Reminds me of the time needed to write an application that would be particularly hard to test (was critical, handled distributed state). Managed to write it so obviously correct, that I couldn't think of anything to test before releasing the clustered version. I only ever managed to find 2 bugs, both of which were silly and unimportant.

During development, I did use REPL to run/evaluate most of the functions, on 1-2 inputs, and found a couple of silly, immediately evident bugs. Development took a couple of days, testing was about as mentioned and I really didn't want to debug.

These days, I'm amazed how people can stand to write anything at all complicated in haphazard ways, but then get to spend similar or greater time playing whack-a-mole with bugs to maybe get most of the important bugs.

Yes, good tooling can make debuggers usable. OTOH, if you need debuggers, I would consider it a shortcoming of the programming methods/structures/styles and usually even entire stacks, where debugging is practically necessary.