[Haskell-cafe] Different forms for RandomT?

[Zemyla]

It occurs to me that this would be best done by a specific method, like:

interleaveRandom :: (Monad m, RandomGen g) => StateT g m a -> StateT g m a
interleaveRandom (StateT m) = StateT $ \g -> let (gl, gr) = split g in
liftM (\p -> (fst p, gr)) $ m gl

It'd act like unsafeInterleaveIO and unsafeInterleaveST, but it'd be safe,
and you would know when it actually was splitting.
On Jul 30, 2015 1:15 PM, "Roman Cheplyaka" <roma at ro-che.info> wrote:

> On 30/07/15 20:38, Zemyla wrote:
> > Normally, a monad transformer to provide a random number generator would
> > be of the form StateT g, where g is a RandomGen. But I've seen some
> > libraries (like QuickCheck) define their RandomT as:
> >
> > newtype RandomT g m a = RandomT { runRandomT :: g -> m a }
> >
> > with their monadic bind operation defined as
> >
> > (RandomT m) >>= f = RandomT $ \g -> let (ga, gb) = split g in m ga >>=
> > (\a -> runRandomT (f a) gb)
> >
> > and return and fail as in ReaderT.
> >
> > Can someone describe the advantages and disadvantages of doing RandomT
> > this way? I mean, if your generator has a subpar split operation (and
> > most do), this will obviously exacerbate any problems with it.
>
> tf-random addresses this.
>
> > Does it give any comparable advantages?
>
> It doesn't introduce data dependencies. Let's say you generate a random
> binary tree. With the split approach, you can take the right subtree
> without evaluating the left one.
>
> Roman
>
>
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