I'm writing a program with several functions, some of which depend on certain fields in a state monad, others of which depend on others, but no routine needs all the fields. So I thought I would declare a two classes, one for each type of data need that a function has: -- as an aside, here's an example of data which is parameterized by two types. data ReportData t1 t2 = ... -- this is rolling my own state monad with a random generator class Monad m => RandMonad m where getGen :: m StdGen putGen :: StdGen -> () -- this is a class of state monad which logs ReportData: class Monad m => LogMonad m where putReport :: ReportData t1 t2 -> m () For a particular use case, I declare a type of State monad: data MyStateData t1 t2 = MyStateData t1 t2 { theGen :: StdGen , theReports :: [StepReport t1 t2] } type MyState t1 t2 = State (MyStateData t1 t2) And I try to define my instances: instance RandMonad (MyState t1 t2) where getGen = gets theGen putGen g = modify (\s -> s { theGen = g}) instance LogMonad (MyState t1 t2) where putReport r = modify (\s -> s { theReports = r : theReports s}) I get an error on the LogMonad instance, saying that there's no instance for (MonadState (MyState t1 t2) (StateT (MyState t1 t2) Identity)) I guess I don't really understand typeclasses once you start using higher kinded types, so please enlighten me. Any reading on this subject would be helpful, too.