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{-# LANGUAGE NoMonadFailDesugaring #-}
module WCompatWarningsOnOff where
import qualified Data.Semigroup as Semi
monadFail :: Monad m => m a
monadFail = do
Just _ <- undefined
undefined
(<>) = undefined -- Semigroup warnings
-- -fwarn-noncanonical-monoid-instances
newtype S = S Int
instance Semi.Semigroup S where
(<>) = mappend
instance Monoid S where
S a `mappend` S b = S (a+b)
mempty = S 0
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