Since the monad type `m` only appears to the right of the arrow, we
can write the instance using `lift`:
instance RandomGen g => MonadRandom (ReaderStateRandom r s g) where
getRandom = RSR . lift . lift $ getRandom
On Tue, Jun 21, 2016 at 6:12 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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