Hiromi ISHII konn.jinro at gmail.com
Wed Sep 26 14:52:02 UTC 2018

Not just a one-shot function, but you can do it with monoids:

```
import Data.Semigroup (Option(..), Semigroup(..))
import Data.DList

maximumsOn :: Ord b => (a -> b) -> [a] -> Maybe [a]
maximumsOn f = fmap (toList . elts) . getOption . foldMap (\a -> Option \$ Just \$ MaxAll (f a) \$ pure a)
-- You don't need `Option`s since GHC 8.4

data MaxAll a b = MaxAll { weight :: a, elts :: DList b }
instance Ord a => Semigroup (MaxAll a b) where
MaxAll l ls <> MaxAll r rs =
case compare l r of
EQ -> MaxAll l (ls <> rs)
LT -> MaxAll l rs
GT -> MaxAll r ls
```

> 2018/09/26 19:27、Tom Ellis <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk>のメール:
>
> Data.List.minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
>
>
> but there are many cases where that's quite unhelpful.  Actually what we
> want is more like
>
>    minimumsBy :: ... => (a -> a -> Ordering) -> t a -> [a]
>
> There can be many distinct minimizers.  For example when I want to get the
> collection of the youngest people from [(Age, Person)] I want
>
>    minimumsBy (compare `on` fst) [(12, alice), (15, balaji), (12, cho)]
>
> to return
>
>    [(12, alice), (12, cho)]
>
> Does "minimumsBy" exist somewhere reasonably standard?  Hoogle doesn't throw
> up anything obvious
>
>    https://www.stackage.org/lts-12.1/hoogle?q=%28a+-%3E+a+-%3E+Ordering%29+-%3E+t+a+-%3E+%5Ba%5D
>
> Thanks,
>
> Tom
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----- 石井 大海 ---------------------------
konn.jinro at gmail.com

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