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

[Mikhail Glushenkov]

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)?