Re: [Haskell-cafe] Is this haskelly enough?

on, which will appear in Data.Function in the next release of base, is defined thusly:

on :: (b -> b -> c) -> (a -> b) -> a -> a -> c (*) `on` f = \x y -> f x * f y

You can also use Data.Ord.comparing, in this case -- comparing is just (compare `on`).

From Ord.hs:

-- | -- > comparing p x y = compare (p x) (p y) -- -- Useful combinator for use in conjunction with the @xxxBy@ family -- of functions from "Data.List", for example: -- -- > ... sortBy (comparing fst) ... comparing :: (Ord a) => (b -> a) -> b -> b -> Ordering comparing p x y = compare (p x) (p y) Shachaf