Question

I am trying to create rewrite rule rewriting node’s text to method’s return value.

But when i use rewriting rule parser parses only first node and then stops without any exception. Without rewrite-rule parser works fine.

Here is my not working example:

grammar test01;

options
{
    output=AST; 
}

@members{
  public String MyTestFun(){
    return "test";
  }
}

test : id+;

   id   : ID -> {MyTestFun()}; // With this rule only first node is parsed
// id   : ID -> {"test"};      // and with this rule too

ID   : ('a'..'z')+;

So question is how to create rewrite rule rewriting node text to method’s value?

Answer 1

You can’t put a String inside the right of a rewrite arrow -> between the { ... }. It needs to be an instance of a Tree (CommonTree is the default).

This works:

grammar test01;

options {
  output=AST;
}

@members{
  public CommonTree myTestFun(){
    CommonToken token = new CommonToken(ID, "test");
    return new CommonTree(token);
  }

  public static void main(String[] args) throws Exception {
    test01Lexer lexer = new test01Lexer(new ANTLRStringStream("foo"));
    test01Parser parser = new test01Parser(new CommonTokenStream(lexer));
    CommonTree tree = (CommonTree)parser.test().getTree();
    System.out.println(tree.toStringTree());
  }
}

test : id+;
id   : ID  -> {myTestFun()};

ID   : ('a'..'z')+;

If you now run the demo by doing:

java -cp antlr-3.3.jar org.antlr.Tool test01.g
javac -cp antlr-3.3.jar *.java
java -cp .:antlr-3.3.jar test01Parser

(on Windows, the last command is: java -cp .;antlr-3.3.jar test01Parser)

the output test is printed to the console.

Note that if you change the return type of myTestFun() to CommonToken, you will get a class cast exception:

Exception in thread “main” java.lang.ClassCastException: org.antlr.runtime.CommonToken cannot be cast to org.antlr.runtime.tree.Tree

Whenever you have output=AST inside your options { ... }, you must use an instance of a Tree.