Re: Bison Question: Ambiguous Grammar?

[Claus Brabrand]

Hans Aberg wrote:

> At 12:49 +0200 2005/05/06, Claus Brabrand wrote:
>
> > I have a question about Bisons GLR parser.
> >
> > Is there a way for Bison (in %glr-parser mode) to return a parse tree
> > (doesn't matter which) when parsing according to an ambiguous grammar?
>
> Bison never returns a parse tree. If you want that, you need to 
> define its construction in the actions. When people ask that 
> question, they often need much less; if you so want, you may precise 
> your programming situation.

Hans,

Thanks for your reply.

Yes, I realize there's no parse tree beyond what I may chose to 
construct in the action code myself.

What I meant is: is there a way for Bison to take the actions 
according to one (any) of the parses?

I am using Bison in GLR-mode for code-generating syntax-directed 
transformations (executed in
the action code). The grammars are highly ambiguous which is fine: I 
just want to execute actions
for one (any) of the parses.

I have a grammar which is capable of parsing a string (and returning 
a transformed string which is
just the result of executing the corresponding action code). Now, 
when I add a certain production to
this grammar, I get a parse error in the middle of the string it was 
able to parse without this
extra production(?!?) What I don't understand is how adding a 
production to a grammar G can cause it
to reject the string - it should just enlarge the language induced by 
the grammar - and so, that if it was
capable of parsing my string before, it should also be able to do so 
after (with the extra production).

This really puzzles me(?)

Is Bison's GLR algorithm intended for ambiguous grammars or will it 
only work on the subset of
grammars that are unambiguous (LALR(1) or not)? Maybe this particular 
production renders the
grammar ambiguous (at least wrt. my input string - i.e. that my 
string has two parses).

I would greatly appreciate any insights you might have!

Thanks,
/Claus.
--
Claus Brabrand, Ph.D.
address@hidden
http://www.daimi.au.dk/~brabrand/