area.ex

defmodule Square do
  defstruct [:side]
end

defmodule Circle do
  defstruct [:radius]
end

defmodule Area do
  def calculate(shape) do
    case shape do
      %Square{side: side} -> side * side
      %Circle{radius: radius} -> radius * radius * 3.14
    end
  end
end

Area.calculate(%Square{side: 10}) #=> 100
Area.calculate(%Circle{radius: 10}) #=> 314.0