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

[Daniel Kahlenberg]

Maybe this is a beginners question... But here my problems description:

> import Random
> import Control.Monad
> import qualified Control.Monad.State as S
> import Test.QuickCheck.Gen
> import Test.QuickCheck.Arbitrary

Each thing can have one of three types:

> data Ontology = Thing1 Bool
>               | ThingK Bool
>               deriving (Show, Eq)

> instance Arbitrary Ontology where
>    arbitrary =
>        oneof [ return $ Thing1 True
>              , return $ ThingK True
>              , return $ Thing1 False
>              , return $ ThingK False ]

Liked to have a state monad runner for my arbitrary things as in "Real World
Haskell", "Chapter 14. Monads" ("Random values in the state monad").

[RWH]:

> -- file: ch14/Random.hs
> type RandomState a = S.State StdGen a

[RWH]:

< -- file: ch14/Random.hs
< getRandom :: Random a => RandomState a
< getRandom =
<   S.get >>= \gen ->
<   let (val, gen') = random gen in
<   S.put gen' >>
<   return val

[RWH]:

< -- file: ch14/Random.hs
< getTwoRandoms :: Random a => RandomState (a, a)
< getTwoRandoms = liftM2 (,) getRandom getRandom

[RWH]:

< -- file: ch14/Random.hs
< runTwoRandoms :: IO (Int, Int)
< runTwoRandoms = do
<   oldState <- getStdGen
<   let (result, newState) = S.runState getTwoRandoms oldState
<   setStdGen newState
<   return result

Thought getRandom function would be the best place to inject my unGen function
call, but cannot get it to type-check:

> getRandom :: Random a => RandomState [a]
> getRandom =
>   S.get >>= \gen ->
>   let (val, gen') = liftM2 (,) (unGen arbitrary gen 12) (random gen) in
>   S.put gen' >>
>   return val

A function not almost fulfilling my needs but the Random Number Generator isn't
updated... I liked to have different [Ontology] occasions per call:

> genArray :: [Ontology]
> genArray = unGen arbitrary (mkStdGen 42) 12 :: [Ontology]

Does anyone have the patience to help me out at least one step further?

Cheers
Daniel