[antlr-interest] Is ANTLR Hallucinating?

[Randall R Schulz]

On Monday 25 June 2007 12:04, Wincent Colaiuta wrote:
> El 25/6/2007, a las 20:33, Randall R Schulz escribió:
> > Hi,
> >
> > I have this production in an ANTLR grammar:
> >
> > clifFile
> >
> >   : clText * EOF
> >
> >   ;
> >
> >
> > It elicits this error from ANTLR:
> >
> > warning(200): CLIF.g:1371:4: Decision can match input such as
> > "{EOF, Open}" using multiple alternatives: 1, 2
> > As a result, alternative(s) 2 were disabled for that input
> > warning(201): CLIF.g:1371:4: The following alternatives are
> > unreachable: 2
> >
> >
> > Why is it talking about alternatives in a rule that has none?
>
> Try looking at the syntax diagram in ANTLRWorks so that you can see
> the two alternative paths.

Of course, there are not two paths. Just the loop back over the closed 
clText production followed by the EOF token.


> ...
>
> Without seeing your entire grammar I can't comment on what the
> problem is but ANTLR evidently thinks there is more than one way to
> match rule "clText" zero or more times followed by an EOF. That is,
> although there are no explicit alternatives in your "clifFile" rule,
> the ambiguity creeps in anyway in the other rule(s) referenced from
> that rule.

It's too large a grammar to include in toto.

The grammar is somewhat Lisp-like (or KIF like, if you're familiar with 
that language), so all the top-level constructs begin with an open 
parenthesis (the Open token in the rule I showed).

It's possible this is the consequence of some other ambiguities I have 
yet to resolve. The people who wrote the BNF from which this ANTLR 
grammar is being derived are not accustomed to dealing with parser 
generators, I don't think.


> Cheers,
> Wincent


Randall Schulz