with_blogpost_4.ex

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