Interfaces for Ruby

defines_interface.rb

# total pseudocode

class StickyNotePlacer < Role
  messages :call, :user, :message, :color?
end

class ApplicationApi < Role
  messages :sticky_note_placer
end

module Application
  def self.new
    Base.new
  end

  class Base
    as_role(ApplicationApi) do
      def sticky_note_placer
        ConsoleStickyNotePlacer.new
      end
    end
  end

  class ConsoleStickyNotePlacer
    as_role(StickyNotePlacer) do
      def call(user, message, color = :yellow)
        puts "#{self} called with #{[user, message, color]}"
      end
    end
  end
end

# This would fail on line 27, because the as_role call would notice that not all the messages 
# for the StickyNotePlacer role had been defined by ConsoleStickyNotePlacer.
# If you did define those methods too, then the thing should work.
#
# The point is that the roles act a bit like Java interfaces, and allow you to explicitly 
# partition up polymorphic behaviour of a class.
#
# The also allow you to get some checking as to whether you're conforming to the role's protocol
# when you make an implementation.
#
# WDYT?