java - ANTLR4 visitor pattern on simple arithmetic example

Question

I am a complete ANTLR4 newbie, so please forgive my ignorance. I ran into this presentation where a very simple arithmetic expression grammar is defined. It looks like:

grammar Expressions;

start : expr ;

expr  : left=expr op=('*'|'/') right=expr #opExpr
      | left=expr op=('+'|'-') right=expr #opExpr
      | atom=INT #atomExpr
      ;

INT   : ('0'..'9')+ ;

WS    : [ 
]+ -> skip ;

Which is great because it will generate a very simple binary tree that can be traversed using the visitor pattern as explained in the slides, e.g., here's the function that visits the expr:

public Integer visitOpExpr(OpExprContext ctx) {
  int left = visit(ctx.left);
  int right = visit(ctx.right);
  String op = ctx.op.getText();
  switch (op.charAt(0)) {
    case '*': return left * right;
    case '/': return left / right;
    case '+': return left + right;
    case '-': return left - right;
    default: throw new IllegalArgumentException("Unkown opeator " + op);
  }
}

The next thing I would like to add is support for parentheses. So I modified the expr as follows:

expr  : '(' expr ')'                      #opExpr
      | left=expr op=('*'|'/') right=expr #opExpr
      | left=expr op=('+'|'-') right=expr #opExpr
      | atom=INT #atomExpr
      ;

Unfortunately, the code above fails because when encountering parentheses the three attributes op,left and right are null (fails with NPE).

I think I could work around that by defining a new attribute, e.g., parenthesized='(' expr ')', and then deal with that in the visitor code. However, it seems overkill to me to have a whole extra node type to represent an expression in parentheses. A simpler but uglier solution is to add the following line of code at the beginning of the visitOpExpr method:

if (ctx.op == null) return visit(ctx.getChild(1)); // 0 and 2 are the parentheses!

I don't like the above at all because it's very fragile and highly dependent on the grammar structure.

I am wondering if there is a way to tell ANTLR to just "eat" the parentheses and treat the expression like a child. Is there? Is there a better way to do this?

Note: My end goal is to extend the example to include boolean expressions that can themselves contain arithmetic expressions, e.g., (2+4*3)/10 >= 11, that is, a relation (<,>,==,~=,etc.) between arithmetic expressions can define an atomic boolean expression. This is straight forward and I already have the grammar sketched out but I have the same problem with parenthesis, i.e., I need to be able to write stuff like (I will also add support for variables):

((2+4*x)/10 >= 11) | ( x>1 & x<3 )

EDIT: Fixed the precedence of the parenthesized expression, parenthesis always have higher precedence.

Answer 1

Sure, just label it differently. After all, the alternative '(' expr ')' isn't a #opExpr:

expr  : left=expr op=('*'|'/') right=expr #opExpr
      | left=expr op=('+'|'-') right=expr #opExpr
      | '(' expr ')'                      #parenExpr
      | atom=INT                          #atomExpr
      ;

And in your visitor, you'd do something like this:

public class EvalVisitor extends ExpressionsBaseVisitor<Integer> {

    @Override
    public Integer visitOpExpr(@NotNull ExpressionsParser.OpExprContext ctx) {
        int left = visit(ctx.left);
        int right = visit(ctx.right);
        String op = ctx.op.getText();
        switch (op.charAt(0)) {
            case '*': return left * right;
            case '/': return left / right;
            case '+': return left + right;
            case '-': return left - right;
            default: throw new IllegalArgumentException("Unknown operator " + op);
        }
    }

    @Override
    public Integer visitStart(@NotNull ExpressionsParser.StartContext ctx) {
        return this.visit(ctx.expr());
    }

    @Override
    public Integer visitAtomExpr(@NotNull ExpressionsParser.AtomExprContext ctx) {
        return Integer.valueOf(ctx.getText());
    }

    @Override
    public Integer visitParenExpr(@NotNull ExpressionsParser.ParenExprContext ctx) {
        return this.visit(ctx.expr());
    }

    public static void main(String[] args) {
        String expression = "2 * (3 + 4)";
        ExpressionsLexer lexer = new ExpressionsLexer(CharStreams.fromString(expression));
        ExpressionsParser parser = new ExpressionsParser(new CommonTokenStream(lexer));
        ParseTree tree = parser.start();
        Integer answer = new EvalVisitor().visit(tree);
        System.out.printf("%s = %s
", expression, answer);
    }
}

If you run the class above, you'd see the following output:

2 * (3 + 4) = 14