gistfile1.rb

module Calculate
  attr_accessor :num1
  attr_accessor :num2

  def calculate
    self.!
  end
end

class Arithmetic < Module
  def self.[](operation_module)
    new operation_module
  end

  def initialize(operation_module)
    mod = operation_module
    define_method :operation_module do
      return mod
    end

    include operation_module
    include Calculate
  end
end

module Add
  def !
    num1 + num2
  end
end

module Multiply
  def !
    num1 * num2
  end
end

class Calculation
  include Arithmetic[Add]
end

calculation = Calculation.new
calculation.num1 = 2
calculation.num2 = 5

puts calculation.calculate
# => 7

class Calculation
  include Arithmetic[Multiply]
end

calculation = Calculation.new
calculation.num1 = 2
calculation.num2 = 5

puts calculation.calculate
# => 10