[antlr-interest] Ambiguity error in lexer generation

[Loring Craymer]

Hmm.  ANTLR _is_ deterministic, so I suspect that not
getting error messages is due to losing them en route
to the display or not getting flushed on exit, not due
to different ANTLR behaviors.

--Loring

--- Alex Kinneer <kinneera at hotmail.com> wrote:

> I understand the warnings (at least I'm pretty sure
> I do), but I'm still not clear on why they are being
> reported, and more importantly why they are not
> being reported consistently. What I am trying to
> emphasize is that if I run antlr on the same grammar
> file multiple times, I sometimes get these warnings,
> and sometimes don't. And that seems like a bug to
> me. Either the lexer rules are ambiguous, or they
> aren't, right? So why would it sometimes say they
> are and sometimes not, when I'm just running antlr
> on the exact same grammar?
>  
> More importantly, I don't think the lexer rules even
> are ambiguous, except to the extent that antlr
> advertises it can resolve automatically without
> warning. For example, as best I can tell, the
> grammar doesn't specify anything more ambiguous than
> what the Java 1.5 grammar for antlr 3.0 does (e.g.
> how does antlr distinguish the keyword 'class' from
> an Identifier in that grammar), yet the Java 1.5
> grammar doesn't seem to produce any warnings (at
> least in that regard).
>  
> -Alex
> 
> 
> 
> > Date: Wed, 19 Sep 2007 10:12:44 -0700> From:
> lgcraymer at yahoo.com> Subject: Re: [antlr-interest]
> Ambiguity error in lexer generation> To:
> kinneera at hotmail.com; antlr-interest at antlr.org> >
> You have a probable error and two warnings here:>
> 1.) The analysis failure is probably an error
> issued> as a warning. You might have to disambiguate
> with a> synpred.> 2.) The two "disabled alternative"
> warnings tell you> that 'v' will be recognized as
> T22 and 'g' will be> recognized as T16. This is the
> type of resolution> that you were looking for.> >
> --Loring> > --- Alex Kinneer <kinneera at hotmail.com>
> wrote:> > > > > I'm hoping somebody can offer some
> insight as to why> > antlr would
> *nondeterministically* report lexer> > ambiguity
> warnings. That is to say, when I run the> >
> following commands:> > > > rm TestLang__.g> > rm
> *.class;> > java org.antlr.Tool TestLang.g> > > > It
> sometimes, but not always, generates warnings of> >
> this sort:> > > > warning(205): TestLang.g:1:8:
> ANTLR could not> > analyze this decision in rule
> Tokens; often this is> > because of recursive rule
> references visible from> > the left edge of
> alternatives. ANTLR will re-analyze> > the decision
> with a fixed lookahead of k=1. Consider> > using
> "options {k=1;}" for that decision and> > possibly
> adding a syntactic predicate.> > warning(209):
> TestLang.g:20:1: Multiple token rules> > can match
> input such as "'v'": T22, T24, T25,> >
> UNQUOTED_STRING, JAVA_ID> > As a result, tokens(s)>
> > JAVA_ID,UNQUOTED_STRING,T24,T25 were disabled for>
> > that input> > warning(209): TestLang.g:13:1:
> Multiple token rules> > can match input such as
> "'g'": T16, T18,> > UNQUOTED_STRING, JAVA_ID> > ...>
> > > > > > It is primarily the inconsistent reporting
> of these> > warnings that is very perplexing to me.
> There's also> > nothing obvious to me in the grammar
> that should be> > causing the warnings. There is a
> generic Java> > identifier lexer rule that could
> cause an ambiguity> > with a set of keywords, but it
> is my understanding> > that antlr should be able to
> resolve this ambiguity> > (by giving preference to
> the keywords). The unquoted> > string rule has a
> semantic predicate that should> > prevent
> ambiguity.> > > > Thanks,> > Alex> > > >>
>
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