adding_perimeter.ex

defmodule Perimeter do
  defstruct []
end

defprotocol PerimeterProtocol do
  def calculate(shape)
end

defimpl Shape, for: Perimeter do
  def calculate(%Perimeter{}, shape) do
    PerimeterProtocol.calculate(shape)
  end
end

# Now we can implement the PerimeterProtocol for any shape:

defimpl PerimeterProtocol, for: Square do
  def calculate(%Square{side: side}) do
    side * 4
  end
end

defimpl PerimeterProtocol, for: Circle do
  def calculate(%Circle{radius: radius}) do
    radius * 2 * 3.14
  end
end

Shape.calculate(%Perimeter{}, %Square{side: 10})
Shape.calculate(%Perimeter{}, %Circle{radius: 10})