whole_shabang.ex

defmodule Square do
  defstruct [:side]
end

defmodule Circle do
  defstruct [:radius]
end

defmodule EquilateralTriangle do
  defstruct [:side]
end

defprotocol Shape do
  def area(shape)
end

defimpl Shape, for: Square do
  def area(shape) do
    shape.side * shape.side
  end
end

defimpl Shape, for: Circle do
  def area(shape) do
    shape.radius * shape.radius * 3.14
  end
end

defimpl Shape, for: EquilateralTriangle do
  def area(shape) do
    # some basic high school math that I've forgotten :(
  end
end

defmodule Project do
  def total_cost(shape, cost_per_square_meter) do
    Shape.area(shape) * cost_per_square_meter
  end
end

triangle = %EquilateralTriangle{side: 10}
square   = %Square{side: 10}
circle   = %Circle{radius: 10}

Project.total_cost(triangle, 20)
Project.total_cost(square, 20)
Project.total_cost(circle, 20)