extend.rb

# illustrates interesting effects of #extend

module Mixin
  def testo
    "in mixin"
  end
end


class SimpleClass
  def testo
    "in X"
  end
end

class ClassIncludingMixin
  include Mixin
  def testo
    "in Y"
  end
end

# no surprises
x = SimpleClass.new
fail unless x.testo == 'in X'

# dynamically mixin module to object. note that #testo resolves to the
# method in Mixin, which is NOT what happens when you #include!
x.extend(Mixin)
fail unless x.testo == 'in mixin'

# proof that #extend didn't change SimpleClass, only the object instance above
x = SimpleClass.new
fail unless x.testo == 'in X'

# mixin is already included in this class, so the method in the class
# takes precedence over mixin.
x = ClassIncludingMixin.new
fail unless x.testo == 'in Y'