When to put dollar sign $ at the reference to symbol in ANTLR?

Question

I have both books by T.Parr about ANTLR and I see dollar sign all over with references to symbols. It work(ed) for me too:

term : IDENT -> { new TokenNode($IDENT) };

or something more complex:

type_enum : 'enum' name=IDENT '=' val+=IDENT (',' val+=IDENT)* ';' 
            -> { new EnumNode($name,$val) };

But this line gives me absurd error:

not_expr :  term 
         | NOT ex=not_expr -> { new UnaryExpression($NOT,$ex) };

The error says missing attribute access on rule scope: ex. You know what the fix is? Removing the dollar sign on "ex". That's it.

Out of curiosity I checked the mentioned rules (above) and removed the dollar sign -- they work as before (i.e. I don't get any error).

QUESTION: so what is this story with dollar sign? Should I not use it? Or should I use it until I get an error?

I would not ask this question, if I not saw this convention almost used as a standard in ANTLR.

Answer 1

QUESTION: so what is this story with dollar sign? Should I not use it? Or should I use it until I get an error?

It depends what you want to reference.

Understand that there are 3 different types of "labels":

1. name=IDENT, the label name references a CommonToken;
2. val+=IDENT, the label val references a List containing CommonToken instances, in this case;
3. ex=not_expr the label ex references a ParserRuleReturnScope

I recommend always using a $. I don't know if it is by design that NOT ex=not_expr -> { new UnaryExpression($NOT,$ex) }; doesn't work, but to get a hold of whatever not_expr matched, I'd simply do this:

not_expr : term 
         | NOT ex=not_expr -> { new UnaryExpression($NOT, $ex.tree) }
         ;

I don't see why you'd want to get a hold of the entire ParserRuleReturnScope: the tree holds all the information you need.

HTH