[k_bx]

I've spent long time writing Haskell and now write Rust professionally, so let me tell you something. The stuff where you really want to "program in types" lives beyond Haskell, in languages like Agda, Idris, some HoTT stuff etc. anyways.

In principle the more developed the type system is – the closer it to not distinct between types and values. Caviat is that its "type inference" gets worse.

So, in those more developed languages, you could have type-level proofs (guarantees) that your calculator produces correct results, as a proof, as theorems. That 2+3 will be 5, not as runtime test assertion, but as a theorem, that no other result is possible no matter what happens. Or that your parser will never fail on valid JSON etc. but nobody guarantees it's going to be a pleasant thing to write, maintain, and debug. And compile times will probably be terrible.