Add 'MonadFail' instance for 'PropertyT'

[geigerzaehler]

Add MonadFail instance for PropertyT so users can write the following tests

property $ do
  Just val <- lookupUnderTest
  assertSomeProperties val

The above test will produce a test failure when lookupUnderTest returns Nothing.

We build on the MonadFail instances for TestT provided by #233.

[moodmosaic]

FWIW, here's why there isn't a MonadFail instance for PropertyT: #233 (comment) /cc @HuwCampbell

[geigerzaehler]

FWIW, here's why there isn't a MonadFail instance for PropertyT: #233 (comment) /cc @HuwCampbell

Thanks for the hint. I didn’t see that.

As I understand the comments the instance was not added because it was not clear what the behavior should be. I would make the case that having the test failure behavior makes a lot of sense. It is basically fail = error (which used to be the default) with more information.

It would be great if the original decision would be reconsidered.

[moodmosaic]

[..]the instance was not added because it was not clear what the behavior should be.

Not quite, I actually think @HuwCampbell makes it clear (in that same thread):

People expect to see MonadFail errors when upgrading to 8.6. It's part and parcel with the upgrade and that's a good thing. We shouldn't add a MonadFail instances just to counter that.

See also nick8325/quickcheck#228 (comment) on a similar discussion in QC space.

[jacobstanley]

I certainly do have reservations about fail meaning discard for properties, and even a little for generators. I see properties as advanced tests, rather than generators, so I think error is a sensible choice.

[geigerzaehler]

🎉