Question

I’m trying to learn to use ANTLR, but I cannot figure out what’s wrong with my code in this case. I hope this will be really easy for anyone with some experience with it. This is the grammar (really short).

grammar SmallTest;

@header {
package parseTest;
import java.util.ArrayList;
}

prog returns [ArrayList<ArrayList<String>> all]
    :(stat { if ($all == null)
               $all = new ArrayList<ArrayList<String>>();
             $all.add($stat.res);
           } )+
    ;

stat returns [ArrayList<String> res]
    :(element  { if ($res == null)
                   $res = new ArrayList<String>();
                 $res.add($element.text);
               } )+ NEWLINE
    |   NEWLINE
    ;

element: ('a'..'z'|'A'..'Z')+ ;
NEWLINE:'\r'? '\n' ;

The problem is that when I generate the Java code there are some empty if conditions, and the compiler displays an error because of that, I could edit that manually, but that would probably be much worse. I guess something is wrong in this.

Sorry for asking, this has to be really stupid, but my example is so similar to those in the site that I cannot imagine a way to atomize the differences any more.

Thank you very much.

Answer 1

You should put the initialization of your lists inside the @init { ... } block of the rules, which get executed before anything in the rule is matched.

Also, your element rule should not be a parser rule, but a lexer rule instead (it should start with a capital!).

And the entry point of your parser, the prog rule, should end with the EOF token otherwise the parser might stop before all tokens are handled properly.

Finally, the @header { ... } section only applies to the parser (it is a short-hand for @parser::header { ... }), you need to add the package declaration to the lexer as well.

A working demo:

SmallTest.g

grammar SmallTest;

@header {
package parseTest;
import java.util.ArrayList;
}

@lexer::header {
package parseTest;
}

prog returns [ArrayList<ArrayList<String>> all]
@init {$all = new ArrayList<ArrayList<String>>();}
  :  (stat {$all.add($stat.res);})+ EOF
  ;

stat returns [ArrayList<String> res]
@init {$res = new ArrayList<String>();}
  :  (ELEMENT {$res.add($ELEMENT.text);})* NEWLINE
  ;

ELEMENT : ('a'..'z'|'A'..'Z')+ ;
NEWLINE : '\r'? '\n' ;
SPACE   : ' ' {skip();};

Main.java

package parseTest;

import org.antlr.runtime.*;

public class Main {
  public static void main(String[] args) throws Exception {
    SmallTestLexer lexer = new SmallTestLexer(new ANTLRStringStream("a bb ccc\ndddd eeeee\n"));
    SmallTestParser parser = new SmallTestParser(new CommonTokenStream(lexer));
    System.out.println(parser.prog());
  }
}

And to run it all, do:

java -cp antlr-3.3.jar org.antlr.Tool parseTest/SmallTest.g 
javac -cp .:antlr-3.3.jar parseTest/*.java
java -cp .:antlr-3.3.jar parseTest.Main

which yields:

[[a, bb, ccc], [dddd, eeeee]]