1_happy.rb

# modules work! :) If we define_method on a module and include it, it works the same.

class A
  include World
end

module World
  def hello
    puts "World"
  end
end

module Hello
  def hello
    print "Hello, "
    super()
  end

  A.send :include, self
end

A.new.hello
  Hello, World
   => nil

1_sad.rb

# Oh no, we can't extend #foo with a module :( This is what happens when we use define_method.

class B
  include Foo

  def foo
    puts "bar"
  end
end

module Foo
  def foo
    print "foo"
    super
  end
end

B.new.foo
  bar
  => nil

define_method_happy.rb

def module_backed_define_method(klass, name, &block)
  mod = Module.new
  mod.send :define_method, name, &block
  klass.send :include, mod
end

module World
  def hello
    puts "World"
  end
end

class A
  include World

  module_backed_define_method(self, :hello) do
    print "Hello, "
    super()
  end
end



A.new.hello
  Hello, World
   => nil