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{-# LANGUAGE TypeFamilies #-}
{-# OPTIONS_GHC -fdefer-out-of-scope-variables #-}
module T12170a where
-- import Control.Monad -- comment this out to cause error
import Data.IORef
class MonadRef m where
type Ref m :: * -> *
newRef :: a -> m (Ref m a)
readRef :: Ref m a -> m a
instance MonadRef IO where
type Ref IO = IORef
newRef = newIORef
readRef = readIORef
foo :: IO ()
foo = newRef (pure ()) >>= join . readRef
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