How to Properly Integrate Effectful with Hedgehog Using H.evalIO and H.evalM?

[newhoggy]

I'm working on implementing a custom interpreter for an effectful effect that integrates with Hedgehog. The goal is to handle an EvalM operation that allows running computations within the current Eff stack and correctly maps them to Hedgehog's evaluation framework using H.evalM or H.evalIO. Here's my effect definition:

https://github.com/haskell-works/effectful-zoo/blob/main/components/hedgehog/Effectful/Zoo/Hedgehog/Effect/Hedgehog.hs#L21

The interpreter I'm working on looks like this:

https://github.com/haskell-works/effectful-zoo/blob/e2d2087c2c69a11d9b1d3d5d0f4e8dd6f9be9bc1/components/hedgehog/Effectful/Zoo/Hedgehog/Run.hs#L56

I'm running into difficulty because evalM takes a callback that is not IO:

https://hackage.haskell.org/package/hedgehog-1.5/docs/src/Hedgehog.Internal.Property.html#evalM

What should I do here?

I'm okay if it calls evalIO instead, but evalM is preferrable.

[newhoggy]

I have a work around:

    EvalM f -> localUnlift env SeqUnlift $ \unlift -> do
      !result <- try @SomeException $ unlift f

      case result of
        Right a -> pure a
        Left e -> liftTestIO $ H.evalIO $ CE.throw e

But I'm still interested to know if the thing I was trying to do is more straightforwardly possible.

[arybczak]

Do you have a MonadTest (Eff es) instance? If you do, it looks like you should be able to call evalM within the unlift.

[newhoggy]

Unfortunately, MonadTest looks like this:

class Monad m => MonadTest m where
  liftTest :: Test a -> m a

The argument TestT Identity and not TestT IO.

[arybczak]

Yeah, but you already wrote liftTest here: https://github.com/haskell-works/effectful-zoo/blob/main/components/hedgehog/Effectful/Zoo/Hedgehog/Api/Lift.hs, so you have all the components.

It looks to me that you should make LiftTest an operation of your effect, then define an instance like described here: https://hackage.haskell.org/package/effectful-core-2.5.0.0/docs/Effectful-Dispatch-Dynamic.html#g:4 and then you can just use evalM from the hedgehog library with Eff.

[newhoggy]

Okay, I'm impressed. Thank you very much!

[arybczak]

Also, looks like you're unnecessarily reimplementing tons of functions from hedgehog. For example, all functions from https://github.com/haskell-works/effectful-zoo/blob/main/components/hedgehog/Effectful/Zoo/Hedgehog/Api/Assert.hs. All of them in hedgehog just need MonadTest and you have MonadTest, you can just call the originals.

Your Hedgehog effect looks to be at a wrong level of granularity. As I suggested, start with it having only LiftTest, with this you get MonadTest (Eff es) and get the ability to use most of the functions from hedgehog with Eff for free, there's no need to reimplement them.

You currently hardcoded the liftTest function in the MonadTest (Eff es) instance, which is the wrong approach. Please follow the guideline in the haddock link I pointed to.

[newhoggy]

The Hedgehog effect is much smaller now, containing only LiftTest, CatchAssertion and ThrowAssertion.

I had to introduce a new MonadTestProxy type class to plumb it up.