main.exs

defmodule Storage do
  use Agent

  def start_link() do
    Agent.start_link(fn -> [] end, name: :storage)
  end

  def add(data) do
    Agent.update(:storage, fn l -> [data] ++ l end)
  end

  def pop() do
    {ret, l} = Agent.get(:storage, fn l -> List.pop_at(l, 0) end)
    Agent.update(:storage, fn _ -> l end)
    ret
  end
end

# module

defmodule Reader do
  def read(path) do
    path
    |> File.stream!()
    |> Enum.each(fn x -> Storage.add(String.trim(x)) end)
  end
end

# module

defmodule Downloader do
  def download(url, dir) do
    path = Path.join(dir, "%(uploader)s-%(title)s.%(ext)s")

    cmd = 
      'youtube-dl #{url} -o "#{path}" --extract-audio --audio-format mp3'
    :os.cmd(cmd)

    IO.puts("Downloaded #{url}")
  end
end

# module

## Main ##
{[dir: dir, input: input], _, _} =
  OptionParser.parse(System.argv(), strict: [dir: :string, input: :string])

Storage.start_link()
Reader.read(input)

create_downloaders = fn f, t ->
  case Storage.pop() do
    nil ->
      t

    x ->
      task = Task.async(fn -> Downloader.download(x, dir) end)
      f.(f, [task] ++ t)
  end
end

create_downloaders
|> create_downloaders.([])
|> Enum.map(&Task.await(&1, :infinity))