Simulacrum VS ScalaCheck

The Cats ecosystem offers mature named abstractions providing algebraic laws virtually identical to those offered by Haskell and PureScript and that have stood the test of time, at the cost of relying on a "design pattern" approach to implementation you have to squint a bit to see ("typeclasses" based on higher-kinded types and implicit arguments) and that sometimes doesn't play nicely with Scala's colored local type inference. The selling point of this, coupled with parametricity ("tagless-final style"), is the ability to reason algebraically about your code.

Regarding numbers 2 and 3, I believe you are describing "property-based testing"[0]. A Scala version of this is ScalaCheck and can be found here[1].

There appears to be at least one Rust library which claims to provide same, but I am not a Rust developer so cannot recommend any for fitness of purpose.

0 - https://hypothesis.works/articles/what-is-property-based-tes...

1 - https://github.com/typelevel/scalacheck/blob/main/doc/UserGu...

And the readmes and tutorials written for the major libraries are some of the best, and most concise out there. Seriously, even the old stuff, like scalacheck's guide is really good. Compare that to hypothesis. The hypothesis uses readthedocs, but other than the window dressing, much better. Try to find similar concise guides for major Java projects outside of spring boot. They're all external to the projects, published on Baeldung.

Write your test as properties that must be held, and let the framework generate random values and apply the function to thus values and compare the output by those properties. Take a look to ScalaCheck for that approach.