data LockerState = Taken | Free deriving (Show, Eq)
type Code = String
type LockerMap = Map.Map Int (LockerState, Code)
lockerLookup :: Int -> LockerMap -> Either String Code
lockerLookup lockerNumber map =
  case Map.lookup lockerNumber map of
    Nothing -> Left $ "Locker number " ++ show lockerNumber ++ " doesn't exist!"
    Just (state, code) -> if state /= Taken
                           then Right code
                           else Left $ "Locker " ++ show lockerNumber ++ " is already taken!"