[Haskell-cafe] How to update the RNG per call (State monad) when generating QuickCheck arbitraries?

[Daniel Kahlenberg]

Oh thanks,

hold on I'd like to have the genArray call generating distinctive
results in one IO execution (meaning when
I load the .lhs file in ghci):

Prelude> genArray
[ThingK True,Thing1 False]

and when calling immediately again e. g.

Prelude> genArray
[Thing1 True]

By now I only get one and the same again, i. e.:

Prelude> genArray
[ThingK True]

Prelude> genArray
[ThingK True]

...

so I thought an adaptation of the `runTwoRandoms` approach as described
in the RWH book could help.

In other words the genArray should have similar behaviour as e. g. a
`runOneRandom` function defined like:

> getOneRandom :: Random a => RandomState a
> getOneRandom = getRandom

> runOneRandom :: IO Int
> runOneRandom = do
>   oldState <- getStdGen
>   let (result, newState) = S.runState getOneRandom oldState
>   setStdGen newState
>   return result

... the rest of the code as in my first post...

Testing it, different numbers expected as the RNG is updated each call:

Prelude> runOneRandom
2033303743

Prelude> runOneRandom
-566930973

...

Cheers
Daniel

2011/4/26 Bryan O'Sullivan <bos at serpentine.com>:
> On Tue, Apr 26, 2011 at 3:04 AM, Daniel Kahlenberg
> <d.kahlenberg at googlemail.com> wrote:
>>
>> Thought getRandom function would be the best place to inject my unGen
>> function
>> call, but cannot get it to type-check:
>
> You haven't described what it is you're actually trying to do, and I'm
> afraid your code doesn't help to understand that.