with_blogpost_6.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
  # Don't care what specific thing failed
  _ -> :error
end