I filed an issue on github (https://github.com/haskell/random/issues). Maybe the authors can shed some light on this. Am 10/30/2015 um 11:40 PM schrieb Lyndon Maydwell:

I've replicated this and it does seem very strange, and possibly even a bug.

I would guess that most people don't encounter this issue as a generator is usually only seeded once, then threaded throughout generation. Not seeded for once for every random output. The other common practice is that generators are usually seeded on far more random input values than a list of ascending ints.

Seed behaviour:

main = mapM_ print (map p l) p x = length $ filter even $ map (\s -> fst $ randomR (1::Int,10) (mkStdGen s)) x l = map ls [1..10] ls x = map (\y -> x * y * 10) [1..1000]

1000 1000 1000 1000 1000 894 766 670 596 536

Still, I am very surprised by this behaviour. I couldn't find any reference to this behaviour in[1], which is supposedly what the System.Random implementation is based on.

Does anyone else have an explanation?

- Lyndon

[1] - https://en.wikipedia.org/wiki/Linear_congruential_generator

On Sat, Oct 31, 2015 at 5:43 AM, martin mailto:martin.drautzburg@web.de> wrote:

Hello all

When I read:

mkStdGen :: Int -> StdGen

The function mkStdGen provides an alternative way of producing an initial generator, by mapping an Int into a generator. Again, distinct arguments should be likely to produce distinct generators.

I thought, that

> fst $ R.randomR (1,10) (R.mkStdGen s)

should get me a random value between 1 and 10 and that I get different values depending on the seed s. But this

> length $ filter even $ map (\s -> fst $ randomR (1::Int,10) (mkStdGen s))[1..1000]

gives my 1000, i.e. all random numbers are even numbers.

However, when I use random instead of randomR

> length $ filter even $ map (\s -> fst $ random (mkStdGen s) ::Int)[1..1000]

I get 499 (okay)

Why is that so? _______________________________________________ Beginners mailing list Beginners@haskell.org mailto:Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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On Sun, 01 Nov 2015 03:24:07 +0100, Andrew Bernard wrote:

According to the package documentation for System.Random:

This implementation uses the Portable Combined Generator of L'Ecuyer [System.Random] for 32-bit computers, transliterated by Lennart Augustsson.

The paper in question is given as:

Pierre L'Ecuyer, /Efficient and portable combined random number generators/, Comm ACM, 31(6), Jun 1988, pp742-749

Does anybody have a copy of this paper?

The first result of a search with startpage was: http://www.iro.umontreal.ca/~lecuyer/myftp/papers/cacm88.pdf Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --