new MonadRandom instance; and maintainership
sjoerd at w3future.com
Fri Apr 26 00:02:57 CEST 2013
Maybe a derived Alternative instance for RandT still makes sense?
On Apr 25, 2013, at 8:28 PM, Brent Yorgey <byorgey at seas.upenn.edu> wrote:
> On Thu, Apr 25, 2013 at 10:47:57AM -0400, Brent Yorgey wrote:
>> On Wed, Apr 24, 2013 at 02:38:01PM -0400, Brent Yorgey wrote:
>>> Hi all,
>>> I would like to add a derived MonadPlus instance for both Rand and
>>> RandT. Also, since it seems that MonadRandom has no maintainer I
>>> propose to take on its maintainership. Any
>>> comments/objections/etc. welcome.
>> Also, ignore what I said about deriving an instance for Rand, that
>> does not make any sense. However, adding an instance for RandT does
>> still make sense. A repository with the changes is here:
>> I'll upload in a few days if no one objects.
> Puzzle time! Can you explain the semantics of the following code
> (given a derived MonadPlus instance for RandT)?
> maybeFail :: RandT StdGen Maybe ()
> maybeFail = do
> r <- getRandomR (0, 1 :: Double)
> guard (r < 1/2)
> succeed :: RandT StdGen Maybe ()
> succeed = maybeFail `mplus` succeed
> I therefore rescind my proposal to add a MonadPlus instance for
> RandT. However, I still volunteer to be the maintainer. =)
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