Add composition of MonadTransformers

[original issue 84 by @Icelandjack]

Add

--      ComposeT :: MonadTransformer -> MonadTransformer -> MonadTransformer

type    ComposeT :: (k3 -> k2 -> Type) -> (k1 -> k3) -> (k1 -> k2 -> Type)

newtype ComposeT trans1 trans2 m a = ComposeT (trans1 (trans2 m) a)

  deriving newtype (Functor, Applicative, Monad)


instance (MonadTrans trans1, MonadTrans trans2) => MonadTrans (ComposeT trans1 trans2) where

  lift :: Monad m => m ~> ComposeT trans1 trans2 m

  lift = ComposeT . lift . lift

where MonadTransformer = (Type -> Type) -> (Type -> Type).

Now we can derive MonadTrans for transformers like

type    OkT :: MonadTransformer

newtype OkT m a = OkT (Int -> Int -> m a)

 deriving (Functor, Applicative, Monad)

 via ReaderT Int (ReaderT Int m)


 deriving MonadTrans

 via ComposeT (ReaderT Int) (ReaderT Int)