uniformly stream from IO and File in Elixir (akin to Ruby's ARGF)

argf.exs

defmodule App do
  defmodule CLI do
    def main(argv) do
      io_stream_from_file_args_or_stdin(argv)
      |> App.process_stream()
    end

    defp io_stream_from_file_args_or_stdin(argv) do
      get_io_source(argv)
      |> IO.stream(:line)
    end

    defp get_io_source([]) do
      :stdio
    end

    defp get_io_source([path]) do
      {:ok, file} = File.open(path)
      file
    end
  end

  @doc """
    stream - should have side effects in the sense of the Iterator design
      pattern or database Cursor, as IO.Stream does and File.Stream does not.
      
      IO.stream/2 documentation reads "Note that an IO stream has side effects
      and every time you go over the stream you may get different results."
  """
  def process_stream(stream = %IO.Stream{}) do
    [first_line] = Enum.take(stream, 1)
    header_result = process_first_line(first_line)

    stream
    |> Stream.chunk_every(2)
    |> Stream.each(fn [odd_line, even_line] ->
      process_line_pairs(odd_line, even_line, header_result)
    end)
    |> Stream.run()
  end

  def process_first_line(line) do
    # ...
  end

  def process_line_pairs(odd_line, second_line, header_result) do
    # ...
  end
end

# Thanks to @fcevado and @bamorim for the fruitful discussion on https://telegram.me/elixirbr