Re: How is ambiguity defined?

Ambiguous in JayParser, since there would be multiple parse trees in the parse forest.

Thanks,
Tom
On 5 Jan 2022, 3:21 PM +0000, Steven Pemberton <steven.pemberton@cwi.nl>, wrote:
>
>
> On Wednesday 05 January 2022 13:35:30 (+01:00), Norm Tovey-Walsh wrote:
>
> > Apologies for what may be a naive question.
> >
> > Consider this grammar:
> >
> > lnnl: "a", number, number, "a" .
> > -number: oddnumber ; evennumber .
> > -oddnumber: "1" ; .
> > -evennumber: "2" ; .
> >
> > The grammar is ambiguous in that there are multiple nonterminals that
> > can match nothing.
> >
> > But there is only one possible parse for “a12a”. I assume that we don’t
> > report any ambiguity in this case.
> Correct.
>
> > There are several possible parses for “aa”, but they’ll all produce
> > exactly the same XML serialization. Does that count as ambiguous?
>
> As Michael recently remarked, this is angels dancing on a pin territory, and I believe my implementation says it's ambiguous, and his says not (but I may be wrong in this particular case).
>
> In your case, the serializations being the same is thanks to the "-" before the rules. Take them away, and you see that the parses are truly ambiguous, and it is that that is reported as an error.
>
> But is "matching nothing" any different from "matching a character"? If I replace the nothings with "0" in your grammar:
>
> lnnl: "a", number, number, "a" .
> -number: oddnumber ; evennumber .
> -oddnumber: "1" ; "0".
> -evennumber: "2" ; "0".
>
> is "a00a" ambiguous? Sure it is.
>
> Steven
>
>