[Haskell-cafe] Bug with QuickCheck 1.1 and GHC 6.8.2
patperry at stanford.edu
Thu Aug 14 13:08:08 EDT 2008
'split' has type
split :: StdGen -> (StdGen, StdGen)
It takes a generator and produces two independent generators. The old
version of variant calls 'split' n times to get a new generator, and
it only calls "split" on the second result. The new version calls
split (log n) times. It gets a unique generator for each value of n
by using both results from split.
rands 2 r0 = let (r1, r2) = split r0
(r3, r4) = split r1
rands 3 r0 = let (r1,r2) = split r0
(r3',r4') = split r2
On Aug 14, 2008, at 4:57 AM, Johan Tibell wrote:
> On Thu, Aug 14, 2008 at 12:15 PM, Patrick Perry
> <patperry at stanford.edu> wrote:
>> Actually, a much better solution is:
>> variant :: Int -> Gen a -> Gen a
>> variant v (Gen m) = Gen (\n r -> m n (rands r v))
>> rands r0 0 = r0
>> rands r0 n = let (r1,r2) = split r0
>> (n',s) = n `quotRem` 2
>> in case s of
>> 0 -> rands r1 n'
>> _ -> rands r2 n'
> It's not really clear to me how this works. Could you please explain?
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