table.hs

import Prelude hiding (lookup)

type Map k a = k -> Maybe a

empty :: Map k a
empty = const Nothing

insert :: Eq k => k -> a -> Map k a -> Map k a
insert k v f = \x -> if x == k then Just v else f x

delete :: Eq k => k -> Map k a -> Map k a
delete k f = \x -> if x == k then Nothing else f x

lookup :: Eq k => k -> Map k a -> Maybe a
lookup k f = f k

main = do
  let t = insert 5 "foo" . insert 2 "bar" $ empty
  print $ lookup 2 t
  print $ lookup 0 t

  let t' = delete 2 t
  print $ lookup 2 t'
  print $ lookup 5 t'