Hello All: Over the past few days I got my first exposure to monad transformers. I worked through the sample below. The witness w displays IC 0 as expected, but even after providing the instance of show for type O I a, witness w' causes ghci to display No instance for (Num (I' a0)) arising from a use of w' Possible fix: add an instance declaration for (Num (I' a0)) Would someone be able to explain why after providing the instance of show on O I a, w' does not print the expected result (OC (IC 0)) ? BTW - You will notice the sample intentionally avoids the use of deriving (Show) on newtype I. -- Rick {-# LANGUAGE NoMonomorphismRestriction, DatatypeContexts, FlexibleContexts, FlexibleInstances #-} module Main where import Control.Monad import Control.Monad.Trans.Class <snip> -- |A parameterized new type I representing an "inner type" with one constructor IC newtype I a = IC a -- deriving (Show) -- |Unwraps the value in the inner type unI (IC x) = x -- |A monad instance on the inner type instance Monad I where return = IC m >>= f = f (unI m) instance Show a => Show (I a) where show (IC x) = "IC " ++ show x -- |Witness on inner type w :: Num a => I a w = (IC 0) >>= return . id -- |A parameterized new type O m a representing an "outer type" with a named constructor newtype O m a = OC {runO :: m (I a)} -- deriving (Show) -- |? instance Show a => Show (O I a) where show (OC x) = "OC (IC " ++ show (unI x) ++ "))" -- show (OC (IC 0)) -- |Unwraps the value in the outer type unO (OC (IC a)) = a -- |A monad transformer instance on type O m a instance Monad m => Monad (O m) where return = OC . return . IC m >>= f = OC $ do {v <- runO m; runO (f (unI v))} -- | Witness on monad transformer w' :: Num (I a) => O I a w' = (OC (IC 0)) >>= return . id -- | Executes addition on the zero in IC w'' :: Num a => I a w'' = liftM (+1) (IC' 0) -- |Instance of monad transformer lift on O m a instance MonadTrans O where lift m = OC (m >>= return . IC) -- |Executes addition on the zero in IC and lifts to O m a w''' :: (Num a, MonadTrans t) => t I a w''' = lift ((IC 0) >>= return . (+1))