Concerns and 'included' hook

module Concerning
  def self.included(klass)
    klass.extend(ClassMethods)
    klass.include(InstanceMethods)
  end

  module ClassMethods
    def bar
      'bar'
    end
  end

  module InstanceMethods
    def foo
      'foo'
    end
  end
end

Usage

class SomeClass
  include Concerning
end