[Haskell-cafe] Re: Importance of MonadRandom

Yitzchak Gale gale at sefer.org
Tue Feb 6 05:30:43 EST 2007

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

Below are the new versions of the classes. Any



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


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