Switch base of the transformer stack to UIO

A lot of the utilities are still in IO for now, and we still provide a
transitional MonadIO instance, but the transformer stack itself is Unexceptional now.

gnutls.cabal

@@ -36,6 +36,7 @@ library
       base >= 4.0 && < 5.0
     , bytestring >= 0.9
     , transformers >= 0.4.0.0
+    , unexceptionalio-trans
 
   extra-libraries: gnutls
   pkgconfig-depends: gnutls

lib/Network/Protocol/TLS/GNU.hs

@@ -54,10 +54,12 @@ import qualified Foreign.C as F
 import           Foreign.Concurrent as FC
 import qualified System.IO as IO
 import           System.IO.Unsafe (unsafePerformIO)
+import           UnexceptionalIO.Trans (UIO, Unexceptional)
+import qualified UnexceptionalIO.Trans as UIO
 
 import qualified Network.Protocol.TLS.GNU.Foreign as F
 
-data Error = Error Integer
+data Error = Error Integer | IOError IOError
 	deriving (Show)
 
 globalInitMVar :: M.MVar ()
@@ -86,7 +88,7 @@ data Session = Session
 	, sessionCredentials :: IORef [F.ForeignPtr F.Credentials]
 	}
 
-newtype TLS a = TLS { unTLS :: ExceptT Error (R.ReaderT Session IO) a }
+newtype TLS a = TLS { unTLS :: ExceptT Error (R.ReaderT Session UIO) a }
 
 instance Functor TLS where
 	fmap f = TLS . fmap f . unTLS
@@ -99,11 +101,12 @@ instance Monad TLS where
 	return = TLS . return
 	m >>= f = TLS $ unTLS m >>= unTLS . f
 
+-- | This is a transitional instance and may be deprecated in the future
 instance MonadIO TLS where
-	liftIO = TLS . liftIO
+	liftIO = TLS . withExceptT IOError . UIO.fromIO' (userError . show)
 
-runTLS :: Session -> TLS a -> IO (Either Error a)
-runTLS s tls = R.runReaderT (runExceptT (unTLS tls)) s
+runTLS :: (Unexceptional m) => Session -> TLS a -> m (Either Error a)
+runTLS s tls = UIO.lift $ R.runReaderT (runExceptT (unTLS tls)) s
 
 runClient :: Transport -> TLS a -> IO (Either Error a)
 runClient transport tls = do