State and Rand Commute

staterand.hs

{-# LANGUAGE GeneralizedNewtypeDeriving, NoMonomorphismRestriction #-}

import Control.Monad
import Control.Monad.State
import Control.Monad.Random

newtype SR g s a = SR { unSR :: RandT g (State s) a }
 deriving (Monad, Applicative, Functor)

runSR m = do
 g <- newStdGen
 return $ runState (runRandT (unSR m) g) False

newtype RS g s a = RS { unRS :: StateT s (Rand g) a }
 deriving (Monad, Applicative, Functor)

runRS m = do
 g <- newStdGen
 return $ runRand (evalStateT (unRS m) False) g

t1 :: RandomGen g => SR g Bool [Either Int Bool]
t1 = do
 r1 <- SR (getRandomR (1::Int,6))
 b1 <- SR (lift get)
 SR (lift $ modify not)
 r2 <- SR (getRandomR (1::Int,6))
 b2 <- SR (lift get)
 r3 <- SR (getRandomR (1::Int,6))
 return [Left r1,Right b1,Left r2,Right b2,Left r3]

t2 :: RandomGen g => RS g Bool [Either Int Bool]
t2 = do
 r1 <- RS (lift $ getRandomR (1::Int,6))
 b1 <- RS (get)
 RS (modify not)
 r2 <- RS (lift $ getRandomR (1::Int,6))
 b2 <- RS (get)
 r3 <- RS (lift $ getRandomR (1::Int,6))
 return [Left r1,Right b1,Left r2,Right b2,Left r3]