Proposal: Move 'split' into a separate class in System.Random

[Jan-Willem Maessen]

On Sat, Oct 9, 2010 at 11:02 AM, Thomas DuBuisson
<thomas.dubuisson at gmail.com> wrote:
> All,
> I have updated the ticket/patch for this proposal [1].  It seems the
> community has agreed on
>
>> class RandomGen g where
>>    next     :: g -> (Int, g)
>>    genRange :: g -> (Int,Int)
>>    genRange _ = (minBound, maxBound)
>
>> class (RandomGen g) => SplittableGen g where
>>    split    :: g -> (g, g)
>
> Notice SPJs good catch that SplittableGen should be constrained by
> RandomGen is there (lacking discussion, but its hard to imagine
> objection).  I'm not sure what you, Jan, mean by removing Read and
> Show constraints as I don't see any.

I was mis-remembering the following as a class constraint on RandomGen:

data StdGen = ...	-- Abstract

  instance RandomGen StdGen where ...
  instance Read      StdGen where ...
  instance Show      StdGen where ...

(http://www.haskell.org/onlinelibrary/random.html)

This makes it hard to impossible to implement splitting of StdGen both
efficiently and correctly.

-Jan