(I split this out of the deep goals thread) @Jack Rusher asks:

@Tom Larkworthy Have you used any systems with faster feedback than hot-reloading?

maybe? Just to clarify what I mean by hot-code reload vs live-reload. Live reload is a full restart on a program on change, which loses state, vs hot-code reloading where only changed code is reloaded which maintains program state between partial restarts. But I would say Observable is a little beyond hot-reload because it's also the notebook format so program output and code is interleaved, so it that removes a context switch between IDE and program output and gives you a REPL vibe inline too. But thats as advanced as I have got so far for fast feedback. Can I do better?!

Ah, that's a better definition of hot-reload than I was expecting! 🙂 The ability to cherry pick which parts of your program you change vs which you hold invariant, including the direct inspection of values in the running system, is the key feature of "interactive programming" (called "live coding" in some circles now). This style of development, which blurs the lines between programming/debugging/testing, is (IMO) the most important feature of any programming environment, and something that too many language designers have never experienced.