[DRMacIver]

> Smarter in what way?

Better at generating values that exhibit interesting behaviour in tests. QuickCheck style testing mostly only works well because it turns out that there are a lot of bugs that are relatively "dense" in the search space of tests, and doesn't do very well at finding hard to reach bugs. This means that some bugs are found with very low probability, which is bad both because you want to find them reliably and because it means that even running your tests for longer is often not enough to find interesting behaviour.

(ETA: This mostly applies to Haskell QuickCheck and derivatives. I believe the Quviq Erlang QuickCheck has had a great deal of hand tuning of its generators to get better behaviour, so its data generation is probably strictly better than Hypothesis's in many cases right now. I'm trying to get a generic mechanism for improving things without requiring this hand tuning, so hopefully at some point I'll be able to reverse that situation)

Security-oriented fuzzers do a lot of clever things to actually determine the shape of the search space and adapt to it so that they can take advantage of the structure of the program under test so that running for longer gives them more power than just repeatedly trying the same thing over and over again. I'm hoping to incorporate some of those ideas into Hypothesis but don't currently.

> What makes QuickCheck's support for this makes it work less than ideal?

Mostly that it plays very badly with shrinking (this is better but still not good in test.check and friends) and the API for it is on the clunky side.

> What do you mean by not having to "touch" the values it generates?

In classic quickcheck, the shrink API is based on taking a value and replacing it with a simpler version of itself. This means that if the value has been mutated (which is mostly not a problem in Haskell, but can be if you're using ioProperty, and is definitely a problem in QuickCheck ports to impure languages) then you run into problems. e.g. a test that appends an element to a variable sized array argument can get the shrinker into an infinite loop.

It also means that QuickCheck is limited by the type constraints on the generated values. You can't e.g. do duplicate detection because you aren't constrained to generate values on which that is meaningful.

In Hypothesis in comparison everything is based off its IR, so it can do manipulations and comparisons on that, and it doesn't matter what type the generated value is.

(Hypothesis also can't do perfect duplicate detection because it has the problem that many IR values may map to the same value, but its duplicate detection still seems to be mostly good enough in practice)