Addition of MonadReader like interface for StatefulGen

[lehins]

No description provided.

[Shimuuar]

That's great!

I played with code a bit. Few observations. MonadStatefulGen does not need env parameter, it's only used for default definition of functions in terms of MonadReader. And it could be dropped at modest cost of not having default implementation of localGenM:

class (R.StatefulGen g m) => MonadStatefulGen g m | m -> g where
  askGen :: m g
  default askGen :: (MonadReader env m, HasGenEnv env g) => m g
  askGen = ...
  localGenM :: (g -> m g) -> m a -> m a
  default localGenM :: (MonadReader env m, HasGenEnv env g) => (g -> m g) -> m a -> m a
  localGenM = ...

Also I don't think m needs to be instance of MonadReader env. Let have

newtype Rand a = Rand (State SplitMix a)

Such type could obviously be instance of MonadStatefulGen StateGenM but there's no good reason to be instance of MonadReader StateGenM as well.

DerivingVia

I have unreasonable fondness of DerivingVia so I checked whether it possible to use it too define instances. It turns out yet.

newtype ViaReader env m a = ViaReader (m a)
  deriving newtype (Functor, Applicative, Monad)

deriving newtype instance MonadReader env m => MonadReader env (ViaReader env m)
deriving newtype instance R.StatefulGen g m => R.StatefulGen g (ViaReader env m)

instance (MonadReader env m, HasGenEnv env g, R.StatefulGen g m) =>
  MonadStatefulGen g (ViaReader env m)

deriving via ViaReader env (Lazy.StateT s m)
    instance ( R.StatefulGen g (Lazy.StateT s m)
             , MonadReader env m
             , HasGenEnv env g
             ) => MonadStatefulGen g (Lazy.StateT s m)

Also I didn't checked whether it actually works but it does compile.