Oracle grammar ambiguities and conflicts

On dealing with non-reserved words as keywords, our moderator wrote:


>>[That's a famous problem. There's a variety of less than fabulous
>>solutions, hacks in the lexer to guess when to return a keyword and
>>when to return a symbol, parser backup if it gags on a keyword, or
>>bushy parsers that put all of the allowed keywords into the grammar
>>in places where they can be used as symbols. -John]




> 1) "parser backup if it gags on a keyword" -- not feasible in
> bison/flex (to my knowledge), and most tools capable of producing backups
> use exception based processing, which results in unacceptable performance
> for my task.


I have actually used a solution that does precisely this. The "trick" is
that you essentially need to copy the parts of the bison skeleton that do
the reduce-actions to find out if your keyword is applicable. (you need to
check acceptable tokens to shift for every state that is reachable without
shifting a token to the stack) This can be done as a sort of intermediate
stage between your lexer and parser.


Unfortunately for you I don't have the code anymore. Another poster to
this newsgroup gave to me, and I don't recall many details about it,
except that it worked well but was seriously hard to read (but then again,
it was most likely copied directly from the seleton file bison uses, and
that is, not optimized for human readability)


The thread about this can be found here: (no code though)
http://compilers.iecc.com/comparch/article/96-08-058




--
Mark Thiehatten

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