Hi all, After working on optimizing the folds defined in Data.Map, I'm not sure that the current definitions are correct. In particular, `fold` is defined as -- | /O(n)/. Fold the values in the map, such that -- @'fold' f z == 'Prelude.foldr' f z . 'elems'@. -- For example, -- -- > elems map = fold (:) [] map -- -- > let f a len = len + (length a) -- > fold f 0 (fromList [(5,"a"), (3,"bbb")]) == 4 It's not true that fold f z == foldr f z . elems in general. It only holds if `z` is an identity for `f` as `z` is used at every leaf in the tree. Using a none identity element for `z` doesn't really work in practice as a user cannot tell how many times `z` will be used, as it depends on the shape of tree which in turn depends on how balanced it is at any given point. Is there a better way to define folds over maps or should we just change the documentation to say that you most likely want `z` to be an identity? Cheers, Johan