[antlr-interest] Ambiguity error in lexer generation

[Loring Craymer]

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
> 
>
_________________________________________________________________
> Gear up for Halo® 3 with free downloads and an
> exclusive offer. It’s our way of saying thanks for
> using Windows Live™.
>
http://gethalo3gear.com?ocid=SeptemberWLHalo3_WLHMTxt_2



      ____________________________________________________________________________________
Fussy? Opinionated? Impossible to please? Perfect.  Join Yahoo!'s user panel and lay it on us. http://surveylink.yahoo.com/gmrs/yahoo_panel_invite.asp?a=7