Given data Tree a = Leaf a | Node (Tree a) (Tree a) Write a leaf counter. Hutton suggests: leaves :: Tree a -> Int leaves (Leaf _) = 1 leaves (Node l r) = leaves l + leaves r I tried: leavesTrent :: Tree a -> Int leavesTrent = leaves' 0 where leaves' n (Leaf a) = n + 1 leaves' n (Node l r) = (leaves' n l), (leaves' n r) The idea is: If it is a leaf, add one to the accumulator. (Following Hutton's explanation of how sum works if defined with foldl.) If it is a tree, proceed down the left subtree recursively, until you get to a leaf, then roll up to the right subtree. The problem (among the problems) is that I don't know how to tell the compiler to do all lefts, before backing up to go right. I only know how to do that using a real operator like "+" or foo (l, r). Is that kind of no-op recursive branching possible? Trent.