iterator.ex

defmodule Advance do

  defp advance_by(stream, count) do
    Enumerable.reduce(stream, count, fn
      x, {acc, 1} -> {:suspend, {[x | acc], 0}}
      x, {acc, counter} -> {:cont, {[x | acc], counter - 1}}
    end)
  end

  defp continue(cont, by) do
    {_, {head, _}, cont} = cont.({:cont, {[], by}})
    {Enum.reverse(head), fn -> continue(cont, by) end}
  end

  def advance(stream, by) do
    continuation = &advance_by(stream, &1)
    continue(continuation, by)
  end
  def advance(stream), do: advance(stream, 1)

end