On Tue, 3 March, 2009 9:07 pm, Jim Idle wrote:

Sam Barnett-Cormack wrote:

Jim Idle wrote:

Sam Barnett-Cormack wrote:

<SNIP>

I was trying to look at your grammar for you but the one you have
attached is missing some token defs:

I presume that you saw I sent you the corrected grammar off list right?


I saw it aftewards - my next task when I return to it this evening will be
to make sure I understand it. What do you think about the fact that
newlines are definitely not significant? Does that make the grammar
inherently ambiguous and one has to choose an interpretation to favour?

Sam

It is only inherently ambiguous because you have not encode the semantics within the grammar. The change I made probably does encode the correct semantics as when things are ambiguous like this, the 'reference parser', should there be such a thing, will be choosing one way or another (although really poor languages/parsers sometimes choose one way over the other inconsistently - too many examples to quote here) .

So, all I did with the predicates is force the parser to consume tokens in the longest match.

Say you can have:

a : r1 | r2;

r1 : A B?;

r2 : B;

Obviously the B can match in rule r1 or rule r2 (although ANTLR can deal with that, it is because your grammar is more complicated that it cannot in your case). So you have to define the behavior. Usually, you will want the B? to belong to r1:

r1 : A ((B)=>B)? ;

Which is the nature of the predicates I added to your grammar.

I finally found a version of the language spec for ASN.1 and I think that the predicates I gave you are correct, but I cannot guarantee either that, or that your grammar is complete and correct ;-)

Jim

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[il-antlr-interest: 22238] Re: [antlr-interest] Error I can't get my head around at all...

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