Switch from Leancheck to Hedgehog.

[patrickt]

Leancheck is a lovely library, but it is not as ergonomic nor as rich in API as Hedgehog. We’re already using Hedgehog for semantic-core, might as well go all the way. This would enable us to get rid of that Instances.hs file that has like 125 orphan instances.

[robrix]

cf #69

[robrix]

I’d like to also suggest QuickCheck.

Pros:

• doctest supports it already (and relies on Arbitrary instances).
• It seems to enjoy better support for testing generated distributions (cf Building on developers’ intuitions to create effective property-based tests and the docs for e.g. label & classify).
• The manual is pretty excellent.

Cons:

• Shrinking has to be defined manually.
• Arbitrary is lawless.

[patrickt]

Compatibility with doctest is indeed a big win. I personally find the limitations of an Arbitrary-based approach very frustrating, but the docs are indeed good and we should definitely look into how the shrinking/generated distribution testing compares to QuickCheck. There’s a package for tasty-hedgehog that provides label and classify, but I’ve never used it so I can’t vouch for it.

One of the big advantages of #120 is that we can drop in tests from any test framework and they’ll DTRT, so we can mix and match as necessary.

[patrickt]

I’ve also found the Hedgehog docs to be excellent.

[robrix]

I’m reasonably convinced that Hedgehog is the way to go now. I just wish there were a doctest equivalent for Hedgehog, as I tend to go out of my way to avoid writing Arbitrary instances for doctests anyway!