[nateabele]

Sure, but the point is, a halfway decent type system will eliminate whole classes of errors—consequently eliminating whole swaths of things you would ever need to write tests for.

What's more, though, is that a good type system will help you 'Make Impossible States Impossible'[0]—even more behavior you don't have to test because bad behavior is simply not representable in your program.

Tests are strictly more powerful in the small, because they can be arbitrarily complex, but I would say a (really good) type system is more powerful in the large, because you get way more leverage in terms of overall program correctness.

A lot of my personal projects are in Elm (yeah, yeah, bring on the hate)—the models are really well-defined, but the test suites usually only have a handful of tests of behavior I really want to verify. Haven't even played with the property checker yet, but again, that's something that is strictly more powerful than unit tests, but you can only get it* if you have a type system.

[0] https://www.youtube.com/watch?v=IcgmSRJHu_8

* Okay that's not really true: see clojure.spec in Clojure—but there are so few counterexamples that it's true for most practical purposes