[Compiler-sig] AST observations

[Jeremy Hylton]

>>>>> "ECN" == Eric C Newton <ecn@metaslash.com> writes:

  ECN> compiler.visitor:

  ECN>    ASTVisitor
  ECN>         As the comment says, it's not a visitor, it's a walker.
  ECN>         Someone mentioned earlier that this is rather
  ECN>         confusing.

I'm sorry this is confusing, but I think it is one of the standard
variations on the visitor pattern.  It's certainly the case that the
visitors we've all been writing have the signs of being a visitor,
e.g. a method for each class of object.

As for how the traversal occurs, GoF (p. 339) says:

"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."

The text goes on to discuss these alternatives and notes that you
could also use an internal iterator that is a kind of hybrid between
having the traversal in the object structure and using an iterator.
In this case, the iterator calls a method on the visitor with the
object as an argument as opposed to calling a method of the object
with the visitor as the argument.

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.

Jeremy