Get next line from the file(stream)

Examples:

Read through datasource reads the file as stream and should give the same result as reading the file directly.

iex> expected = File.read!("README.md")
...> num_lines = (String.split(expected, "\n") |> Enum.count) - 1 
...> 
...> # Start the datasource for file "README"
...> {:ok, file_source} = Datasource.start_link("README.md", &Datasource.File.next/1)
...>
...> # Call `next()` number of lines times
...> subject = 
...>   Enum.map((1..num_lines), fn(_) -> Datasource.next(file_source) end)
...>   |> Enum.join
...>
...> assert subject == expected
true

You can use Enum.flat_map_reduce to read from the datasource until it returns :no_data.

iex> {:ok, file_source} = Datasource.start_link("README.md", &Datasource.File.next/1)
...> { _, cnt } = Enum.flat_map_reduce((0..1000), 0, fn x, acc ->
...>   if Datasource.next(file_source) == :no_data, do: {:halt, acc}, else: {[x], acc + 1}
...> end)
...> assert cnt <= 1000
true

When next is called for the first time, the filename is passed as a string. In this case next will create the Stream for this file and calls next again, this time with the stream to use and reads the first line.