This is marginally clearer, but not all that great. import Data.List as L import Control.Monad.State import Data.Maybe (isJust) data PlayerInfo = PlayerInfo { playerNumber :: Integer } instance Eq PlayerInfo where p1 == p2 = playerNumber p1 == playerNumber p2 data Game = Game { players :: [PlayerInfo] } addPlayer newPlayer = do exists <- fmap (isJust . find (==newPlayer)) $ gets players when exists $ gets players >>= \oldPlayers -> modify (\game -> game { players = newPlayer:oldPlayers }) return exists So, add an Eq instance to players so that you can just compare them simply without deconstructing them all the time. However, if you were to give an Ord instance to players, you could replace lists with sets, which will allow you to insert without caring whether the player is already in the set. If you are dead set on getting back a boolean, then you'll still have to check for existence with member, so you won't gain much in brevity. But if you weren't, you could shorten it considerably to: import Control.Monad.State import Data.Set as S data PlayerInfo = PlayerInfo { playerNumber :: Integer } deriving Ord instance Eq PlayerInfo where p1 == p2 = playerNumber p1 == playerNumber p2 data Game = Game { players :: Set PlayerInfo } addPlayer newPlayer = modify $ \game@(Game { players = oldPlayers }) -> game { players = S.insert newPlayer oldPlayers } On Wed, Jul 25, 2012 at 5:02 PM, Corentin Dupont wrote:

Hi, I always write functions like this:

addPlayer :: PlayerInfo -> State Game Bool addPlayer pi@(PlayerInfo {playerNumber = pn}) = do pls <- gets players case find (\(PlayerInfo {playerNumber = myPn}) -> pn == myPn) pls of Nothing -> do modify (\game -> game { players = pi : pls}) return True otherwise -> return False

It simply adds a new PlayerInfo to a list contained in Game, with the condition that it doesn't exists already. Do you see a more elegant way to do this?

Best, Corentin

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