Earlier quoted context omitted.
> programming from inside the system. Can you elaborate on that? I think you are saying that REPLs in other languages don't do what you can do in Lisp, but I don't think I know enough to fully understand why.
Imagine this: You start a fresh REPL session with a vague idea of something you want to build. You have good tooling that enables you to write code in your favourite editor and pipe it to the REPL through a socket. You write a couple of functions and send them to the REPL. You call them a few times with some test arguments, maybe define a couple global variables while testing, and realize one of the functions doesn't…
Because there's a problem I've run into several times when writing Lisp (I use SLIME): I decide to refactor some stuff, so I rewrite some stuff, maybe remove a function or two from the source as part of that. Things seem to be working fine. I save my .lisp files and then at some point end up restarting the Lisp process (often because McCLIM can be kind of flaky and lose its connection to the X server). When I go to load my code back in, I get errors, because oops I was actually still calling that deleted function somewhere -- it "worked" before because the old function was still hanging out in the image.
When I develop in a compiled language, I just accept that I'm going to be re-building the entire thing from scratch (equivalent to restarting the lisp process & re-loading the source files) every time I want to try a change; if I broke something, I'll find out the next time I run make/go build/whatever. When I'm developing in Lisp, and I'll freely accept that my Lisp dev flow probably isn't ideal, it's so easy to screw up but be unaware of it for hours if not days: I write some code, I'm happy, I close Emacs, then a week later I come back and try to load my package and it's just broken.
I could solve it for myself by just using images: the "deleted" function would persist in the image, and every time I started working again it would be there. But if I ever want to let somebody look at the code, hack on it, make changes, what's that look like?