I know, and that is exactly my point. It isn't like "send to REPL" in pretty much any half-decent LISP. And it hurts development comfort, speed and maintenance

I know, and that is exactly my point. It isn't like "send to REPL" in pretty much any half-decent LISP. And it hurts development comfort, speed and maintenance is way more burdensome than what it could be. Here, you would need to involve a compiler of course and do more due diligence and then run ftrace or something to exchange the code. Also, you would probably in some cases need to make sure, the ordering of code is right as to not hurt performance. (See Emery Berger's talk https://www.youtube.com/watch?v=7g1Acy5eGbE https://www.youtube.com/watch?v=7g1Acy5eGbE which probably applies here as well.) Of course, with great power comes great responsibility. You could add some optional checking facility, that would be able to roll back the changes to code if some conditions were met.