Just for the heck of it, I tried out an implementation of scanl using Joachim Breitner's magical oneShot primitive. Using the test scanlA :: (b -> a -> b) -> b -> [a] -> [b] scanlA f a bs = build $ \c n -> a `c` foldr (\b g x -> let b' = f x b in (b' `c` g b')) (const n) bs a scanlB :: (b -> a -> b) -> b -> [a] -> [b] scanlB f a bs = build $ \c n -> a `c` foldr (\b g -> oneShot (\x -> let b' = f x b in (b' `c` g b'))) (const n) bs a f :: Int -> Bool f 0 = True f 1 = False {-# NOINLINE f #-} barA = scanlA (+) 0 . filter f barB = foldlB (+) 0 . filter f with -O2 (NOT disabling Call Arity) the Core from barB is really, really beautiful: it's small, there are no lets or local lambdas, and everything is completely unboxed. This is much better than the result of barA, which has a local let, and which doesn't seem to manage to unbox anything. It looks to me like this could be a pretty good tool to have around. It certainly has its limits—it doesn't do anything nice with reverse . reverse or reverse . scanl f b . reverse, but it doesn't need to be perfect to be useful. More evaluation, of course, is necessary.to make sure it doesn't go wrong when used sanely. David