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Illogical parser hang

From: Karel Kulhavy
Subject: Illogical parser hang
Date: Tue, 9 Oct 2007 12:12:24 +0200
User-agent: Mutt/1.5.12-2006-07-14


I wrote a simple grammar that has only one symbol 'a' and a reduce reduce
conflict. Yet, if I feed a single 'b' as a text to the input, the parser
generated by Bison hangs. Shouldn't it be immediately obvious to the parser
that if there is no 'b' in the grammar and the first token he gets on the input
is 'b' that the input is invalid?

See attached pinnacle.tgz, unpack it and enter. Then type "make". It will
demonstrate the hang.

How I came to the problem - I am writing a BASIC interpreter. I discovered it
hangs if someone accidentally types ';' after a command even when ';' is an
invalid token in the BASIC and is never used in the grammar. I simplified the
compiler as much as was possible while it still hangs and got this.


Attachment: pinnacle.tgz
Description: application/tar-gz

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