[antlr-interest] How can a syntactic predicate not be satisfied if the input matches it?

[Martijn Reuvers]

Hello Gavin,

It may have something to do with a token that is the 'largest' token
which is consumed first and automatically 'enforces' the other rule
(making the predicate fail).

E.g. this very simple grammer:

grammar Test;

a: (r1)=>r1
   | r2;

r1 : Y Z;

r2: X;

X: 'ab';

Y: 'a';

Z: 'b';

If you provide this grammar with 'ab'. This will always end up in r2,
as the token X is always matched first (returned by the lexer). That's
how the antlrs lexer works by default.

Regards,
Martijn


On Sat, Dec 17, 2011 at 1:54 AM, Gavin King <gavin.king at gmail.com> wrote:
> Hi folks,
>
> I have a large grammar with a rule like this:
>
>   z: (x)=>x | y
>
> And I have some input that I know matches x. I know this for sure,
> because when I change z to read:
>
>   z: x
>
> Then ANTLR parses that input successfully with no errors.
>
> To my surprise, the input does not match z, and the errors generated
> come from attempting to match y. So apparently the syntactic predicate
> (x)=> failed to match the input, even though I'm quite certain that x
> does match it. I even tried changing z to:
>
>    z options { backtrack=true; } : x | y
>
> which, unsurprisingly, failed to help.
>
> What could possibly be the cause of this?
>
> Thanks,
> Gavin
>
> --
> Gavin King
> gavin.king at gmail.com
> http://in.relation.to/Bloggers/Gavin
> http://ceylon-lang.org
> http://hibernate.org
> http://seamframework.org
>
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