I just discovered the highly useful function Data.Function.on. I vaguely recall a few people muttering a couple of years back that this would be a useful thing to have, but I had no idea it was in the standard libraries now. Anyway, while using it, I discovered a small omission from the Haskell libraries: We have min, max, minimum and maximum. We also have minimumBy and maximumBy. But there's no sign of minBy or maxBy. You can do, for example, (min `on` postcode) customer1 customer2 but that gives you the lowest postcode, not the customer to which this postcode belongs. By contrast, minimumBy (compare `on` postcode) [customer1, customer2] gives you the corresponding customer. But it seems silly to have to actually construct a list just for this. So... is there any danger of getting minBy and maxBy added? (I don't actually know off the top of my head where min and max are defined - the Prelude?) Also, constructions like sortBy (compare `on` foo) must surely be very common. Perhaps it would be an idea to define a family of functions like sortOn :: (Ord y) => (x -> y) -> [x] -> [x] sortOn foo = sortBy (compare `on` foo) Just an idea. Finally, take a look at this: newtype SwapOrd x = SwapOrd (unswap_ord :: x) deriving Eq instance Ord x => Ord (SwapOrd x) where compare x y = swap_ord $ compare x y swap_ord :: Ordering -> Ordering swap_ord o = case o of EQ -> EQ GT -> LT LT -> GT Just in case you wanted to sort things in reverse order. I think having swap_ord would be nice if nothing else...