new MonadRandom instance; and maintainership
Felipe Almeida Lessa
felipe.lessa at gmail.com
Thu Apr 25 20:40:49 CEST 2013
So if r >= 1/2, then it loops? Ouch!
On Thu, Apr 25, 2013 at 3:28 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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