with_blogpost_3.ex

def join_game(user_id, game_id) do
  with {:ok, user} <- Users.get(user_id), 
       {:ok, game} <- Games.get(game_id),
       false       <- Game.is_full?(game),
       false       <- Game.is_started?(game),
       true        <- User.has_permission?(user, game)
do
  Game.add_user(game, user)
else
  :not_found -> {:error, "User or game not found"}
  true       -> {:error, "Game is full, or game has already started"}
  false      -> {:error, "User does not have permission"}
end