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{-# 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
main = print test3_3
 
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