[antlr-interest] Confusing, hopefully-final, problems

[Sam Barnett-Cormack]

Sam Barnett-Cormack wrote:
> Hey all,
> 
> With Gavin's generous help, I'm down to one error and a couple of 
> warnings, which is nice.
> 
> In the attached grammar, I haven't fixed the lexer issues Gavin 
> identified yet, as I'm focused on the parser for the moment.
> 
> The error is on line 473, as are two of the warnings. I can't say as I 
> can figure out what it's getting at all, but I'm probably just being dumb.
> 
> The rule is
> 
> extensionAndException : '...' exceptionSpec? ;
> 
> and exceptionSpec is:
> 
> exceptionSpec : ('!' exceptionIdentification)? ;
> 
> exceptionIdentification : signedNumber | definedValue | type ':' value ;
> 
> The error and warnings are:
> 
> warning(200): workspace/asn.1-parse/grammars/ASN_1.g:346:82: Decision 
> can match input such as "'('" using multiple alternatives: 1, 2
> As a result, alternative(s) 2 were disabled for that input
> warning(200): workspace/asn.1-parse/grammars/ASN_1.g:473:31: Decision 
> can match input such as "'}'" using multiple alternatives: 1, 2
> As a result, alternative(s) 2 were disabled for that input
> warning(200): workspace/asn.1-parse/grammars/ASN_1.g:473:31: Decision 
> can match input such as "','" using multiple alternatives: 1, 2
> As a result, alternative(s) 2 were disabled for that input
> error(201): workspace/asn.1-parse/grammars/ASN_1.g:473:31: The following 
> alternatives can never be matched: 2
> 
> My best guess is that the problem comes from "type" in the 
> exceptionIdentification rule, but then I'd expect the error to be there, 
> and not in extensionAndException.

No, Sam, it's because a rule is referenced as optional, and that rule 
can match nothing as well, so there's no point it being optional...

So I'm now down to a warning about

type : ((typeWithConstraint)=>typeWithConstraint | builtinType | 
referencedType) constraint* ;

And a load of parser warnings and errors that it wasn't bothering to 
generate before... *sigh*

Sam