fibonacci.ex

  def  fibonacci_stream do
    Stream.unfold( [1], fn
      [prev, curr] ->
          next = prev + curr
          {next, [curr, next]}
        ([1]) ->
          {1, [1,1]}
        end)
    end
    
  def euler do
    fibonacci_stream 
    |> Enum.take_while(fn x -> x < 4000000 end)
    |> Enum.filter(fn x -> Integer.is_even(x) end)
    |> Enum.sum()
  end