Using with a base monad other that IO

[newhoggy]

I'm interested in using effectful with a base monad other than IO - in particular PropertyT IO from the hedgehog library.

Is this possible?

[ocharles]

This isn't possible, but PropertyT is quite a strange monad that might not even possible to capture as an effect with effectful. What I have had success with is defining a Test effect that models Hedgehog's TestT monad, along with evalEff. This lets you combine effectful and MonadTest, which has felt like the goth balance. If this is interesting let me know, as I've been meaning to tidy this up and open source it

[newhoggy]

Yes, please.

[arybczak]

Btw, doesn't PropertyT (Eff es) work? Or it just doesn't fit with what your need to do?

[newhoggy]

Ideally, I should be able to use hedgehog and effectful functions together without lifting.

[newhoggy]

I'm now interested in implementing your suggestion, but I'm not sure where to start.

[newhoggy]

I have this:

type TestEffects2 = '[IOE]

property :: ()
  => H.PropertyT (Eff TestEffects2) ()
  -> H.Property
property f =
  H.Property H.defaultConfig $
    withFrozenCallStack (H.evalM (hoist runEff f))

[newhoggy]

BTW, I've managed to do this with polysemy: https://github.com/haskell-works/hw-polysemy/blob/main/test/HaskellWorks/Polysemy/HedgehogSpec.hs

[newhoggy]

I was able to get effectful to worth with TestT IO: https://github.com/haskell-works/effectful-zoo

Thanks for the suggestion @ocharles !

[arybczak]

Nice 👍 one suggestion: please don't put INLINE pragmas on functions unless you've benchmarked and concluded they help. GHC inlining heuristics are pretty good, so most of the time these pragmas just lead to longer compile times and code bloat.

[newhoggy]

I'm trying to implement EvalM to call evalM from hedgehog and have trouble aligning the types:

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

The case that needs to be implemented is here:

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.

[newhoggy]

I moved my question to its own Q&A: #273

[newhoggy]

I figured out a way to support hedgehog property tests with effectful:

haskell-works/effectful-zoo#12