@@ -7,7 +7,7 @@ import Control.Monad (MonadPlus)
77import Control.Monad.Fix (MonadFix)
88import Control.Monad.Trans.Class (MonadTrans, lift)
99import Control.Monad.IO.Class (MonadIO)
10−import Control.Monad.Morph (MFunctor(..), MMonad)
10+import Control.Monad.Morph (MFunctor(..), MMonad(..))
1111import Control.Monad.Catch (MonadThrow, MonadCatch, MonadMask)
1212
1313import qualified Control.Monad.Base as MB
@@ -33,7 +33,7 @@ newtype TaggedTrans tag trans m a = TaggedTrans (trans m a)
3333 deriving
3434 ( Generic
3535 , Functor, Applicative, Alternative, Monad, MonadPlus
36− , MonadFix, MonadTrans, MonadIO, MMonad
36+ , MonadFix, MonadTrans, MonadIO
3737 , MonadThrow, MonadCatch, MonadMask )
3838
3939type Pack tag trans m a = trans m a -> TaggedTrans tag trans m a
@@ -179,3 +179,10 @@ instance
179179-- trigger GHC Trac #11837 on GHC 8.0.1 and older.
180180instance MFunctor trans => MFunctor (TaggedTrans tag trans) where
181181 hoist f (TaggedTrans t) = TaggedTrans (hoist f t)
182+
183+-- NB: Don't use GeneralizedNewtypeDeriving to create this instance, as it will
184+-- trigger GHC Trac #11837 on GHC 8.0.1 and older.
185+instance MMonad trans => MMonad (TaggedTrans tag trans) where
186+ embed f (TaggedTrans t) = TaggedTrans (embed (runTaggedTrans . f) t)
187+ where
188+ runTaggedTrans (TaggedTrans x) = x