client/src/Util/List.hs


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module Util.List
  ( groupBy
  ) where

import           Control.Arrow ((&&&))
import           Data.Function (on)
import qualified Data.List     as L

groupBy :: forall a b. (Ord b) => (a -> b) -> [a] -> [(b, [a])]
groupBy f =
  map (f . head &&& id)
    . L.groupBy ((==) `on` f)
    . L.sortBy (compare `on` f)