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

On Sep 26, 2018, at 6:27 AM, Tom Ellis wrote:

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)]

It is a rather elementary function: import Data.Foldable import Data.Ord -- Stable version that keeps the input order for elements that are -- equal. If this were to be a library function, I'd drop the -- 'reverse' post-processing step, and leave the choice of stability -- to the caller. -- minimumsBy :: Foldable t => (a -> a -> Ordering) -> t a -> [a] minimumsBy cmp xs = reverse $ foldl' acc [] xs where acc [] x = [x] acc mins@(m:_) x = case cmp m x of LT -> mins EQ -> x:mins GT -> [x] -- Viktor.