004 23:53:14 EST


I'm updating a language grammar I wrote a while ago using MKS Yacc,
which consists of one or more statements separated by semicolons, i.e.

One of the things I want to do is to allow for superfluous embedded
and trailing semicolons, as in S1;;S2;

I had a bit if trouble expressing this in rules without getting
shift/reduce conflicts. I found that if I simplify the grammar down
to the following:

| statements semicolon
| statements semicolon statement

yacc reports no conflicts, but it looks a bit clumsy to me. However,
if I replace "someterminal" by "expression", which has a number of
alternative definitions (simpleexpression, term, factor, etc) I
immediately get a whole raft of shift/reduce conflicts, which leads me
to think the simplified version above isn't the best way to express
the "superfluous semicolons" point mentioned above.

Can anyone suggest a better formulation of "statements" or explain how
I go about getting rid of the s/r conflicts in the fuller version.

Cheers & TIA, Martyn (you can probably guess that this isn't my usual
subject, but it isn't homework, either)

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