Proposal: Add getStdGenState and stdGenFromState

[John Meacham]

64 bits of entropy doesn't seem like much at all for a RNG. I thought
the idea was StdGen could be fully abstract.

    John

On Mon, Feb 6, 2012 at 9:07 AM, Bas van Dijk <v.dijk.bas at gmail.com> wrote:
> Dear Ryan, all,
>
> To quote the documentation of System.Random: "The Show and Read
> instances of StdGen provide a primitive way to save the state of a
> random number generator."
>
> Primitive it is indeed. Currently the only way to serialize (using
> cereal or binary) a StdGen is to convert it to a String and then
> serialize this String. This takes up many more bytes than are actually
> needed.
>
> I would like to propose a more efficient way of serializing the state
> of the random number generator by providing the following two
> functions:
>
> getStdGenState :: StdGen -> (Int32, Int32)
> getStdGenState (StdGen s1 s2) = (s1, s2)
>
> stdGenFromState :: (Int32, Int32) -> StdGen
> stdGenFromState (s1, s2) = StdGen s1 s2
>
> Of course this satisfies:
> (stdGenFromState . getStdGenState) g == g
>
> Alternatively, instead of using (Int32, Int32) we can use an Int64:
>
> getStdGenState :: StdGen -> Int64
> getStdGenState (StdGen s1 s2) =
>    fromIntegral s1 `unsafeShiftL` 32 .|.
>    fromIntegral s2
> #if !MIN_VERSION_base(4,5,0)
>    where unsafeShiftL = shiftL
> #endif
>
> stdGenFromState :: Int64 -> StdGen
> stdGenFromState s = StdGen
>    (fromIntegral (s `unsafeShiftR` 32))
>    (fromIntegral s)
> #if !MIN_VERSION_base(4,5,0)
>    where unsafeShiftR = shiftR
> #endif
>
> I don't really care which one we use but I think the latter is a bit
> easier to use.
>
> Oh, I don't really care about the names either.
>
> Regards,
>
> Bas
>
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