Re: Proposal: Generalize the RandomGen and Random classes

On Wed, Oct 6, 2010 at 5:53 PM, Thomas DuBuisson wrote:

All, There has been precious few comments on this proposal (ending in 2 days).  Conversation has thus far been:

Accept (1 - me) No - Random is H98 and changing it is not acceptable (2 - Milan, Yitz) Unknown Stance (2 - SPJ, Antonie Latter)

Mostly I'm hoping for more comments.  If you found the splittable debate of value then I'd imagine this aspect of Random would concern you too.

In response to the idea that we can't or shouldn't change H98:

Yitz:

...

If those extensions are all added to Haskell 2011 or whatever, then it could be considered.

AFAIK, all accepted parts of Haskell standards are implemented FIRST (typically as a GHC extension or package) then considered for Haskell'.  The Haskell2010 and Haskell98 packages, which you can import, can contain their own definition of System.Random - this could be static without stagnating all libraries that are mentioned in a Haskell standard.

Milan:

...

I personally do not think it is worth to modify the Random module, which dates back to at least Haskell 98. Maybe a new package on the Hackage?

So because it is old we can't modify it?  The point of changing the library is to benefit a broader portion of the community.  We could stop any and all changes to package X and make a new package every time but this isn't a sufficient argument to me.

If peoples only objections are compatibility then we can queue this change up with the next API breaking change, unless there will never again be such a change.

Cheers, Thomas

On Tue, Sep 14, 2010 at 5:11 PM, Thomas DuBuisson wrote:

...

Hello, RandomGen and Random classes assume generators produce Int values. This is non-ideal as many high speed generators produce special values (ex: doubles) or generic values (bit streams / bytestrings) that can be converted directly to types easier than coercing to Int then to an 'a' via the Random class.

See 4315 [1] for the patch.

Period of discussion: Till October 8 (3.5 weeks, giving a little time after ICFP for last minute debate)

Specifically, this proposal:

1) Alters RandomGen:

from:  class RandomGen g where      next :: g -> (Int, g)      genRange :: g -> (Int, Int)

to

class RandomGen g v | g -> v where    next :: g -> (v, g)    genRange :: g-> (v,v)

2) Alters Random:

From:

class Random a where  randomR :: RandomGen g => (a,a) -> g -> (a,g)  random  :: RandomGen g => g -> (a, g)  randomRs :: RandomGen g => (a,a) -> g -> [a]  randoms  :: RandomGen g => g -> [a]  randomRIO :: (a,a) -> IO a  randomIO  :: IO a

to

class Random a where  randomR :: RandomGen g v => (a,a) -> g -> (a,g)  random  :: RandomGen g v => g -> (a, g)  randomRs :: RandomGen g v => (a,a) -> g -> [a]  randoms  :: RandomGen g v => g -> [a]

Additional Points of Debate 1) Because random[R]IO can not be part of the new Random instance with a sensible default, these have been moved to top-level functions:

 randomRIO :: (Random a Int) => (a,a) -> IO a  randomIO  :: (Random a Int) => IO a

Other options exist and I'm open to them.  I'm just being upfront about what the patch currently does.

2) All pre-existing instances of "Random x" for some concrete 'x' have been modified to be "instance Random x Int".  As 'Int' was the previous (hardcoded) default for RandomGen this is simply matching the behavior.  More instances are possible and probably make sense now. Alternatively, one could argue for zero default instance as they can collide with how a particular user wishes types to be coerced.

3) Not-so-new extensions are used to enable these changes.  Extensions include MultiParamTypeClasses, FlexibleContexts, and FunDeps.

4) A patch is included bumping the version from 1.0.0.x to 1.1.0.0.

Cheers, Thomas

[1] http://hackage.haskell.org/trac/ghc/ticket/4315

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I guess the new class interface feels somehow off to me. As a developer, I would now say: "The function requires a random generator which exclusively produces Word16s" which I find oddly specific, and lowers the chances of me being able to use a single generator with disparately authored components without piles of newtype shims (and who know what else). Maybe a better solution would be for the RandomGen class to provide a multitude of 'next' functions: next :: g -> (Int, g) nextInt :: g -> (Int, g) nextInt8 :: g -> (Int8, g) nextInt16 :: g -> (Int16, g) nextInt32 :: g -> (Int32, g) . . . nextWord :: g -> (Word, g) nextword8 :: g -> (Word8,g) . . . nextDouble :: g -> (Double, g) The class methods other than 'next' would have a default implementation equivalent to the above value's Random instance. Then as a consumer of a RandomGen, I can ask for the values I want, and the implementation of the RandomGen can provide efficiently what it can. And then as a downstream developer I can benchmark like crazy to find the combination that works for me. Plus, I don't think this requires additional extensions. Antoine