Hegel, a universal property-based testing protocol and family of PBT libraries

For what it's worth the devs say their "current long-term plan is to implement a second Hegel server in Rust" [0], so the current state of affairs is probably a compromise between getting something usable for end users out and something more "sane", as you put it.

[0]: https://antithesis.com/blog/2026/hegel/#what%E2%80%99s-next

The biggest thing that leaps out at me looking at it is that Hegel is very built around flexible user-specified data generation (using the library's base generators and combinators) mixed freely with test execution. Validity in contrast looks extremely type-based, which is convenient when you're only testing fairly general properties of built-in types, but I've never found flexible enough to be a really good basis for property-based testing once your testing needs get even a bit more complicated. e.g. a lot of tests want some sort of upper and lower bounds on their numbers, and I don't want to define a type for each.

For an only slightly more involved example of this, suppose you've got, say, a Project type, and Projects have an owner that is a User. You might want a test that is about a single user that has some number of projects. In a generator-based approach, this is easy: You just generate a User object, then you generate a bunch of Project objects that have to have that User as their owner. Just works.

In contrast, in a type based approach, there's basically no way to express this without e.g. defining a new ProjectsOwnedByASingleUser type and defining what it means to be a valid instance of that type... It's a lot of machinery for what is IMO a strictly worse user experience.

The generators of T, typically called an Arbitrary<T> can be constrained, e.g Arbitrary<number> could generate any float or just non-negative integers. Ideally we would define an Arbitrary<float64> or Arbitrary<u64>.

A sufficiently expressive type system makes it easier to narrow the gap between what is generated and what is assignable to type T, making it clearer what the valid input domain to the SUT is.

I often gesture towards this phenomenology when religious folk casually attempt to claim "spirit" as some form of belief they hold over me. I honestly don't know if I've developed the position well, it is almost entirely through the lens of continental philosophy absorbing Hegel, but I use it to illustrate that my concept of spirit, as an atheist, may not be a different phenomenological occurrence than that of a religious framing and even shares the quality of a rich historical lineage I can draw from. I could just as easily retreat into untranslated German that sounds poetic or prophetic to the uninitiated, but that would be doing exactly what I'm asking them not to do, leaning on a vocabulary the other person can't engage with without first conceding the ground it's built on. This seems to effectively persuade them to adjust their vocabulary to a register I can actually engage with without needing to hedge for the axiomatic differences we have.

This is a comfortable mode of engagement and it is one I can share with religious folk, but I do find they often refuse this register and I will admit I can't always articulate why I find their refusal frustrating either.

(I strongly recommend the latter.)