The splittable idea isn't mine, it looks like perhaps Remi Turk did it.
One thing which I'd recommend is including getRandom, getRandomR as
implemented in terms of getR and the ordinary random operations,
simply because they're the two most common uses, it's probably worth
it to define them separately. Also, getRandoms and getRandomRs would
still be convenient to have in the library, and should do appropriate
splitting of the generator.
- Cale
On 06/02/07, Yitzchak Gale <[EMAIL PROTECTED]> wrote:
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}
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