ANTLR v4 : How to implement deactivation of rule alternatives in listener like embedded semantic predicate in grammar file?

Question

I have a java target v4 ANTLR grammar. I want to implement the same functionality of embedded semantic predicate using a listener to free my grammar from language specific embedded actions. The propose is to deactivate an alternate sub rule matching. I know how to write an extend BaseListener and overide that but really do not know how to do this since I am a beginner.

grammar MyParserGrammar;
@parser::members {
    public static boolean singularSub, pluralSub;
    }
sentence: (subject beVerb)+
            {
            singularSub=false;
            pluralSub=false;
            }
            ;
subject: singularSub {singularSub=true;}|
         pluralSub {pluralSub=true;};
singularSub : 'He';
pluralSub : 'They';
beVerb: {singularSub}? 'is'|
        {pluralSub}? 'are';
 WS: [ \t\r\n]->skip;

The exact part I want to sift and very hard for me is:

beVerb: {singularSub}? 'is'|
        {pluralSub}? 'are';

My Listener

public MyGListener extends MyParserGrammarBaseListener{
        @Override 
        public void exitBeVerb(MyParserGrammarParser.BeVerbContext ctx) {

        }
}

Answer 1

You could do something like this:

sentences
 : sentence+ EOF
 ;

sentence
 : subject beVerb
 ;

and then override the enterSentence(...) method and inspect the subject and beVerb from it:

class MyGListener extends MyParserGrammarBaseListener {

    @Override
    public void enterSentence(MyParserGrammarParser.SentenceContext ctx) {

        boolean isPluralSubject = ctx.subject().getText().equals("They");
        boolean isPluralVerb = ctx.beVerb().getText().equals("are");

        if (isPluralSubject != isPluralVerb) {
            // throw an exception?
        }
    }
}

Note that for parsing real human languages, ANTLR is not a good fit. In such cases, look into using something like Stanford's Natural Language Processing tools: https://nlp.stanford.edu/software