[antlr-interest] Way to send two different trees based on input to rule?

[Terence Parr]

On Nov 25, 2006, at 5:47 AM, Brian DeVries wrote:

> Hello all,
>
> I have an ANTLR v3 parser rule that takes a boolean parameter, does  
> some stuff, and outputs (part of) an AST. When that parameter is  
> true, I want it to output a certain node in the tree, and something  
> else when it's false.

a : A -> {p}? x
         -> y
    ;

That should be in the manual...

Ter
> Is there a way to do this? I'm thinking semantic predicates, but  
> the v2 documentation from the website states that predicates in the  
> middle of the rule are validating, and I couldn't get the  
> disambiguating one to work on another rule a few days ago (it was  
> still treating it as a validating predicate).
>
> Toy example (should be syntactically right):
>
> rule [boolean b]
>     :    X y  -> ^(R X y)
>     ;
>
> I'm wanting something like
>
> rule [boolean b]
>     :    X y /* if $b == true */ -> ^(R X y)
>                /* else */            -> ^(R Z y)
>     ;
>
> Thanks!
> ~Brian