[Haskell-cafe] Using MonadRandom with QuickCheck

[Magnus Grindal Bakken]

Thanks, Li-yao! For anyone else reading this, this is the MonadRandom 
instance
I ended up with:

mkGen :: (QCGen -> a) -> Gen a
mkGen f = MkGen $ \g _ -> f g

instance MonadRandom Gen where
    getRandom = mkGen (fst . random)
    getRandoms = mkGen randoms
    getRandomR range = mkGen (fst . randomR range)
    getRandomRs range = mkGen (randomRs range)

With that said, I found that in many cases to get the game states I needed 
for
my property tests I had to use more specific generators that took input 
such as
a range of number of moves, whether the game was finished or not, the 
current
action, etc., so it didn't end up being as useful for me as I hoped.

>
On Sunday, August 21, 2016 at 10:37:35 PM UTC+2, Li-yao Xia wrote:
>
> Hi Magnus,
>
> Import Test.QuickCheck.Gen to get the internals of Gen, which AFAIK is 
> currently the only way to get to the underlying random generator (QCGen).
>
> getRandom = MkGen $ \g _ -> fst (random g)
> -- repeat with randomRs, randoms.
>
> I don't know why Gen is not an instance of MonadRandom either. It seems 
> reasonable to add one, though it might go against some design principle 
> of QuickCheck by ignoring the size parameter.
>
> Regards,
> Li-yao
>
> On 08/21/2016 09:38 PM, Magnus Grindal Bakken wrote:
> > Is there some easy way to use the Gen type from QuickCheck with the
> > MonadRandom class?
> >
> > Links to the packages in question:
> > https://hackage.haskell.org/package/MonadRandom
> > https://hackage.haskell.org/package/QuickCheck
> >
> > Some background in case there's an easier way to solve the problem:
> > I'm toying around with an API for a simple turn-based board game. The
> > main type in my API is called GameState, and I'm trying to define an
> > Arbitrary instance for it so I can set up invariants with QuickCheck.
> > The Arbitrary instance should produce game states that are the result
> > of playing a random number of random moves.
> >
> > At certain points in the gameplay the game itself needs to invoke some
> > randomness. I've designed the public game API to have basically just
> > two functions:
> >
> > playAction :: ActionData -> GameState -> GameState
> > evaluateGameState :: MonadRandom m => GameState -> m GameState
> >
> > The usage is supposed to be that the client first calls playAction
> > with an ActionData value that depends on the current move, and
> > afterwards calls evaluateGameState to evaluate whatever random events
> > need to happen at that point (e.g. reshuffling a deck of cards). That
> > way my main "action" logic doesn't need to concern itself with any
> > form of randomness.
> >
> > I'm using MonadRandom from Control.Monad.Random because it seems like
> > a much nicer way to generate random numbers than using RandomGen
> > directly. However I can't figure out how to make it play nice with
> > QuickCheck. I can easily set up my Arbitrary instance to call
> > playAction an arbitrary number of times, but I can't call
> > evaluateGameState since it lives in the other monad. It would work if
> > Gen were an instance of MonadRandom, but I can't quite work out how to
> > write that instance since most functions on Gen only work if the
> > output type is an instance of Arbitrary, while MonadRandom needs to
> > work with any types that are part of the Random class. This part
> > works:
> >
> > instance MonadRandom Gen where
> >     getRandomR = choose
> >
> > But I don't know how to define the other functions on MonadRandom for 
> Gen.
> >
> > I think I might also be able to use the PropertyM monad transformer:
> > 
> https://hackage.haskell.org/package/QuickCheck-2.9.1/docs/Test-QuickCheck-Monadic.html
> ,
> > but monad transformers always confuse me so I haven't been able to
> > figure out how to do that yet either.
> >
> > I've thought about rewriting the API to use the Gen type instead of
> > MonadRandom, but it feels kind of iffy to have the API depend on a
> > testing library.
> >
> > Has anyone tried to use these libraries together before?
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