modules.rb

module Alfa
  def self.included(base)
    puts "#{self} included in #{base}"
    unless base.const_defined?(:PAPPA)
      base.const_set :PAPPA, [:a, :b]
    end
  end

  def self.extended(base)
    puts "#{self} extended in #{base}"
    unless base.const_defined?(:PAPPA)
      base.const_set :PAPPA, [:a, :b]
    end
  end

  def papa
    p self::PAPPA
  end
end

module Beta
  PAPPA = [1, 2]

  extend Alfa

  def self.hola
    papa
  end
end

class Teta
  include Alfa

  def initialize()
    # just to keep in mind that this is a class that we'll instantiate
  end

  def hola
    papa
  end
end

puts "from Beta #{Beta.hola}"
puts "from Teta #{Teta.new().hola}"

This is what I get:

Alfa extended in Beta
Alfa included in Teta
[1, 2]
from Beta [1, 2]
puppa.rb:17:in `papa': #<Teta:0x007f9b30b27028> is not a class/module (TypeError)
    from puppa.rb:36:in `hola'
    from puppa.rb:41:in `<main>'