Several functions on containers used to have types like lookup :: (Ord k) => k -> Map k a -> Maybe a but these were generalized to lookup :: (Monad m, Ord k) => k -> Map k a -> m a The only methods of Monad used are return and fail. The problem is that, depending on the monad, fail can be an ordinary value or a runtime error: this device makes it harder to check whether a program is safe, because it hides possible runtime errors among testable conditions. The proposal is to change these signatures back, specializing them to Maybe. The functions involved are: lookup :: Ord k => k -> Map k a -> Maybe a lookupIndex :: Ord k => k -> Map k a -> Maybe Int minViewWithKey :: Map k a -> Maybe ((k,a), Map k a) maxViewWithKey :: Map k a -> Maybe ((k,a), Map k a) minView :: Map k a -> Maybe (a, Map k a) maxView :: Map k a -> Maybe (a, Map k a) lookup :: Key -> IntMap a -> Maybe a maxViewWithKey :: IntMap a -> Maybe ((Key, a), IntMap a) minViewWithKey :: IntMap a -> Maybe ((Key, a), IntMap a) maxView :: IntMap a -> Maybe (a, IntMap a) minView :: IntMap a -> Maybe (a, IntMap a) minView :: Set a -> Maybe (a, Set a) maxView :: Set a -> Maybe (a, Set a) maxView :: IntSet -> Maybe (Int, IntSet) minView :: IntSet -> Maybe (Int, IntSet) No information is lost, because in each case there is a single failure mode.