Perhaps you just want monad transformers [1, 2]. If you're not familiar with them, you should probably read [3].
With transformers you'd be able to do this (and more):
$ return 2 :: IO Int $ return 2 :: ListT Maybe Int -- this works like Maybe [Int] $ return 2 :: MaybeT [] Int -- this works like [Maybe Int]
In fact, I read and used transformers a bit. If I understand correctly,
I can only do such things for monads, supported by transformers. So,
I cant get (return 2) :: MyWierdMonad (MyAnotherMonad 2)
Yes, I know, it have little practical use, but I am insterested about
teoretical part of question, magic of ghc.
--
Best regards, Dmitry Bogatov