No, but you can do this:

{-# LANGUAGE GeneralizedNewtypeDeriving #-} import Control.Monad (liftM)

newtype Functorize m a = F { unF :: m a } deriving (Eq, Show, Monad)

Insert any other type classes you care about potentially inheriting from the parent Monad into the deriving clause, like MonadPlus. Unfortunately it's not possible to derive "everything my internal type has", which makes it difficult to include things like MonadState or other multiparameter classes.

inF f = F . f . unF instance Monad m => Functor (Functorize m) where fmap f = inF (liftM f)

-- ryan On Sun, Jan 11, 2009 at 5:12 AM, Mikhail Glushenkov wrote:

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)?

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