Pattern match on specific keys in order to handle correctly user input

game.ex

# Handle keydown events
@spec on_input(Socket.t(), String.t()) :: Socket.t()
defp on_input(socket, key) when key in @left_keys,
  do: move_paddle(socket, :left)

defp on_input(socket, key) when key in @right_keys,
  do: move_paddle(socket, :right)

defp on_input(socket, _), do: socket

# Handle keyup events
@spec on_stop_input(Socket.t(), String.t()) :: Socket.t()
defp on_stop_input(socket, key) when key in @left_keys,
  do: stop_paddle(socket, :left)

defp on_stop_input(socket, key) when key in @right_keys,
  do: stop_paddle(socket, :right)

defp on_stop_input(socket, _), do: socket