play chord(:d4, :minor)

ii_V_I = [
  {chord(:d4, :minor7),    2},
  {chord(:g5, :dominant7), 2},
  {chord(:c5, :major7),    4}
]

progression = fn -> Enum.map(Stream.cycle(ii_V_I), fn {chord, beats} ->
    play chord, duration: beats, attack: 0.2, decay: 1
    Process.sleep beats
  end)
end

pid = spawn progression

Process.exit(pid, :kill)