ちゅーんさんにとって本当に必要だったもの

main.hs

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE RankNTypes #-}
module Main where
import Control.Lens
import Data.List

finding :: (a -> Bool) -> Lens' [a] (Maybe a)
finding f = lens (find f) $ setToList f

setToList :: (a -> Bool) -> [a] -> (Maybe a) -> [a]
setToList _ xs Nothing = xs
setToList _ [] (Just y) = []
setToList f (x:xs) m@(Just y) =
  if f x then y : xs else x : setToList f xs m

main :: IO ()
main = do
  print $ [1..10]^.finding (==5) -- Just 5
  print $ [1..10]&finding (==5)._Just.~999 -- [1,2,3,4,999,6,7,8,9]
  print $ [1..10]^.finding (==0) -- Nothing
  print $ [1..10]&finding (==0)._Just.~999 -- [1,2,3,4,5,6,7,8,9]