All, This issue was brought up again recently by Milan's questions about what to do with folds for the containers package. Currently the Foldable type class has: class Foldable t where fold :: Monoid m => t m -> m foldMap :: Monoid m => (a -> m) -> t a -> m foldr :: (a -> b -> b) -> b -> t a -> b foldl :: (a -> b -> a) -> a -> t b -> a foldr1 :: (a -> a -> a) -> t a -> a foldl1 :: (a -> a -> a) -> t a -> a with default implementations for each in terms of the others. Then it defines: foldr' :: Foldable t => (a -> b -> b) -> b -> t a -> b foldr' f z0 xs = foldl f' id xs z0 where f' k x z = k $! f x z foldl' :: Foldable t => (a -> b -> a) -> a -> t b -> a foldl' f z0 xs = foldr f' id xs z0 where f' x k z = k $! f z x That is, they are fixed definitions so specialised implementations cannot be provided. Note also that these are the classic higher-order "foldl in terms of foldr" definitions. Current releases of GHC cannot optimises these higher-order definitions into efficient versions using accumulating parameters. Since one of the main purposes of foldl' is performance (other purpose being to avoid space leaks) then that's rather unfortunate. The proposal is simple: move these two functions into the Foldable type class itself. They would keep their existing default definitions but since they are then class methods they can have efficient implementations provided by the class instances. This should not break much code. In particular it should not break existing type class instance declarations since there is a default definition for instances that don't defined the new methods. The only potential breakage is that foldl' and foldr' are exported via Foldable(..) rather than directly. This could affect modules that use explicit imports. (I consider this fact to be a slightly unfortunate quirk of the Haskell module system). Patch attached. Deadline: 2 weeks: Monday 4th July. Unresolved: what is a good concise specification of foldr' to use in the documentation? For foldl' we can say: foldl' f z = List.foldl' f z . toList Related issues not covered by this simple proposal: providing foldl1' and foldr1', updating instances to define foldl' and foldr' if possible (e.g. array could provide an efficient impl of foldr'). Duncan