fib.ex

defmodule Fib do
  def seq(n) when n >= 0, do: reverse_seq(n, [0], 1) |> Enum.reverse

  defp reverse_seq(0, sequence, _next), do: sequence

  defp reverse_seq(n, sequence = [last | _], next) do
    reverse_seq(n - 1, [next | sequence], next + last)
  end
end