protocol.rb

module Ext
  class Protocol
    def initialize(meths, dispatch={})
      @dispatch = dispatch
      @meths = meths
    end

    def adapt(obj)
      w = obj.class.ancestors.detect do |klass|
        @dispatch[klass]
      end
      adapter = @dispatch[w]
      adapted = adapter.new(obj)

      #silly-ish I know:
      unless (@meths - (adapted.methods - Object.new.methods)).empty?
        raise "Adapter #{adapter} doesn't implement full protocol interface, #{@meths.inspect}"
      end

      adapted
    end

    def extend(dispatch)
      Protocol.new(@meths, @dispatch.merge(dispatch))
    end
  end

  EachProtocol = Protocol.new([:each])
end

module MyAppOrLib
  class StringEach
    def initialize(str)
      @str = str
    end

    def each(&block)
      @str.each_char(&block)
    end
  end

  EachProtocol = Ext::EachProtocol.extend({
    String => StringEach
  })

  class Doer
    def do
      EachProtocol.adapt("abc").each do |part|
        puts part
      end
    end
  end
end

MyAppOrLib::Doer.new.do