I'm writing a program which uses a lot of pseudorandom numbers, and for that reason it would be nice to put the StdGen in a state monad. like let's say I want to combine error handling and storing the StdGen state. import Control.Monad.Error import Control.Monad.State data RandState = RandState StdGen -- Er is a monad that combines error handling and pseudorandom state type Er a = ErrorT String (State RandState) a -- to access random numbers, I could define things like erRandomR :: Random r => (r,r) -> Er r erRandomR (lo, hi) = do RandState g <- get let (value, g') = randomR (lo, hi) g put $ RandState g' return value erRandoms :: Random r => Er [r] erRandoms = do RandState g <- get let (g1, g2) = split g let values = randoms g1 put $ RandState g2 return values erRandomRs :: Random r => (r,r) -> Er [r] erRandomRs (lo,hi) = do RandState g <- get let (g1, g2) = split g let values = randomRs (lo,hi) g1 put $ RandState g2 return values -- I could define new ways of using random values, like choosing a random element of a list erChooseList :: [a] -> Er a erChooseList xs = do let l = length xs when (l==0) (throwError "in randomChooseList, passed null list") idx <- erRandomR (0,l-1) return $ xs !! idx However, after I got done with that, I realized that I wanted to add additional state, maybe a ReaderT , stuff like that--different in different parts of the program. But I always want access to random numbers with the same functions: erRandomR, erRandoms, etc. So I thought class Monad m => RandMonad m where putGen :: StdGen -> m () getGen :: m StdGen Then I could make Er an instance of RandMonad, like this type Er a = ErrorT String (State RandState) a instance RandMonad Er where putGen g = put (RandState g) getGen = do RandState g <- get return g Clearly I don't know what I'm doing, because when I tried to run this much I got the error "Type synonym Er should have 1 argument, but has been given none." I tried a bunch of variations of this but got nowhere. Can someone explain how I should conceive of this?