On 25 Apr 2007, at 17:39, Evan Lavelle wrote:
I'm just about to write a yylex wrapper to let me push back a token
so that Bison can re-read it. Before I do this, can anyone tell me
if there's a better way to handle this?
My problem is that I need to write a partial parser, which ignores
large chunks of a language. The chunks are delimited by known
tokens, so I need to do something like
ignore_block
: STARTBLOCK {
// check yylex input for ENDBLOCK, push it back
} ENDBLOCK ;
ie. something like a flex exclusive start state, but initiated from
Bison.
You can still alter the lexer start conditions from the Bison
generated parser. Make a variable that Bison can alter, and which the
lexer checks every time it is being called (by some code right before
the rules in the .l file). The biggest problem is that the parser may
or may not need a token for lookup. So it is best to put lexer state
changes before a token in the parser .y grammar. Then check that it
works in practice (that is, the context changes before any new tokens
are being read), as the LALR(1) parsing algorithm that Bison uses is
somewhat unpredictable.
The problem you mention, making a partial parser, has been discussed
in this list before, I think a couple of times. I do not immediately
recall when. But perhaps you mighth find it by a search in the archives.
Hans Aberg
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