minimumOn function

[Boarders]

It would be nice to have a minimumOn function of the form:

minimumOn :: (Ord b, Generic.Vector v a) => (a -> b) -> v a -> a 

A possible implementation could just adapt the existing one for minimumBy:

minimumOn :: (Ord b, Vector v a) => (a -> b) -> v a -> a
{-# INLINE minimumOn #-}
minimumOn f = Bundle.foldl1' minBy . stream
 where
 {-# INLINE minBy #-}
 minBy x y = case compare (f x) (f y) of
 GT -> y
 _ -> x

[lehins]

@Boarders I can see how such function can be useful (especially for Storable, Unbox and Prim, when b doesn't have an instance, while a does), but there is a problem with your suggested implementation. Namely function f will be evaluated 2*n - 1 times and in case when f is really expensive this could be very noticeable.

A possibly better solution would be to first zip with that function over:

minimumOn :: (Ord b, Vector v a) => (a -> b) -> v a -> a
{-# INLINE minimumOn #-}
minimumOn f = fst . Bundle.foldl1' minBy . Bundle.map (\a -> (a, f a)) . stream
 where
 {-# INLINE minBy #-}
 minBy x y = case compare (snd x) (snd y) of
 GT -> y
 _ -> x

If anyone is willing to implement this and maximumOn for all vector types with docs, tests and at least one benchmark, I would not be opposed to merging such addition.

[Shimuuar]

Fixed by #356