How to use custom listOf in QuickCheck

Question

In my code, I need to generate a list of Point

I've instanced the custom type Point in the Arbitrary Type Class. So far so good.

Now I need to generate a list: [Points]

But the default random list generated by QuickCheck(using listOf I believe?) is not working for me. Because I need some special relations between the points . I know of the Generator combinators, ex. suchAs But they are too slow(because of very rare relation)

I've defined a customed Gen [Point] to suit my needs.

randomBoard :: Gen [Point]
randomBoard = ...

But I have no idea how to let my test to use this custom Gen in the property test because it defaults to the listOf generated list.

Answer 1

IIUC, the issue is that your tests are using Arbitrary instances, by way of the Testable instance for (Arbitrary a, Show a, Testable prop) => Testable (a -> prop) .

A common pattern to get custom generators is to throw around some newtype s with the desired Arbitrary instances. So you'd define something like

newtype Board = Board [Point]

instance Arbitrary Board where
  ...

That would work. But I'd recommend bypassing Arbitrary altogether and writing your properties with forAll and its variants .

genBoard :: Gen [Point]
genBoard = ...

shrinkBoard :: [Point] -> [[Point]]
shrinkBoard = ...

myProperty :: Property
myProperty = forAllShrink genBoard shrinkBoard (\board -> ...)