Hello all, This is a really helpful list: I've learned half a dozen new things just by reading this month's traffic. Anyway...I have the following bit of code that updates a tree structure given a route to a leaf: data Tree a = Leaf a | Node (Tree a) (Tree a) deriving (Show, Eq) data PathSelector = GoLeft | GoRight deriving (Show, Eq) type Path = [PathSelector] selectChild (Node left _) GoLeft = left selectChild (Node _ right ) GoRight = right updateNode (Node _ right) GoLeft newLeft = Node newLeft right updateNode (Node left _) GoRight newRight = Node left newRight updateLeaf new (Leaf previous) = Leaf new updateTree :: Tree a -> Path -> a -> Tree a updateTree tree path newValue = case path of [] -> updateLeaf newValue tree (p:ps) -> updateNode tree p (updateTree' (selectChild tree p) ps newValue) I wanted to rewrite updateTree without using explicit recursion. Unfortunately, the best I could come up with is: upDownRecurse :: (a -> b -> a) -> (a -> c) -> (a -> b -> c -> c) -> a -> [b] -> c upDownRecurse down bottoming up = upDownRecurse' where upDownRecurse' acc [] = bottoming acc upDownRecurse' acc (x:xs) = up acc x (upDownRecurse' (down acc x) xs) updateTree' :: Tree a -> Path -> a -> Tree a updateTree' tree path newValue = upDownRecurse selectChild (updateLeaf newValue) updateNode tree path So what's the sexier way of doing this? Cheers, -- Matt