[antlr-interest] How to best resolve ambiguity?

[Martin Traverso]

Ok, I did some testing, and antlr seems to be doing the natural thing even
without explicitly setting k=1. What threw me off was that when I compiled
the grammar I got the following errors:

test.g:6:7: Decision can match input such as "'+'" using multiple
alternatives: 1, 2
As a result, alternative(s) 2 were disabled for that input
test.g:7:7: Decision can match input such as "'*'" using multiple
alternatives: 1, 2
As a result, alternative(s) 2 were disabled for that input

But looking at the generated code it doesn't seem like any alt was disabled.

If I change the (..)* to (..)?, antlr does seem to disable the alt by
converting the (..)? to (..). But then, setting k=1 as suggested will have
no effect whatsoever.


On a related note, any idea why antlr complains with "2:16: The following
alternatives are unreachable: 2" for the following grammar? A bug, maybe?

grammar T;
a    :  'x' ('+' a)? ;

BTW, this is with the latest version in perforce.

Martin

On 12/30/05, Terence Parr <parrt at cs.usfca.edu> wrote:
>
>
> On Dec 30, 2005, at 5:06 PM, Martin Traverso wrote:
>
> > Howdy,
> >
> > What is the best way to resolve the ambiguities caused by 'D' a in
> > rule c in the following grammar?
> >
> > grammar T;
> > a: b ('+' b)*;
> > b: c ('*' c)*;
> > c: 'D' a
> >    | 'x';
> >
> Which rule has the ambiguity?  All the (...)* loops, right?  That
> unterminated 'a' in 'c' will cause trouble...a true ambiguity I
> think.  That said, set k=1 in the (...)* subrules and it will
> greedily and happily do the "natural" thing I think.
>
> Ter
>
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