maybe you will like my way of parsing the input?

      File.read!("day_5.txt")
      |> String.split(["\n", ",", " -> "], trim: true)
      |> Enum.map(&String.to_integer/1)
      |> Enum.chunk_every(4)
      |> Enum.map(&List.to_tuple/1)

We can split the file in all the special content we have, and then just take numbers 4 at a time. No need to nest anything

My solution

Today was significatively simpler than yesterday, but I had to remember my vector algebra
To calculate the intermediate points, I used:

  def fill_points({x1,y1,x2,y2}) do
    {dx, dy} = {x2 - x1, y2 - y1}
    slope = {step(dx), step(dy)}
    do_fill({x1, y1}, {x2, y2}, slope, [])
  end

  defp step(0), do: 0
  defp step(x) when x > 0, do: 1
  defp step(x) when x < 0, do: -1

  defp do_fill(point, point, _slope, acc) do
    [point | acc]
  end

  defp do_fill({x1, y1} = point, end_point, {dx, dy} = slope, acc) do
    new_point = {x1 + dx, y1 + dy}
    do_fill(new_point, end_point, slope, [point | acc])
  end

IO.puts would work, I think:

iex(5)> input = "1.1...11.\n.111...2...\n...2.1.111.\n...1.2.2...\n.112313211\n...1.2...\n...1...1...\n.1...1...\n1...1.\n222111..."
iex(6)> IO.puts input
1.1...11.
.111...2...
...2.1.111.
...1.2.2...
.112313211
...1.2...
...1...1...
.1...1...
1...1.
222111...