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{-# LANGUAGE TypeFamilies, ScopedTypeVariables,
GeneralizedNewtypeDeriving #-}
module T7148a where
import Control.Monad.ST
data Proxy a = Proxy
type family Result a b
class Convert a where
coerce :: Proxy b -> a -> Result a b
newtype SAFE a = SAFE a
type instance Result (SAFE a) b = a
instance Convert (SAFE a) where
coerce _ (SAFE a) = a
newtype IS_NO_LONGER a = IS_NO_LONGER a deriving Convert
type instance Result (IS_NO_LONGER a) b = b
--infered type is
unsafeCoerce :: forall a b. a -> b
unsafeCoerce = coerce (Proxy :: Proxy b) . IS_NO_LONGER . SAFE
--use it safely
id' :: a -> a
id' = unsafeCoerce
--segfault (with high probability)
crash :: segfault
crash = unsafeCoerce . tail . tail . tail . unsafeCoerce $ True
--time for side effects
unsafePerformIO :: IO a -> a
unsafePerformIO x = runST $ unsafeCoerce x
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