[wtetzner]

Tests can't replace types, just like types can't replace tests. You need both.

Types can't check the correctness of everything, but they do prove that certain classes of errors don't exist in your program.

Tests, on the other hand, can test for many more types of bugs, but they can only look for errors, they can't prove correctness (except in very small, closed environments where you can literally test every possible combination of inputs and outputs).

[jdm2212]

> they do prove that certain classes of errors don't exist in your program

That's particularly important when refactoring because you want to assert that you haven't introduced new bugs, and the type system will often let you prove that with almost zero effort on your part.