dynamically including a module depending on an instance property

dynamic_modules.rb

module DynamicSetup
  def dynamic_setup_complete?
    @dynamic_setup_complete
  end

  def method_missing name, *args, &block
    if dynamic_setup_complete?
      super
    else
      perform_dynamic_setup
      @dynamic_setup_complete = true
      send name, *args, &block
    end
  end
end

class Foo
  include DynamicSetup

  module Normal
    def greeting
      "Hello"
    end
  end

  module Silly
    def greeting
      "Wibble"
    end
  end

  def initialize silly = false
    @silly = silly
  end

  def perform_dynamic_setup
    extend @silly ? Silly : Normal
  end
end

puts Foo.new.greeting       # => "Hello"
puts Foo.new(true).greeting # => "Wibble"