I've used utility functions in Erlang where I make changes, then compile and load all modified modules...
It's absolutely hot loading, there's persistent state, any fully qualified function calls run in the newest module. The gen_server pattern always calls into your behavior module with fully qualified calls, so those are pretty easy to get into new code at a reasonable time. If you write your own service loop, it's pretty common to call ?MODULE:loop() to enable hotloading there too.
There's footguns; hotloading is a way to make rapid changes to your system, but sometimes rapid changes isn't what's needed and sometimes the rapid change you make is to go from a partially broken system to a fully broken system. But, there's a virtuous circle when you can make production changes quickly, because you can make a small change and observe and make many follow ups in a single day. With push processes that take a long time, you end up encouraged to make a bigger change one time, otherwise you spend all day waiting for traffic to move between old and new versions, or waiting for instances to deploy, etc.
It's absolutely hot loading, there's persistent state, any fully qualified function calls run in the newest module. The gen_server pattern always calls into your behavior module with fully qualified calls, so those are pretty easy to get into new code at a reasonable time. If you write your own service loop, it's pretty common to call ?MODULE:loop() to enable hotloading there too.
There's footguns; hotloading is a way to make rapid changes to your system, but sometimes rapid changes isn't what's needed and sometimes the rapid change you make is to go from a partially broken system to a fully broken system. But, there's a virtuous circle when you can make production changes quickly, because you can make a small change and observe and make many follow ups in a single day. With push processes that take a long time, you end up encouraged to make a bigger change one time, otherwise you spend all day waiting for traffic to move between old and new versions, or waiting for instances to deploy, etc.