{-# LANGUAGE OverloadedStrings #-}

import Control.Lens

test3s = [Left (1::Int), Left (2::Int),Right 3]

test3_3 :: Maybe [Either Int Int]
test3_3 =
  case preview (_head . _Left) test3s of
    Just _  -> Just (set (_head . _Left) 200 test3s)
    Nothing -> Nothing
    
data MaybeMatched a = Matched a | NotMatched a deriving (Show,Functor)

instance Applicative MaybeMatched where
  pure a = NotMatched a
  NotMatched a <*> NotMatched b = NotMatched $ a b
  NotMatched a <*> Matched b = Matched $ a b
  Matched a <*> NotMatched b = Matched $ a b
  Matched a <*> Matched b = Matched $ a b

test_mm :: MaybeMatched [Either Int Int]
test_mm = let
  myset lens x = lens (\_ -> Matched x)
  in myset (_head . _Left) 5 test3s   -- try replacing with []

main = print test_mm