Untitled

import Control.Arrow

data Trie a = Trie (Maybe a) [(Char, Trie a)] deriving Show

find :: Trie a -> String -> Maybe a
find (Trie v _)  []     = v
find (Trie v ts) (c:cs) = case lookup c ts of
                               Just t  -> find t cs
                               Nothing -> Nothing

instance Functor Trie where
    fmap f (Trie v ts) = Trie (fmap f v) (map (second (fmap f)) ts)