[fiddlerwoaroof]

For me, even when working on something besides my editor configuration, having access to the parts of the “editor primitives” makes for a lot of powerful one-off editing tools. It was relatively easy, for example, for me to us lsp features to get a list of undefined JS variables in the current scope and add them to the function argument list. And, since lsp and the other bits I put together are all in elisp, I could use jump-to-definition to quickly find the itnernals I need to make the change.

[klibertp]

What you're describing is a feature of the system as a whole. You can have the same workflow with any dynamic, reflective environment. All Smalltalks give you the same ability to "jump to definition" of anything, turtles all the way down, and fiddle with those definitions. You could have the same ability in a system written in Lua - you just generally don't, because it requires designing the system as a whole specifically to allow it.

[fiddlerwoaroof]

Sure, but I’m not particularly defending emacs lisp (would prefer Common Lisp). It’s not exactly true that any language can have it, though: the language has to be designed to handle redefinition correctly.

[klibertp]

> the language has to be designed to handle redefinition correctly.

I believe it's more a question of how easy the redefinition is to implement. You can live-update a running Java or C programs, it's just less convenient/harder to pull off than with Forth, Smalltalk, Lisp, Prolog, and the like. So I think that yes, in principle, every language could have it - it's just that you'd need a huge pile of hacks for some and a few simple instructions for others to get it.

[fiddlerwoaroof]

I think it’s basically impossible to safely live-patch part of a compilation unit in most programming languages: you’d have to account for inlining and other optimizations to do this correctly. You _can_ patch at linkage seams and other places, but this is a fraction of the sorts of redefinitions that you get easily in systems designed for it. (And I’ve spent a lot of time trying to make various programming languages more Lispy so I can get stuff done: you always discover there are static presumptions that make it impossible to get the full experience)

[klibertp]

> I think it’s basically impossible to safely live-patch part of a compilation unit in most programming languages

There's no argument there; you're right. That's why V and Nim, for example, put reloadable things in a separate compilation unit and handle some things (global state at least) specially upon reload (if I understand what they do correctly.)

My point was that you can get quite close (sometimes with a massive pile of hacks and/or developer inconvenience), not that you can get the full experience (as in Smalltalk or Lisp) everywhere. Especially since the reloading being convenient is a large part of the experience, I think.