[Haskell-cafe] Re: Importance of MonadRandom

[Cale Gibbard]

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 <gale at sefer.org> 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}
>