gistfile1.rb

# based on http://gist.github.com/339028

module Base
  module Plugins
    def plugins
      @plugins ||= []
    end
 
    def activate( plugin_name )
      plugin = Base::Plugins.const_get plugin_name.capitalize
      if plugin
        plugins << plugin_name
        extend plugin
      else
        raise "no such plugin #{plugin_name}"
      end
    end

    module Backward
      def say(what)
        super what.reverse
      end
    end
    
    module Loudness
      def say(what)
        super what.upcase
      end
    end
    
  end
end

module Base
  class Server
    include Plugins

    def say(what)
      p "say: #{what}"
    end
  end
end


s = Base::Server.new 
s.say        'hello world'
s.activate   'backward'
s.say        'hello world'
p s.plugins

puts

s2 = Base::Server.new
s2.say       'hello world'
s2.activate  'loudness'
s2.say       'hello world'
p s2.plugins

puts

s2.activate 'backward'
s2.say      'hello world'
p s2.plugins