defmodule Plane do
  defstruct is_fueled?: false, has_pilot?: false

  def take_off(%Plane{ is_fueled?: false }), do: :needs_fuel
  def take_off(%Plane{ has_pilot?: false }), do: :needs_pilot
  def take_off(%Plane{}), do: :whoosh!
end

plane = %Plane{is_fueled?: true}
Plane.take_off(plane)
# => :needs_pilot

plane = %Plane{has_pilot?: true}
Plane.take_off(plane)
# => :needs_fuel

plane = %Plane{has_pilot?: true, is_fueled?: true}
Plane.take_off(plane)
# => :whoosh!