Using ST with polymorphic test templates for QuickCheck

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#1

I have a class that has an associated monad, so that instances can be created for different monads.

class MutRef a where
  type MutRefM a :: * -> *
  type MutRefVal a

  readMutRef :: a -> MutRefM a (MutRefVal a)
  writeMutRef :: a -> MutRefVal a -> MutRefM a ()
  modifyMutRef :: a -> (MutRefVal a -> MutRefVal a) -> MutRefM a ()

I would like to test these with QuickCheck.

I want to create functions representing ‘laws,’ like so:

readsAndWrites :: (MutRef a, Monad (MutRefM a), Eq (MutRefVal a))
    => TestHarness a -> MutRefVal a -> Property 
readsAndWrites (getRef, runner) a = runner $ do
  a' <- run $ do
    ref <- getRef
    writeMutRef ref a
    readMutRef ref
  assert (a == a')

TestHarness is a tuple of helper functions:

type TestHarness a = (MutRefM a a, PropertyM (MutRefM a) () -> Property)

providing the test with a ‘Ref’ to use, and turning the action into a Property respectively.

I would then be able to instantiate a QuickCheck test property using the ‘law’ and a harness.

For IO, this works without a problem:

ioRefTestHarness :: TestHarness (IORef Int)
ioRefTestHarness = (newIORef 0, monadicIO)

prop_ioRefReadsAndWrites :: MutRefVal (IORef Int) -> Property
prop_ioRefReadsAndWrites = readsAndWrites ioRefTestHarness

compiles and runs.

However, I also have an ST instance:

instance MutRef (STRef s a) where
  type MutRefM (STRef s a) = ST s
  type MutRefVal (STRef s a) = a

  readMutRef = readSTRef
  writeMutRef = writeSTRef
  modifyMutRef = modifySTRef

Attempting to naively define a test harness however,

stRefTestHarness :: TestHarness (STRef s Int)
stRefTestHarness = (newSTRef 0, monadicST)

fails with the error message:

    • Couldn't match type ‘PropertyM (ST s) ()’
                     with ‘forall s1. PropertyM (ST s1) a0’
      Expected type: PropertyM (MutRefM (STRef s Int)) () -> Property
        Actual type: (forall s. PropertyM (ST s) a0) -> Property
    • In the expression: monadicST
      In the expression: (newSTRef 0, monadicST)
      In an equation for ‘stRefTestHarness’:
          stRefTestHarness = (newSTRef 0, monadicST)
    • Relevant bindings include
        stRefTestHarness :: TestHarness (STRef s Int)
          (bound at test/quickcheck.hs:79:1)

I’m guessing I need a quantification somewhere, but I have not been successful out where through trial and error.

Is it possible to parameterize my harness type over ST like this?

How do I go about what I am trying to do?