They are indeed checked at compile-time if you use the optional type checker. It's not on by default because there are still a few odd corner-case bugs in it. B

They are indeed checked at compile-time if you use the optional type checker. It's not on by default because there are still a few odd corner-case bugs in it. But the Run button remembers which state you are in, so once you switch to using the type-checker, you'll always use it unless you stop using it. What it should do is a decision that should be made by the individual curriculum/instructor.