CODEWITHSUNDEEP
antlrreturn-valuevoidantlr4
FelRPI
FelRPI

Reputation: 429

Getting a return value from Antlr Listener?

i have a Antlr generated Listener, and i call my tree walker to go through the tree from a parse function in another class. Looks like this: 

public double calculate(){

     ANTLRInputStream input = new ANTLRInputStream("5+2");
     Lexer lexer = new Lexer(input);
     CommonTokenStream tokens = new CommonTokenStream(lexer);
     Parser parser = new Parser(tokens);
     ParseTree tree = parser.calculate();
     ParseTreeWalker walker = new ParseTreeWalker();
     walker.walk(new Listener(), tree);
     return 0;
}

So the listener works perfect with the enter() and quit() Functions and prints the correct value in the end: 

public void exitParser(ParserContext ctx) {
    result = stack.peek();
    System.out.println(result);
}

But i wanna receive the final value in my calculate() function to return it there. Since exitParser(...) is void i dont know how to deal with it.

With the visitor i was able to do it like that: 

public double calculate(){
    // ...
    String value = new WRBVisitor().visit(tree);
    return Double.parseDouble(value);
}

Hope someone understands my problem and knows a solution for it.

Best regards

Upvotes: 4

Views: 2062

Answers (1)

Bart Kiers
Bart Kiers

Reputation: 170158

As mentioned in the comments: a visitor might be a better option in your case. A visitor's methods will always return a value, which is what you seem to be after. That could be a Double if your expressions always evaluate to a numeric value, or some sort of home-grown Value that could represent a Double, Boolean, etc.

Have a look at my demo expression evaluator (using a visitor) on GitHub: https://github.com/bkiers/Mu

Upvotes: 1

Related Questions