I'm glad I finally see this being raised. I don't know if I can make the claim that if a program behaves to spec, the types must be right (that seems like an a

I'm glad I finally see this being raised. I don't know if I can make the claim that if a program behaves to spec, the types must be right (that seems like an academic statement out of my own wheelhouse). But I've always scratched my head at the canard of "static types saves time from obviating the need to write a whole class of type validation tests", that just doesn't map to my own experience. I challenged my intuitions on this recently and checked out some of my existing unit tests suites to see what my type error coverage looked like. It was exceedingly rare that I was able to fine a behavior assertion which after introducing an intentional change to cause a type error, didn't blow up the test. The cases where I did find type errors that snuck through with false negatives, the test itself was actually written poorly and the same behavior assertion would fail in other ways unrelated to types. I'm sure there are situations (in my own stack and in other folks') where my above findings are contradicted but, given that static typing for something like JS is not in any way a zero-cost abstraction, I'm not sure it's really a net win.