fib.exs

defmodule Fib do
  def fib(0), do: 0
  def fib(1), do: 1
  def fib(2), do: 1
  def fib(n), do: fib(n, 1, 1)

  defp fib(3, prev, cur), do: prev + cur
  defp fib(n, prev, cur), do: fib(n - 1, cur, prev + cur)
end

IO.puts Fib.fib(1000000)