calculate_for.ex

defmodule Square do
  defstruct [:side]
end

defmodule Circle do
  defstruct [:radius]
end

defprotocol Area do
  def calculate_for(shape)
end

defprotocol Perimeter do
  def calculate_for(shape)
end

defimpl Area, for: Square do
  def calculate_for(square) do
    square.side * square.side
  end
end

defimpl Perimeter, for: Circle do
  def calculate_for(circle) do
    circle.radius * 2 * 3.14
  end
end

Area.calculate_for(%Square{side: 10})
Perimeter.calculate_for(%Circle{radius: 10})