Since I realized my code was always using infinite lists, I replaced it by Data.Stream. However, my code stopped working. The problem is with these functions: scan :: (a -> b -> a) -> a -> Stream b -> Stream ascan f z (Cons x xs) = z <:> scan f (f z x) xs-- | @scan'@ is a strict scan.scan' :: (a -> b -> a) -> a -> Stream b -> Stream ascan' f z (Cons x xs) = z <:> (scan' f $! (f z x)) xs They are too strict I think. My code works again when I add a lazy pattern match: scan f z ~(Cons x xs) = z <:> scan f (f z x) xs scan' f z ~(Cons x xs) = z <:> (scan' f $! (f z x)) xs This is justified since they then behave like scanl on lists. However it seems this package is used a lot, so maybe some code depends on this strictness. What to do? PS: Why does scanl' not exist in Data.List?