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)

lookupLocker :: Int -> LockerMap -> Either String Code
lookupLocker 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."

main =
  let
    lockers = Map.fromList
      [
        (100, (Taken, "A100")), 
        (101, (Taken, "A101")), 
        (102, (Free, "A102")),
        (103, (Free, "A103"))
      ]
  in 
    print $ lookupLocker 100 lockers