one-punch

To use lix instead of nix, set `nix.package = pkgs.lix` in your NixOS/home-manager configurations.

You are right that the generators in Hypothesis correspond to `Functor`s, so you can map over it. Indeed, in QuickCheck, its `Gen` [1] (type of `arbitrary` [2], the generator in QuickCheck) is a `Functor`, and moreover a `Monad`, so you can do monadic binds on `Gen`.

> So in Hypothesis you can take a generator that produces integers, and create one that produces only even numbers by (in Haskell terms) mapping (2) over it. Now, the shrinker you get for free will also only produce even numbers.

> That's not possible (or at least not easily possible) in Haskell. You would have to define at least a newtype wrapper.

Since `Gen` in QuickCheck is a `Functor`, to multiply its output by 2 to generate only even numbers, you can do this in Haskell as:

    fmap (\x -> x*2) (arbitrary @Int)

    arbitrary @Int <&> (*2)

My understanding is that Haskell’s QuickCheck can shrink functions, and with parametric polymorphism, can work with vector types and more [1], which would be difficult if not impossible in Python.

And in general, the base types covered by the shrinking in Python’s Hypothesis should have analogues by the shrinking in Haskell’s QuickCheck. And with deriving, Haskell’s QuickCheck should get shrinking ‘for free’ as in Python’s Hypothesis.

Also, Haskell’s QuickCheck extends to stateful and parallel checking, which is not supported in Python [2].

It makes it easy to understand how individual steps are progressing, and how individual steps relate to the overall plan. It enables me to locate expensive build steps, and possibly to avoid them if steps are failing.