[Haskell-cafe] Join a transformer

[Ross Paterson]

On Wed, Apr 11, 2018 at 11:17:54AM +0000, David Feuer wrote:
>    If something like MMonad gets added to transformers, then its MFunctor
>    version should really be double-ended to work with things like the
>    final version of FreeT.

Since transformers is targeted at standard Haskell, it wouldn't be able
to define MFunctor or embed.  It would have to be something like

class (MonadTrans t) => MonadMonad t where
    joinT :: Monad m => t (t m) a -> t m a