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 wrote:

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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On 2016-06-21 at 02:12, Christopher Howard wrote:

However, it seems that I must implement MonadRandom myself, as there is no instance for this sort of arrangement already.

Deriving the class also works, at least with GHC-7.10.3: newtype ReaderStateRandom r s g a = RSR { rSR :: ReaderT r (StateT s (Rand g)) a } deriving (Functor, Applicative, Monad, MonadReader r, MonadState s, MonadRandom) bergey