triple.ex

defmodule AmazeOn do
  defstruct [price: 10]
end

defmodule Area do
  defstruct []
end

defmodule Square do
  defstruct [:side]
end

defprotocol ShapePricePerShop do
  def calculate(calculation, shape, shop)
end

defprotocol AreaProtocol do
  def calculate(shape, shop)
end

defprotocol SquareProtocol do
  def calculate(shop, shape)
end

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

defimpl AreaProtocol, for: Square do
  def calculate(shape = %Square{}, shop) do
    SquareProtocol.calculate(shop, shape)
  end
end

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

ShapePricePerShop.calculate(%Area{}, %Square{side: 10}, %AmazeOn{})