Yeah, Clojure code is typically built with this in mind, and the data structures as well.

So in a JavaScript server, you might have the database connection set in some config/thing you pass around to request handlers to use, and if you change a request handler, you typically stop the entire application, then start it again. Then the database connection (and the entire config in fact) would need to again connect.

In a Clojure application, first you either start the repl from the server, or start the server from the repl, then you might instead keep the database connection in an "atom" that you keep around for as long as you have the repl/server running. And instead of restarting the process when you change a handler, you'd only change the handler.

And yeah, the libraries and ecosystem at large is built with this (mostly) in mind for everything. The language also supports this by letting you "redefine" basically anything in a running Clojure application, which would be harder in other languages.

I've done some experiments years ago for doing the same in JavaScript, and it kind of works, but every library/framework in the ecosystem is not built with this in mind, so you'll be writing a lot of your own code. Which, now with LLMs, maybe is feasible, but can't say it was at the time exactly.

You're on to something. It's the lisp machine of it all. Hot reloading is nothing that requires anything special, so you can redefine a callback or dependency with ease in the repl and the system chugs along. You can theoretically do something similar in ruby, but it's the opposite of elegant, you'd be forced to re implement methods with different dependencies etc. It's also a function of being "functional" in the lisp sense, that things are lists, and lists can be replaced, functions or otherwise.

The fun way to get a feel for lisp machines is emacs, it's so easy to fall of a language and especially hand-coding in a language if you don't have to.