{-# 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 (traversed . _Left) 200 test3s)
    Nothing -> Nothing

main = print test3_3