1 {-# OPTIONS_GHC -Wall #-}
7 data JoinList m a = Empty
9 | Append m (JoinList m a) (JoinList m a)
13 tag :: Monoid m => JoinList m a -> m
16 tag (Append t _ _) = t
18 (+++) :: Monoid m => JoinList m a -> JoinList m a -> JoinList m a
21 alst1 +++ alst2 = Append ((tag alst1) `mappend` (tag alst2)) alst1 alst2
25 indexJ :: (Sized b, Monoid b) =>
26 Int -> JoinList b a -> Maybe a
27 indexJ _ Empty = Nothing
28 indexJ n _ | n < 0 = Nothing
29 indexJ n (Single _ a) | n == 0 = Just a
31 indexJ n (Append t l r) | n >= getSize (size t) = Nothing
33 | n < (getSize (size (tag l))) = indexJ n l
34 | otherwise = indexJ (n - getSize (size (tag l))) r