import Control.Monad (foldM)

--jeste jednou jednoducha verze
lookUp :: Eq k => k -> [(k, v)] -> Maybe v
lookUp _ [] = Nothing
lookUp k ((l, v):r)
  | k == l = Just v
  | otherwise = lookUp k r

--lookup4 napsany podstatne lip pomoci Maybe monady a do-notace
--(funguje uplne stejne jako verze s andThen)
lookUp4M k s = do
  x <- lookUp k s
  x <- lookUp x s
  x <- lookUp x s
  lookUp x s

--to same, akorat rozepsane s explicitnim pouzitim >>=
--(lepsi je samozrejme pouzit `do', ale kod je jinak ekvivalentni)
lookUp4M' k s =
  lookUp k s >>= \x ->
  lookUp x s >>= \x ->
  lookUp x s >>= \x ->
  lookUp x s

--docela fajn verze
lookup4MBonus k = foldM lookUp k . replicate 4