Hi, I have been thinking about how foldl' can be turned into a good consumer, and I came up with something that I thought would work. So I'd like to ask for opinions from the ghc devs: if this idea looks good, if it is a known bad idea, if there is a better way to do it, etc. The main idea is to have an extended version of foldr: -- | A mapping between @a@ and @b@. data Wrap a b = Wrap (a -> b) (b -> a) foldrW :: (forall e. Wrap (f e) (e -> b -> b)) -> (a -> b -> b) -> b -> [a] -> b foldrW (Wrap wrap unwrap) f z0 list0 = wrap go list0 z0 where go = unwrap $ \list z' -> case list of [] -> z' x:xs -> f x $ wrap go xs z' This allows the user to apply an arbitrary "worker-wrapper" transformation to the loop. Using this, foldl' can be defined as newtype Simple b e = Simple { runSimple :: e -> b -> b } foldl' :: (b -> a -> b) -> b -> [a] -> b foldl' f initial xs = foldrW (Wrap wrap unwrap) g id xs initial where wrap (Simple s) e k a = k $ s e a unwrap u = Simple $ \e -> u e id g x next acc = next $! f acc x The wrap and unwrap functions here ensure that foldl' gets compiled into a loop that returns a value of 'b', rather than a function 'b -> b', effectively un-CPS-transforming the loop. I put preliminary code and some more explanation on Github: https://github.com/takano-akio/ww-fusion Thank you, Takano Akio