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
;

# Where mscHead is a shortcut to :
mscHead:
  mscName mscParameterDecl? timeOffset? end
  mscInstInterface? mscGateInterface
;

I know the built-in ANTLR AST building feature allows the user to declare intermediate AST nodes that won't be in the final AST. But what if you build the AST by hand?

messageSequenceChart returns [msc::MessageSequenceChart* n = 0]:
  'msc' mscHead bmsc'endmsc' end
  {
    $n = new msc::MessageSequenceChart(/* mscHead subrules accessors like $mscHead.mscName.n ? */
                                       $bmsc.n);
  }
;

mscHead:
  mscName mscParameterDecl? timeOffset? end
;

The documentation does not talk about such a thing. So it looks like I will have to create nodes for every intermediate rules to be able to access their subrules result.

Does anyone know a better solution ?

Thank you.

score 2 · Accepted Answer · answered May 17 '11 at 18:51

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.

Awesome ! It works ! :D You just made me save sooooo much time ! — Julio Guerra, May 17 '11 at 20:27
Another problem ! http://stackoverflow.com/questions/6047127/ignore-tokens-in-the-token-characters I believe in you :p — Julio Guerra, May 18 '11 at 15:22

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

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