Hi, I browsed the current AVL tree implementation in Hackage http://hackage.haskell.org/packages/archive/AvlTree/4.2/doc/html/src/Data-Tr... AVL tree denote the different of height from right sub-tree to left sub-tree as delta, to keep the balance, abs(delta)<=1 is kept as invariant. So the typical implementation define N (Negative), P (Positive), and Z (zero) as the tree valid nodes (and the Empty as the trivial case). When a new element is inserted, the program typically first check if the result will break the balance, and process rotation to keep the balance of the tree. Some other pure functional implementation takes the same approach, for example: Guy Cousineau and Michel Mauny. ``The Functional Approach to Programming''. pp 173 ~ 186 Consider the elegant implementation of Red-black tree in pattern matching way by Chris Okasaki, I tried to use the same method in AVL tree, and here is the result. module AVLTree where -- for easy verification, I used Quick Check package. import Test.QuickCheck import qualified Data.List as L -- for verification purpose only -- Definition of AVL tree, it is almost as same as BST, besides a new field to store delta. data AVLTree a = Empty | Br (AVLTree a) a (AVLTree a) Int insert::(Ord a)=>AVLTree a -> a -> AVLTree a insert t x = fst $ ins t where -- result of ins is a pair (t, d), t: tree, d: increment of height ins Empty = (Br Empty x Empty 0, 1) ins (Br l k r d) | x < k = node (ins l) k (r, 0) d | x == k = (Br l k r d, 0) -- For duplicate element, we just ignore it. | otherwise = node (l, 0) k (ins r) d -- params: (left, increment on left) key (right, increment on right) node::(AVLTree a, Int) -> a -> (AVLTree a, Int) -> Int -> (AVLTree a, Int) node (l, dl) k (r, dr) d = balance (Br l k r d', delta) where d' = d + dr - dl delta = deltaH d d' dl dr -- delta(Height) = max(|R'|, |L'|) - max (|R|, |L|) -- where we denote height(R) as |R| deltaH :: Int -> Int -> Int -> Int -> Int deltaH d d' dl dr | d >=0 && d' >=0 = dr | d <=0 && d' >=0 = d+dr | d >=0 && d' <=0 = dl - d | otherwise = dl -- Here is the core pattern matching part, there are 4 cases need rebalance balance :: (AVLTree a, Int) -> (AVLTree a, Int) balance (Br (Br (Br a x b dx) y c (-1)) z d (-2), _) = (Br (Br a x b dx) y (Br c z d 0) 0, 0) balance (Br a x (Br b y (Br c z d dz) 1) 2, _) = (Br (Br a x b 0) y (Br c z d dz) 0, 0) balance (Br (Br a x (Br b y c dy) 1) z d (-2), _) = (Br (Br a x b dx') y (Br c z d dz') 0, 0) where dx' = if dy == 1 then -1 else 0 dz' = if dy == -1 then 1 else 0 balance (Br a x (Br (Br b y c dy) z d (-1)) 2, _) = (Br (Br a x b dx') y (Br c z d dz') 0, 0) where dx' = if dy == 1 then -1 else 0 dz' = if dy == -1 then 1 else 0 balance (t, d) = (t, d) -- Here are some auxiliary functions for verification -- check if a AVLTree is valid isAVL :: (AVLTree a) -> Bool isAVL Empty = True isAVL (Br l _ r d) = and [isAVL l, isAVL r, d == (height r - height l), abs d <= 1] height :: (AVLTree a) -> Int height Empty = 0 height (Br l _ r _) = 1 + max (height l) (height r) checkDelta :: (AVLTree a) -> Bool checkDelta Empty = True checkDelta (Br l _ r d) = and [checkDelta l, checkDelta r, d == (height r - height l)] -- Auxiliary functions to build tree from a list, as same as BST fromList::(Ord a)=>[a] -> AVLTree a fromList = foldl insert Empty toList :: (AVLTree a) -> [a] toList Empty = [] toList (Br l k r _) = toList l ++ [k] ++ toList r -- test prop_bst :: (Ord a, Num a) => [a] -> Bool prop_bst xs = (L.sort $ L.nub xs) == (toList $ fromList xs) prop_avl :: (Ord a, Num a) => [a] -> Bool prop_avl = isAVL . fromList . L.nub And here are my result in ghci: *AVLTree> test prop_avl OK, passed 100 tests. The program is available in github: http://www.google.com/url?sa=D&q=https://github.com/liuxinyu95/AlgoXY/blob/algoxy/datastruct/tree/AVL-tree/src/AVLTree.hs I haven't provided delete function yet. Cheers. -- Larry, LIU https://github.com/liuxinyu95/AlgoXY