How to avoid building intermediates and useless AST nodes with ANTLR3?

Question

I wrote an ANTLR3 grammar subdivided into smaller rules to increase readability. For example:

messageSequenceChart:
  \'msc\' mscHead bmsc \'endmsc\' end
;

         


        

Answer 1

You can solve this by letting your sub-rule(s) return multiple values and accessing only those you're interested in.

The following demo shows how to do it. Although it is not in C, I am confident that you'll be able to adjust it so that it fits your needs:

grammar Test;

parse
  :  sub EOF {System.out.printf("second=\%s\n", $sub.second);}
  ;

sub returns [String first, String second, String third]
  :  a=INT b=INT c=INT
     {
       $first = $a.text;
       $second = $b.text;
       $third = $c.text;
     }
  ;

INT
  :  '0'..'9'+
  ;

SPACE
  :  ' ' {$channel=HIDDEN;}
  ;

And if your parse the input "12 34 56" with the generated parser, second=34 is printed to the console, as you can see after running:

import org.antlr.runtime.*;

public class Main {
  public static void main(String[] args) throws Exception {
    TestLexer lex = new TestLexer(new ANTLRStringStream("12 34 56"));
    TokenStream tokens = new TokenRewriteStream(lex);
    TestParser parser = new TestParser(tokens);
    parser.parse();
  }
}

So, a shortcut from the parse rule like $sub.INT, or $sub.$a to access one of the three INT tokens, in not possible, unfortunately.