How about :
 

> data InteractiveState io = InteractiveState {
> state_read :: io Command
> , state_write :: Result -> io ()
> }

Then you don't even depend on some specific monad. I understand you can always (always?) encapsulate what is done through a type class by using a data containing functions. But then, is this not even closer to OO programming, an object that carries its own methods with itself, possibly with the additional overhead that *each* instance would have its own private references to possibly identical functions.

Thanks,
Arnaud