I think the confusion may be come from the understanding of "distinct". The documentation is right that the generators are not equal which is easily checked e.g. using their Show instance. They *will* produce different random numbers. The user of the library might OTOH assume that "distinct" mean "producing uncorrelated output". This is harder and may simply not hold, especially that it doesn't mention sequentially increasing integers or any other kinds of sequences. The property you seem to be looking for is "have vastly different output for similar numbers". Sounds a lot like a hash function to me.
import Data.Hashable -- hashable import System.Random -- random hGen :: (Hashable a) => a -> StdGen hGen = mkStdGen . hash
GHCi:
mkStdGen <$> [1..10] [2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 1,10 1,11 1] hGen <$> [1..10] [2113803271 1,707778093 1,377439146 1,354368904 1,1689773631 1,1515981814 1,1419492475 1,1232077631 1,2037530173 1,1078099554 1]
You can also ask System.Random for a required set of random numbers for use as seeds.
map mkStdGen <$> replicateM 10 randomIO [817657009 1,491059977 1,1962205061 1,375413047 1,626395891 1,1694342924 1,1145131839 1,441215930 1,1278080790 1,1524285256 1]
Finally, StdGen provides 'split' since it implements RandomGen typeclass. Using Data.List.unfoldr:
take 10 $ unfoldr (Just . split) (mkStdGen 1) [3 40692,80029 2147442707,1054756830 2147402015,545291968 2147361323,879767459 2147320631,1464499717 2147279939,2107652444 2147239247,1777851630 2147198555,1414574869 2147157863,1574498162 2147117171]
The question is what really are your needs here. Different applications
will require different properties. I hope the above will give some hints.
On Fri, Jan 3, 2014 at 5:28 PM, Rob Arthan
Either I am misunderstanding something or there is an infelicity in the implementation of StdGen. The documentation on mkStdGen says that distinct arguments should be likely to produce distinct generators. This made me think that I would get a reasonable pseudo-random function to simulate n rolls of a die by using n to seed the random number generator:
import System.Random roll :: Int -> String roll n = take n . randomRs ('1', '6') . mkStdGen $ n
However, this produces a string beginning with a '6' for 0 <= n <= 53667. In fact the dependency of the first value on the seed seems to be far from random:
map (\l -> (head l, length l)) . group . map (fst . randomR (1, 6) . mkStdGen) $ [0..25*53668+6]
returns:
[(6,53668),(5,53668),(4,53668),(3,53669),(2,53668),(1,53668),(6,53669),(5,53668),(4,53668),(3,53669),(2,53668),(1,53668),(6,53668),(5,53669),(4,53668),(3,53668),(2,53669),(1,53668),(6,53668),(5,53669),(4,53668),(3,53668),(2,53669),(1,53668),(6,53668)]
The behaviour seems to be related to the length of the range. You get similar behaviour for ranges of length 2, 3, 6 and 9 for example, but not for 4, 5, 7 or 8.
If it is relevant, I am using ghc version 7.6.3.
Regards,
Rob.
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