[Compiler-sig] AST observations

[Eric C. Newton]

> I'm sorry this is confusing, but I think it is one of the standard
> variations on the visitor pattern.

Clearly, the class with "visit" methods is a Visitor.  Now this other
thing is also called ASTVisitor, even though it delagates visitation
to a third class with visit methods.

Reminds me of java code, where the same word appears 4 times:

        Thing thing = new Thing(thingy);

> "Who is responsible for traversing the object structure?  A visitor
> must visit each element of the object structure.  The question is, how
> does it get there?  We can put the responsibility in any of three
> places: in the object structure, in the visitor, or in a separate
> iterator object."

Ok, TreeIterator works for me, too. 8-)

> It might be clearer to merge the walker and the visitor into a single
> class using inheritance.  (I think the Walkabout variant described by
> Palsberg and Jay does this,
>     cf. http://citeseer.nj.nec.com/palsberg97essence.html.)  But I
> thought delegation would be clearer and would avoid the need for a
> magic base class that all visitors must inherit from.

The only advantage I can see for this approach is faster visitation:
the base class could have default visit methods that would know how to
iterate over the child nodes.  getChildNodes() would no longer be
necessary.

-Eric