Proposal: Generalize the RandomGen and Random classes

[Milan Straka]

Hi,

as far as I know, a random generator usually creates an int or
a sequence of bits, right? Or do you have some specific random
generators in mind?

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?

Cheers,
Milan

> 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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