Just a small experiment after reading "Design Patterns in Ruby", by Ross Olsen.

composite.rb

# Small experiment in which I try to implement the composite pattern without
# resorting to two classes.

class Composite
  attr_accessor :action, :subcomposites
  
  # Cool thing: if we don't provide a block, it'll be passed nil. So there's no
  # need to make it a "default parameter" (you can't do it, actually).
  def initialize(&action)
    @action = action
    @subcomposites = []
  end
  
  # Add another composite to this one.
  def <<(subcomposite)
    @subcomposites << subcomposite
  end
  
  # If this object has an action, it's a leaf.
  # Else, it's a node of the tree and has a bunch of subcomposites.
  def run
    if @action
      @action.call
    else
      @subcomposites.each do |s|
        s.run
      end
    end
  end
end

# Let's test it.

clean_house = Composite.new

clean_room = Composite.new
clean_room << Composite.new { puts "[room] I'm cleaning my bed!" }
clean_room << Composite.new { puts "[room] I'm cleaning my desk!" }

clean_kitchen = Composite.new
clean_kitchen << Composite.new { puts "[kitchen] I'm cleaning my knife drawer!" }
clean_kitchen << Composite.new { puts "[kitchen] I'm cleaning my spices drawer!" }

clean_fridge = Composite.new
clean_fridge << Composite.new { puts "[kitchen][fridge] Throwing away old stuff!" }
clean_fridge << Composite.new { puts "[kitchen][fridge] Organizing my bottles!" }

clean_kitchen << clean_fridge

clean_house << clean_room
clean_house << clean_kitchen

clean_house.run