Re: Intricate problem with scannerless LALR(1) parser

"Paul B Mann" <paul@paulbmann.com>
Thu, 26 Jun 2008 23:14:08 -0600

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From: "Paul B Mann" <paul@paulbmann.com>
Newsgroups: comp.compilers
Date: Thu, 26 Jun 2008 23:14:08 -0600
Organization: Compilers Central
References: 08-06-010 08-06-063 08-06-066
Keywords: parse
Posted-Date: 27 Jun 2008 00:40:57 EDT

>>>start: a
>>>a: b 'XX'
>>>b: c | '[' b ']'
>>>c: 'X' | c 'X'
>
>> XX is valid, but ambiguous, either a keyword or two X's.


> Are you sure it is ambiguous? Maybe it isn't LALR(1), though.
> Since there is no a in b or c, it looks to me that it only matches
> strings ending in 'XX'. If b is 'XX' then it still must be followed
> by another 'XX'.


You are right. I was wrong.


X XX is valid
XX XX is valid
XXX XX is valid.


The parser needs 2 characters of lookahead to correctly
parse this language, depending on how spaces are processed.




Paul B Mann

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