Breaking head over how to get position of token with a rule - ANTLR4 / grammar

Question

I'm writing a little grammar using ANLTR, and I have a rule like this:

operation   : OPERATION (IDENT | EXPR) ',' (IDENT | EXPR);
...

OPERATION   : 'ADD' | 'SUB' | 'MUL' | 'DIV' ;
IDENT       : [a-z]+;
EXPR        : INTEGER | FLOAT;
INTEGER     : [0-9]+ | '-'[0-9]+
FLOAT       : [0-9]+'.'[0-9]+ | '-'[0-9]+'.'[0-9]+

Now in the listener inside Java, how do I determine in the case of such a scenario where an operation consist of both IDENT and EXPR the order in which they appear? Obviously the rule can match both ADD 10, d or ADD d, 10 But in the listener for the rule, generated by ANTLR4, if there is both IDENT() and EXPR() how to get their order, since I want to assign the left and right operands correctly.

Been breaking my head over this, is there any simple way or should I rewrite the rule itself? The ctx.getTokens () requires me to give the token type, which kind of defeats the purpose, since I cannot get the sequence of the tokens in the rule, if I specify their type.

Answer 1

You can do it like this:

operation : OPERATION lhs=(IDENT | EXPR) ',' rhs=(IDENT | EXPR);

and then inside your listener, do this:

@Override
public void enterOperation(TParser.OperationContext ctx) {
  if (ctx.lhs.getType() == TParser.IDENT) {
    // left hand side is an identifier
  } else {
    // left hand side is an expression
  }

  // check `rhs` the same way
}

where TParser comes from the grammar file T.g4. Change this accordingly.

Another solution would be something like this:

operation
 : OPERATION ident_or_expr ',' ident_or_expr
 ;

ident_or_expr
 : IDENT
 | EXPR
 ;

and then in your listener:

@Override
public void enterOperation(TParser.OperationContext ctx) {
  Double lhs = findValueFor(ctx.ident_or_expr().get(0));
  Double rhs = findValueFor(ctx.ident_or_expr().get(1));

  ...
}

private Double findValueFor(TParser.Ident_or_exprContext ctx) {
  if (ctx.IDENT() != null) {
    // it's an identifier
  } else {
    // it's an expression
  }
}