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

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.

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.