IO in Propertys

[well-typed]

IO in Propertys

[edsko]

Indeed, the choice not to have IO in Gen, or support a general transformer API in Gen, is quite deliberate, as you already point out. In principle we have more leeway with Property', although we do have to be a bit careful.

However, I agree with you that a separation is the way to go here; this is the route taken for example by libraries such as quickcheck-state-machine, quickcheck-dynamic/quickcheck-lockstep, and others.

I don't think it would be that hard actually, because Property and its infrastructure is already quite general. Here's where I would start experimenting. In the tasty driver, we have

data Test = Test TestOptions (Property' String ())

We can add another

data TestIO = TestIO TestOptions (Property' String (IO ()))

alongside, with correspoinding

testPropertyIO :: TestName -> Property' String (IO ()) -> TestTree

(Perhaps it would even make sense to generalize the existing Test datatype rather than define a second one, not 100% sure.) The IsTest instance for this would do what the existing one does and share most of its code, but would perform one additional step for the test to pass: the returned IO action should run without exceptions. I think this would achieve the separation you mention, and would then also open the door to more full-fledged state modelling infrastructure (I'd love to see something like quickcheck-lockstep in falsify at some point).

What do you think? If you'd like to give it a go and get stuck somewhere, let me know and I'll try to think along.

[voidus]

Thank you for the encouragement and guidance. I'll try to look into it some more in the next days 🙂

[ejconlon]

+1 from me on this feature. I have some serialization code that is "morally pure" but allocates and fills buffers in IO during encoding, and I would love to be able to drop my unsafePerformIO!