I wrote:
Cale Gibbard's MonadRandom... I would like to suggest a change to the interface... class (Monad m) => MonadRandom m where nextR :: m Int splitR :: m (m ()) rangeR :: m (Int, Int) getR :: (forall g . RandomGen g => g -> a) -> m a
I see that I have inadvertently done two things differently than Cale with regard to split: Cale used a different type, and he put it into a separate monad. The separate monad idea is a very good one. My type is bit more general than Cale's, and it emphasizes the amusing fact that split is a kind of inverse to monadic join. (Actually, a section.) But Cale's type looks more convenient to use. I am modifying my proposal accordingly on both points. Below are the new versions of the classes. Any comments? Thanks, Yitz \begin{code} class Monad m => MonadRandom m where nextR :: m Int rangeR :: m (Int, Int) getR :: (forall g . RandomGen g => g -> a) -> m a -- Minimum complete definition: nextR and rangeR getR f = do r <- nextR (lo, hi) <- rangeR return $ f $ TrivalGen r lo hi class MonadRandom m => MonadRandomSplittable m where splitR :: m a -> m (m a) splitRandom :: m a -> m a -- Use the following default method definitions only -- when splitting is a trivial operation, such as for -- hardware-based random generators. splitR = return splitRandom = id instance Monad m => MonadRandomSplittable (RandT m) where splitR x = RandT (StateT split) >>= return . (>> x) . RandT . put splitRandom x = RandT (StateT split) >>= lift . evalRandT x instance MonadRandomSplittable Rand where splitR = liftState split >>= return . liftState . put splitRandom x = Rand (State split) >>= return . evalRand x \end{code}