Roles in Ruby

gistfile1.rb

class Role < Module
  IncompleteInterface = Class.new(RuntimeError)

  def included(receiver)
    missing_methods = @public_api.map(&:to_sym) - receiver.public_instance_methods.map(&:to_sym)
    unless missing_methods.empty?
      raise IncompleteInterface,
        "#{receiver} must implement these methods: #{missing_methods.inspect}"
    end

    # other plumbing...
  end

  def public_api(*methods)
    @public_api = methods.flatten
  end
end

StickyNotePlacer = Role.new do
  public_api :place_sticky_note
end

class Thingy
  def place_sticky_note
    "yup"
  end

  include StickyNotePlacer
end

puts Thingy.new.place_sticky_note

class OtherThingy
  include StickyNotePlacer # => raises Role::IncompleteInterface
end

I like this a lot. I may have to try it.