mixin.rb

module Mixin
  def foo
    puts "foo"
  end
end


class A; end

a = A.new

a.foo # => NoMethodError

a.extend(Mixin)
a.foo # => "foo"


class B
  include Mixin

  # ...
end

b = B.new

b.foo # => "foo"


class C
  extend Mixin
end

c = C.new

c.foo # => NoMethodError
C.foo # => "foo"