Proposal: Generalize the RandomGen and Random classes

Thomas DuBuisson thomas.dubuisson at
Tue Sep 14 20:11:19 EDT 2010

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:

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


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

2) Alters Random:


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


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



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