full_solution.ex

defmodule Square do
  defstruct [:side]
end

defmodule Area do
  defstruct []
end

defprotocol Shape do
  def calculate(calculation, shape)
end

defprotocol AreaProtocol do
  def calculate(shape)
end

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

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

Shape.calculate(%Area{}, %Square{side: 2})