module Mod
  def x
     puts “x”

class C1
  extend Mod

class C2
  include Mod

C1.x – will work

C2.x – will not, but will

In essence:

  • extend – class level
  • include – instance level

None of the books explain this properly. They go into great detail with lots of pictures, but this simple thing is not explained anywhere, or is wrapped up in some stupid example that is really trying to make some other point. In the end I looked at the supplied code and worked out how to do it. Cost me a couple of hours and really very trivial.

You can also have private methods in your modules and they all go in nice ’n easy.