[If this should have gone to a different list, please tell me and I will subscribe to that.] Hi! There is no rule in standard libs related to unfoldr. From short googling I know that there may be several approaches to this, but as long as nothing fancy is introduced, the following might be a simple improvement: unfoldr :: (b -> Maybe (a,b)) -> b -> [a] unfoldr u x = build (g x) where g x c n = f x where f x = case u x of Just (h,t) -> h `c` f t Nothing -> n {-# INLINE unfoldr #-} It makes unfoldr a good producer and works nicely with the following example from `The Under-Appreciated Unfold' (Gibbons&Jones): === module Tree(Tree(..), bftf) where import Unfold data Tree a = Nd a [Tree a] root (Nd r _) = r kids (Nd _ ks) = ks bftf = concat . levelf levelf = unfold null (map root) (concat . map kids) unfold :: (b -> Bool) -> (b -> a) -> (b -> b) -> b -> [a] unfold p h t = unfoldr u where u x | p x = Nothing | otherwise = Just (h x, t x) === Greetings, Carsten -- Carsten Schultz (2:38, 33:47), FB Mathematik, FU Berlin http://carsten.codimi.de/ PGP/GPG key on the pgp.net key servers, fingerprint on my home page.