Question

Please explain why self is used in def self.included (klass) below.

module A
  def self.included(klass)
    puts "A -> #{klass}"
    puts A
    puts self
  end
end

class X
  include A
end

Answer 1

By writing def self.included you are defining a method that is part of the singleton class of module A. In general, singleton methods can only be called by doing A.included() but this singleton method has a special name included that causes the Ruby interpreter to call when the module gets included in to a class.

A normal method in a module (defined with def foo) can only be called if the module gets included in to something else.