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{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE BlockArguments #-}
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE StandaloneKindSignatures #-}
{-# LANGUAGE UnboxedTuples #-}
{-# LANGUAGE UnliftedDatatypes #-}
module Main where
import GHC.Exts
import GHC.IO
type U :: UnliftedType
data U = X | Y
showU :: U -> String
showU X = "X"
showU Y = "Y"
main :: IO ()
main = do
res <- IO \ s0 ->
case newMutVar# X s0 of
(# s1, mvar #) ->
case readMutVar# mvar s1 of
(# s2, r1 #) ->
case writeMutVar# mvar Y s2 of
s3 -> case readMutVar# mvar s3 of
(# s4, r2 #) ->
(# s4, [ showU r1, showU r2 ] #)
putStrLn (unwords res)
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