Hello Cafe, left- and rightwards folds come in strict and lazy variants foldl/fold' and foldr/foldr' which makes sense because strict versions sometimes use less stack space while lazy versions support infinite data. For example, head (foldr (:) [] [1..]) returns in an instant while head (foldr' (:) [] [1..]) diverges. On the other hand foldl' (flip (:)) 0 [1..10^9] runs in constant space while foldl (flip (:)) 0 [1..10^9] consumes all memory available on my machine (at least without optimizations. With optimizations GHC's strictness analyzer seems to be smart enough to make the accumulator strict.) Data.Foldable also provides the monoidal fold function foldMap. It is left unspecified whether the elements are accumulated leftwards, rightwards or in some other way, which is possible because the combining function is required to be associative. Does this additional freedom for implementors go so far as to allow for strict accumulation? Is it safe to implement foldMap (without prime) with a strict accumulator or are there examples where lazy accumulation is useful like in the above foldr example and distinguishable from strict accumulation without breaking the monoid laws? Sebastian P.S. I thought the `Last` monoid would be an example that requires a lazy accumulator but it is not because the `Just` constructor guards list elements.