context-sensitive generation using quick check

Question

I would like to generate random terms based on some sort of "context" and I was wondering if this is possible using quickcheck. Basically I would like to have an additional data type passed around so that the arbitrary function can generate terms based on the additional parameter... Is this possible with quickcheck or should I just write my own definition of Gen?

Answer 1

It's possible, though not really sane, to do this from within arbitrary. But if you step out of arbitrary, you can literally just pass an extra parameter around.

-- do whatever you want inside the implementation of these two
chooseIntRange :: Context -> Int
updateContext :: Int -> Context -> Context

arbitraryIntWithContext :: Context -> Gen (Context, Int)
arbitraryIntWithContext ctx = do
    n <- choose (0, chooseIntRange ctx)
    return (n, updateContext n ctx)

The plumbing of the context can be relieved somewhat with StateT, e.g.

-- do whatever you want inside the implementation of this
chooseIntRangeAndUpdate :: MonadState Context m => m Int

arbitraryIntStateT :: StateT Context Gen Int
arbitraryIntStateT = do
    hi <- chooseIntRangeAndUpdate
    lift (choose (0, hi))