Re: [Haskell-cafe] Where is "minimumsBy"?

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 のメール:

Data.List.minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a

https://www.stackage.org/haddock/lts-12.1/base-4.11.1.0/Data-List.html#v:min...

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...

Thanks,

Tom _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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