make the key customizable/restrictable.

hs/DataStructures.hs

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}
{-|
    Module: DataStructures
    Description: Having fun with (functional) data structures
        case last ft of
            Just x -> Just (x, rest ft) -- broken, we'd need a different version of rest
class Associative as where
class Associative as k where
    get :: (Ord k) => k -> as k v -> Maybe v
    insert :: (Ord k) => k -> v -> as k v -> as k v
-- examples
rb = fromPairs $ zip [1..10] [2..11] :: RBTree Int Int
fromPairs :: (Associative as, Ord k) => [(k, v)] -> as k v
fromPairs :: (Associative as k, Ord k) => [(k, v)] -> as k v
fromPairs [] = empty
fromPairs ((k, v):kvs) = insert k v $ fromPairs kvs
    RBNode Black (RBNode Red a xk xv b) yk yv (RBNode Black c zk zv d)
balance c l k v r = RBNode c l k v r
instance Associative RBTree where
instance Associative RBTree k where
    get ik (RBLeaf _) = Nothing
    get ik (RBNode c l k v r) | k == ik = Just v
    get ik (RBNode c l k v r) =