Hello I have a question regarding monad transformers and how to design an API with a transformer. I have a narrowed code example of the question. Please see the questions in the comments below. import Data.Monoid import Control.Monad -- Suppose some data type newtype Inter a = Inter (Int -> a) -- and a monad transformer for that data type. newtype InterT m a = InterT (m (Inter a)) -- It's easy to implement this type-class instance (Monoid a) => Monoid (Inter a) where mempty = Inter (const mempty) Inter a `mappend` Inter b = Inter (a `mappend` b) -- and for the transformer too by lifting into the monad instance (Monad m, Monoid a) => Monoid (InterT m a) where mempty = InterT (return mempty) InterT a `mappend` InterT b = InterT (liftM2 mappend a b) -- But what about this type-class? class Ints a where ints :: a -> Int -> Int -- Seems easy enough instance (Integral a) => Ints (Inter a) where ints (Inter a) n = fromIntegral (a n) -- OH NO! {- instance (Monad m, Integral a) => Ints (InterT m a) where ints (InterT a) n = error "OH NO!" -} -- We could try this class Copointed f where copoint :: f a -> a -- but it seems rather impractical instance (Copointed m, Integral a) => Ints (InterT m a) where ints (InterT a) = ints (copoint a) {- So it seems that for some type-classes it is possible to implement for both the data type and the transformer, but not all type-classes. Is there a general approach to this problem? -} -- Tony Morris http://tmorris.net/