add incomplete version of patricia trie.

e.g. we can lookup, but not insert because merge & matchPrefix weren't
explained. laters?

hs/DataStructures.hs

module DataStructures where
import Prelude hiding (concat, drop, last, length, reverse, take)
import Data.Bits (Bits, (.&.), zeroBits)
-- examples
sl = fromList [1..10] :: List Integer
        else balance c l k v (insert ik iv r)
    empty = RBLeaf Black
data PatriciaTrie k v =
      PTEmpty
    | PTLeaf k v
    | PTBranch { ptPrefix :: k, ptMask :: k, ptLeft :: PatriciaTrie k v, ptRight :: PatriciaTrie k v } deriving Show
instance Associative PatriciaTrie Int where
    get ik PTEmpty = Nothing
    get ik (PTLeaf k v) = if ik == k then Just v else Nothing
    get ik (PTBranch p m l r) = if (ik .&. m) == zeroBits then get ik l else get ik r
    insert ik iv b@(PTBranch p m l r) =
        if ptMatchPrefix ik p m
        then if (ik .&. m) == zeroBits
             then PTBranch p m (insert ik iv l) r
             else PTBranch p m l (insert ik iv r)
        else ptMerge ik (PTLeaf ik iv) p b
    empty = PTEmpty
ptMatchPrefix = undefined
ptMerge = undefined