Haskell: RandomGen drops half of values - haskell

Question

I am writing a simple deterministic Random Number generator, based on the xorshift. The goal here is not to get a cryptographically secure or statistically perfect (pseudo-)random number generator, but to be able to archieve the same deterministic sequence of semi-random numbers across programming languages.
My Haskell program looks like follows:
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
module SimpleRNG where
import Data.Word (Word32)
import Data.Bits (xor, shift)
import System.Random (RandomGen(..))
import Control.Arrow
(|>) :: a -> (a -> b) -> b
(|>) x f = f x
infixl 0 |>
newtype SeedState = SeedState Word32
deriving (Eq, Show, Enum, Bounded)
seed :: Integral a => a -> SeedState
seed = SeedState . fromIntegral
rand_r :: SeedState -> (Word32, SeedState)
rand_r (SeedState num) = (res, SeedState res)
where
res = num
|> xorshift 13
|> xorshift (-17)
|> xorshift 5
xorshift :: Int -> Word32 -> Word32
xorshift amount x = x `xor` (shift x amount)
instance RandomGen SeedState where
next seed_state = (first fromIntegral) $ rand_r seed_state
where
genRange seed_state = (fromEnum (minBound `asTypeOf` seed_state),
fromEnum (maxBound `asTypeOf` seed_state))
split seed_state#(SeedState num) = (seed_state', inverted_seed_state')
where
(_, seed_state') = next seed_state
(_, inverted_seed_state') = next inverted_seed_state
inverted_seed_state = SeedState (maxBound - num)
Now, for some reason, when running
take 10 $ System.Random.randoms (seed 42) :: [Word32]
it returns only the 'odd' results, compared to the output of the following Python program:
class SeedState(object):
def __init__(self, seed = 42):
self.data = seed
def rand_r(rng_state):
num = rng_state.data
num ^= (num << 13) % (2 ** 32)
num ^= (num >> 17) % (2 ** 32)
num ^= (num << 5) % (2 ** 32)
rng_state.data = num
return num
__global_rng_state = SeedState(42)
def rand():
global __global_rng_state
return rand_r(__global_rng_state)
def seed(seed):
global __global_rng_state
__global_rng_state = SeedState(seed)
if __name__ == '__main__':
for x in range(0, 10):
print(rand())
It seems like the internals of the System.Random module do some weird trickery with the return result of the generator
(calling
map fst $ take 10 $ iterate (\(_, rng) -> rand_r rng) (rand_r $ seed 42)
gives the result I'd expect).
This is odd, since the type returned by the generator is already a Word32, so it could/should just be passed on unaltered without any remapping happening.
What is going on here, and is there a way to plug this xorshift-generator into System.Random in a way that returns the same results?

Answer 1

This is to do with the behaviour of System.Random.randoms, which repeatedly applies random to a RandomGen, not next.
class Random a where
...
random :: (RandomGen g) => g -> (a, g)
The Random class is what allows you to reuse RandomGen instances across different enums, and the instance for Word32 (as well as nearly all other types) is defined as
instance Random Word32 where randomR = randomIvalIntegral; random = randomBounded
randomBounded just calls randomR, so the behaviour of random is decided by `
randomIvalIntegral (l,h) = randomIvalInteger (toInteger l, toInteger h)
randomIvalInteger is an interesting function, you can read the source here. It's actually causing your problem because the function will discard a certain number of intermediate values based on the range of the generator and the range being generated over.
To get the values you want, you just need to use next instead - the easiest way would just be to define
randoms' g = x : (randoms' g') where (x, g') = next g