antlr listener... which production?

Question

Consider:

my_rule: a_thing 'as' zoot
       | a_different_thing 'as' blurfl zoot
;

In my listener, how do I distinguish the particular production that was matched? Am I reduced to checking for the presence of blurfl, or is there a more elegant way?

Answer 1

Label the alternatives like this:

grammar T;

my_rule
 : a_thing 'as' zoot                  #AltWithoutBlurfl
 | a_different_thing 'as' blurfl zoot #AltWithBlurfl
 ;

...

That way, your listener will contain methods like these:

public interface TListener extends ParseTreeListener {

    void enterAltWithoutBlurfl(TParser.AltWithoutBlurflContext ctx);
    void exitAltWithoutBlurfl(TParser.AltWithoutBlurflContext ctx);
    
    void enterAltWithBlurfl(TParser.AltWithBlurflContext ctx);
    void exitAltWithBlurfl(TParser.AltWithBlurflContext ctx);

    ...
}

Note that there is no more enterMy_rule and exitMy_rule anymore.