the AST walker currently provides no way to find out how the

the AST walker currently provides no way to find out how the
nodes in the tree are nested with respect to one another.
a simple change to the Visitor interface makes it possible
to do this (for example to maintain a current node-depth, or a
knowledge of the name of the current function).

Visit(nil) is called at the end of a node's children;
this make possible the channel-based interface below,
amongst other possibilities.

It is still just as simple to get the original behaviour - just
return the same Visitor from Visit.

Here are a couple of possible Visitor types.

// closure-based
type FVisitor func(n interface{}) FVisitor
func (f FVisitor) Visit(n interface{}) Visitor {
	return f(n);
}

// channel-based
type CVisitor chan Visit;
type Visit struct {
	node interface{};
	reply chan CVisitor;
};
func (v CVisitor) Visit(n interface{}) Visitor
{
	if n == nil {
		close(v);
	} else {
		reply := make(chan CVisitor);
		v <- Visit{n, reply};
		r := <-reply;
		if r == nil {
			return nil;
		}
		return r;
	}
	return nil;
}

R=gri
CC=rsc
https://golang.org/cl/166047