From: Brian Candler <b.candler@...>
Date: 2010-12-26T19:27:15+09:00
Subject: Re: converting traversal fn into Enumeration

It's easier than you think. Just define a method called 'each' which 
enumerates the objects, and mix-in Enumerable into your class.

class TNode
  include Enumerable
  alias :each :traverse
end

> For somewhat didactic reasons, I'd prefer a method TNode#traversal(),
> that returns an Enumerable

You cannot return an Enumerable, because it is a Module, not a Class. 
Hence you can't create instances of Enumerable.

, so I could do something like:
> -----
>   def find_node(value)
>     traversal.find {|n| n.value == value}
>   end
> -----

What I explained above lets you do it directly on the node:

  rootnode.find { |n| n.values == value }

If you don't want to do this on the node class itself, you can instead 
return some other object which has a #each method:

class TNode
  class Traversal
    include Enumerable
    def initialize(root)
      @root = root
    end
    def each
      ... define it here
    end
  end
  def traversal
    Traversal.new(self)
  end
end

Or there is class Enumerator in the standard library which you can use 
(in 1.8 it's Enumerable::Enumerator). It just gives a wrapper object 
which maps a call to :each to some other method in the underlying 
object.

require 'enumerator'
class TNode
  def traversal
    Enumerable::Enumerator.new(self, :traverse)
  end
end

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