new MonadRandom instance; and maintainership

[Sjoerd Visscher]

Maybe a derived Alternative instance for RandT still makes sense?

Sjoerd

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:
>> 
>>  http://github.com/byorgey/MonadRandom
>> 
>> 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. =)
> 
> -Brent
> 
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