Just manually unrolling the definitions for ReaterT and StateT, I think
this is correct.
instance RandomGen g => MonadRandom (ReaderStateRandom r s g) where
getRandom = RSR (ReaderT (\r -> StateT (\s -> getRandom >>= (\random ->
return (random,s)))))
Or, using TupleSelections,
RSR (ReaderT (\r -> StateT (\s -> (,s) <$> getRandom)))
You could also write this out in terms of execReaderT, execStateT, etc. but
I couldn't be arsed.
You also have to add Applicative and Functor instances to RSR.
Cheers,
Will
On Mon, Jun 20, 2016 at 11:12 PM, Christopher Howard
Hi, I was expanding on my earlier learning, to try a triple monad stack:
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
<...snip...>
import System.Random import Control.Monad.Random import Control.Monad.State.Lazy import Control.Monad.Reader
newtype ReaderStateRandom r s g a = RSR { rSR :: ReaderT r (StateT s (Rand g)) a } deriving (Monad, MonadReader r, MonadState s)
However, it seems that I must implement MonadRandom myself, as there is no instance for this sort of arrangement already. Probably this is trivial, but I'm having trouble wrapping my mind around how to do it. Would anybody perhaps assist me in implementing one function, to help guide me in the correct direction?
instance MonadRandom (ReaderStateRandom r s g) where
getRandom = ...?
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