Make generic visitor with parameterized return values: XVisitor<T> upon -visitor option

[parrt]

add the following double dispatch:

public void accept(XVisitor visitor) { visitor.visit(this); }

and generate XVisitor. Make a BaseXVisitor that has a

void visitChildren(ParserRuleContext ctx) {
  for each child c: visit(c);
}

And generates the following default implementations:

void visit(slistContext ctx) { visitChildren(p); }

I guess we also need a generic method in ParserRuleContext:

public abstract void accept(XVisitor visitor);

People can do things like this:

int visit(NodeType n) { return n.left.accept(this) + n.right.accept(this); }

[DJCordhose]

Cool stuff. Also BaseVisitor is a good thing.

For generic return types this would be in each context class

    public <R> R accept(Visitor<R> visitor) {
        return visitor.visit(this);
    }

and something like this in a visitor implementation provided by the user

public Integer visit(startContext ctx) {
    return ctx.e.accept(this);
}

Am I right?

Visitor interface and blank default implementation would be generated by ANTLR I guess.

[DJCordhose]

The generated visitor interface might look like this

public interface Visitor {
R visit(rule1Context ctx);
R visit(rule2Context ctx);
...
}

[parrt]

Fixed with parrt/antlr4@725b105. Merged into master in #16.