hello_macro.rb

module SaysHi
  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods
    def hi_to
      @hi_to ||= ""
    end

    def says_hi_to(name)
      @hi_to = name
    end
  end

  def say_hi
    puts "Hello, #{self.class.hi_to}"
  end
end

class HelloReader
  include SaysHi
  says_hi_to "dear reader"
end

class HelloWorld
  include SaysHi
  says_hi_to "world"
end

HelloReader.new.say_hi # => "Hello, dear reader"
HelloWorld.new.say_hi # => "Hello, world"