Support Targeted Property-Based Testing

[zkessin]

We should work to include the new features of PropER 1.3

[alfert]

Correct. I have an WIP feature branch already in my local repo, which I should publish. I would like to get PRs on that.

[zkessin]

I will be happy to help out

[alfert]

@zkessin, I published my current state of work on this on its own branch and I am open for reviews, PRs and the like. Currently the tests fail. Any hints why and what to do are more than welcome.

[zkessin]

Awesome, I will take a look in the next few days if I have a chance

[alfert]

I updated the branch. What works now is the level example with forall_sa and forall_targeted. We are in now in sync with the tutorial from Proper in both variants, web page and source code, which interestingly differ. Stil falling is a level implementation based on exists or not_exists.

Generally, I am struggling with the entire concept. The idea of having properties with a negative logic (if they fail, everything is fine) irritates me. And to find a useful metric expressed in a real number to optimise for and which is related with the correctness of software in general, is by no means obvious and I doubt if this is generally a valid approach. It reminds to program synthesis with genetic algorithms in the 90ies: is a sorting algorithm which sorts in 95% of all cases correctly better than an algorithm with sorts only in 94% of all cases? So, what to optimise for? But it might be that I look from the wrong point of view on this approach. If anybody can explain in which cases the targeted approach helps, I would be delighted.

[alfert]

Just to copy the final comment from #77:

This was a quite a tour. The biggest issue - beside understanding and valuing the entire approach - was is little copy bug that changed the semantics of exists and not_exists. So it is finally done.

Documentation can be improved tremendously, but currently I would suggest to read Fred Hebert's book Property based testing, which include a chapter on targeted PBT.

It it available in the master now, a new release hex is coming soon.