L.S., I'm currently writing code where I want some ADTs to be parametric with a monad. There are some extra conditions that I place on my parameter, but I've boiled things down to minimal reproducibility. When I define MyADT as follows: import Data.Typeable data MyADT m = MyADT (m ()) instance (Typeable1 m, Monad m) => Typeable (MyADT m) where typeOf t@(MyADT _) = mkTyCon "MyADT" `mkTyConApp` [typeOf1 ((return :: Monad m => MyADT m -> m (MyADT m)) t)] it compiles fine and GHCi works as expected on a single instance of MyADT: *Main> typeOf (MyADT (return () :: IO ())) MyADT IO However, as soon as I place my data type in a structure of sorts, things break down: *Main> typeOf [(MyADT (return () :: IO ()))] *** Exception: Prelude.undefined *Main> typeOf (Just (MyADT (return () :: IO ()))) *** Exception: Prelude.undefined *Main> typeOf ((return :: a -> IO a) (MyADT (return () :: IO ()))) *** Exception: Prelude.undefined But if I stick a number in a similar structure, typeOf works just fine: *Main> typeOf ((return :: a -> IO a) 5) IO Integer I don't quite understand where the undefined comes from. I'm certainly not using it anywhere (as shown by the minimal reproduction above). Weirder still, when I *do* introduce a nice error, that too does not come up, viz. import Data.Typeable data MyADT m = MyADT (m ()) instance (Typeable1 m, Monad m) => Typeable (MyADT m) where typeOf t@(MyADT _) = error "foobar" with GHCi-session: *Main> typeOf (MyADT (return () :: IO ())) *** Exception: foobar *Main> typeOf ((return :: a -> IO a) (MyADT (return () :: IO ()))) *** Exception: Prelude.undefined *Main> typeOf $ Just (MyADT (return () :: IO ())) *** Exception: Prelude.undefined *Main> typeOf $ Just 42 Maybe Integer FYI, I'm using GHC 7.0.3, as installed with the Haskell Platform 2011.2.0.1. Am I overlooking something? Any help would be appreciated. Regards, Philip