So if r >= 1/2, then it loops? Ouch! On Thu, Apr 25, 2013 at 3:28 PM, Brent Yorgey 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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-- Felipe.

Wouldn't Alternative lead to the same bug? On Thu, Apr 25, 2013 at 7:02 PM, Sjoerd Visscher wrote:

Maybe a derived Alternative instance for RandT still makes sense?

Sjoerd

On Apr 25, 2013, at 8:28 PM, Brent Yorgey 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

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

-- Felipe.

Right, the solution is that with RandT Maybe, upon failure the seed gets thrown away. That is, mplus runs both its arguments with the same seed. What I actually wanted was MaybeT Rand. That requires a MonadRandom instance for MaybeT, which I will add. -Brent On Fri, Apr 26, 2013 at 12:30:51AM +0200, Sjoerd Visscher wrote:

Uhm, yeah, I forgot that you can still use monadic bind together with the Alternative methods.

Never mind me!

Sjoerd

On Apr 26, 2013, at 12:22 AM, Felipe Almeida Lessa wrote:

...

Wouldn't Alternative lead to the same bug?

On Thu, Apr 25, 2013 at 7:02 PM, Sjoerd Visscher wrote:

...

Maybe a derived Alternative instance for RandT still makes sense?

Sjoerd

On Apr 25, 2013, at 8:28 PM, Brent Yorgey 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

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

-- Felipe.