How to iteratively test in Haskell using QuickCheck?

Question

How do you generate a random instance in Haskell QuickCheck and then run further tests on this very instance?

Let me demonstrate by giving an example. Within the following code snippet I generate an Int and then want to run further tests on this very Int :

{-# OPTIONS -Wall #-}

import           Test.Tasty
import           Test.Tasty.QuickCheck as QC
import qualified Debug.Trace as Trace

main :: IO()
main = defaultMain tests

test1 :: Int -> Property
test1 i = withMaxSuccess 2 (test2 i)

test2 :: Int -> (Int, Int) -> Property
test2 i (a, b) =
  Trace.trace ("(n,a,b) = " ++ show (i, a, b)) $
  property True

tests :: TestTree
tests =
  testGroup
  "Test suite"
  [
    QC.testProperty "Test" (withMaxSuccess 3 test1)
  ]

As output I want to have something like:

(n,a,b) = (0,0,0)
(n,a,b) = (0,1,2)
(n,a,b) = (-1,-1,-1)
(n,a,b) = (-1,-2,-3)
(n,a,b) = (2,0,-2)
(n,a,b) = (2,1,-1)

But instead I get:

(n,a,b) = (0,0,0)
(n,a,b) = (-1,-1,-1)
(n,a,b) = (2,0,-2)

I found this post (How can QuickCheck test all properties for each sample) , but it didn't really help me.

Answer 1

Consider the withMaxSuccess function:

withMaxSuccess :: Testable prop => Int -> prop -> Property

This is a function that takes any Testable and turns it into a Property . The resulting Property has the behaviour that it'll run for a maximum number as specified by the Int .

The test1 function is a Testable instance. When you call withMaxSuccess 3 test1 you change the behaviour of test1 , so that when it runs it runs for a maximum of three times.

While test1 returns a Property with the behaviour that it runs only two times ( withMaxSuccess 2 ), it gets overridden by withMaxSuccess 3 .

The withMaxSuccess function doesn't run the test, it just changes the definition of a property. withMaxSuccess 3 is the latest modification of the property, so that's why you see it running three times.