perimeter.ex

defmodule Perimeter do
  def calculate(shape) do
    case shape do
      %Square{side: side} -> side * 4
      %Circle{radius: radius} -> radius * 2 * 3.14
    end
  end
end

Perimeter.calculate(%Square{side: 10}) #=> 40
Perimeter.calculate(%Circle{radius: 10}) #=> 62.8