標準ライブラリに on あるの知らなかった

gistfile1.hs

import Data.Function
import Data.List

data Person = Person { name :: String, age :: Int } deriving Show

people = [
    Person "Joe"   16
  , Person "Mary"   8
  , Person "Bob"    9
  , Person "Mike"   8
  , Person "Alice" 11
  ]

groupBy ((==) `on` age) . sortBy (compare `on` age) $ people