Comment by ndriscoll

Which is a stupid argument that completely misses the point. If you let the compiler prove that nulls (or other invalid states) can't happen (because null isn't a value of that type), then you don't need to test these impossible cases. So it's easier to ensure your tests are solid because there's not a ton of noise checking what happens inside of invalid program paths.

This does not require a new language feature every time there is a bug, as he asserts. It requires language features to let you be descriptive in your type definitions so that invalid program states don't exist by definition. You literally cannot write one down. It's the same idea as saying you can't assign a Monkey to an int64. Scala's ZIO also shows that in fact you can type-infer whether a given path will produce errors or nulls, so you don't need to have perfect knowledge up front or go back and change tons of code if that changes. Errors can automatically propagate, and you need to handle them once, somewhere. Checked exceptions were a fantastic idea; you just need to let the compiler infer them everywhere.