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.