On 09.02.2012 15:32, Jerzy Karczmarczuk wrote:
Aleksey Khudyakov :
1. Mersenne Twister, AND congruential generators AND the Marsaglia
stuff, all use some kind of "seed", all are stateful. There are no
miracles. Just look the agressive monadization, the form of defaultSeed,
etc. within MWC.hs, before saying that this generator doesn't rely on
some global state.
I think you are missing the point here. Surely all PRNG carry some state
around. But both StdGen and mwc-random (and likely many others)
allow to have many generators at once (for use in different threads)
mersenne-random is just wrapper around vastly impure library (as
documentation says) and allows only one genrator per program.
This is why I said it uses *global* state
2. In the standard generator stuff the state is stored in IoRefs and is
not "copied". Did Aleksey ever look inside the sources of these
generators? In any case, the seed changes after each generation, and
must be stored somewhere.
No. It doesn't and cannot
> data StdGen
> = StdGen Int32 Int32
If generator state is stored in IORef it's not possible to implement
`next :: g → (Int,g)'. To do something useful with it one have to
go to IO monad but we can't. So state have to be copied.
3. The API question is a conventional one. People who are really
unhappy, may make their own interfaces, or give such a mini-project as a
students' assignment, instead of weeping that the library is lousy and
should be ignored. E. g., I wanted random numers in some purely
functional, lazy context, and I didn't want the existing interface ; I
manufactured a lazy stream interface, and that was all. "Look Ma!: no
global state..."
Usually. But sometimes it's not possible to implement some algorithms
for particular API. For example if PRNG rely on in-place array updates
it cannot implement Random type class.
4. L'Ecuyer signalled some 15 years ago that MWC generators introduce
some bias on the most significant bits (complementary MWC are safer).
This is less annoying that the last bits periodicity of simple
congruential generators, but for SOME Monte-Carlo it may be harmful.
Thank you for reminder. I wanted to read their paper for some time.
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