I'm afraid, but you are missing a case here. For example the tree Branch (Leaf 1) (Branch (Leaf 3) (Leaf 2)) is not constructed by your program. Correct is insert x t@(Leaf y) = [Branch s t, Branch t s] where s = Leaf x insert x t@(Branch l r) = [Branch l' r | l' <- insert x l] ++ [Branch l r' | r' <- insert x r] ++ {- missed this: -} [Branch s t,Branch t s] where s = Leaf x With this modification, your program becomes essentially the same as my version but with suboptimal sharing (you construct Leaf x twice and with the correction even three times). As a consequence my version is faster and eats less memory. /BR -- -- Mirko Rahn -- Tel +49-721 608 7504 -- --- http://liinwww.ira.uka.de/~rahn/ ---