Hi, see http://hackage.haskell.org/trac/ghc/ticket/4313. The following text is the description of ticket 4313: This proposal depends on #4278 and #4280. In accordance with a poll on libraries@haskell.org (see point 3 of http://article.gmane.org/gmane.comp.lang.haskell.libraries/13273) I propose to add strict folds to the containers. The Data.Map is getting left, right and strict folds in #4278. This proposal adds left, right and strict folds for Set, IntMap and IntSet. The naming is a bit tricky. The folds in IntMap mimics Map (ie. foldrWithKey, foldlWithKey, foldlWithKey'; fold and foldWithKey are deprecated in favor of foldrWithKey). The folds in Set and IntSet are classic (foldr, foldl, foldl'; the old fold is deprecated in favor of foldr). The repository of the containers package with these patches (and also several others) is at http://fox.auryn.cz/darcs/containers/. The patches are also attached (including #4280).