Right. The tests come from the same mind(s) that wrote the code. Some kinds of bugs won't occur to them at the time, and may also be missed by the corresponding

Right. The tests come from the same mind(s) that wrote the code. Some kinds of bugs won't occur to them at the time, and may also be missed by the corresponding tests. Testing upfront is just another tool, whether by unit tests, static typechecking, or a REPL. Often useful, but not a panacea.