Lockers.hs

import qualified Data.Map as Map

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" ++ show lockerNumber 
                                 ++ " doesn't exist!"
    Just (state, code) -> if state /= Taken 
                            then Right code 
                            else Left $ "Locker " ++ show lockerNumber 
                                        ++ " is already taken!"

lockers :: LockerMap
lockers = Map.fromList 
    [ (100, (Taken, "ZD39I"))
    , (101, (Free, "JAH3I"))
    , (103, (Free, "IQSA9"))
    , (105, (Free, "Q0TSA"))
    , (109, (Taken, "893JJ"))
    , (110, (Taken, "99292"))
    ]