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

[Brian DeVries]

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. 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
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