[Haskell-cafe] Declaring each instance of a typeclass to be also an instance of another typeclass

Hi, Is it possible to write something like this:

{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}

import Control.Monad (liftM)

instance (Monad a) => Functor a where fmap = liftM

without having to use UndecidableInstances (and preferably, other type system extensions too)?