The System Paradigm
I have used a REPL connected to a production system to change code. Yes, I have changed code in a running production system. Does that terrify you? Honestly, it kind of terrifies me a little, too! Regardless, sometimes the best way—the only way—to diagnose a particularly nasty bug is to poke a living organism and observe the result. I am more like a doctor diagnosing a patient and less like a detective trying to piece together a sequence of events from the clues left behind.
There's that and plenty of fascinating observations about the difference between a system constructed as a organism (system paradigm), and constructed as a cathedral (or a pyramid: that's the language paradigm).
I am fascinated, but I'd like to talk to someone who had this kind of experience. I understand the beauty and the utility of the REPL, yet intervening in a system in such a organic and (apparently, to me) not organized fashion seems more dangerous than it's compelling. How one can ensure the changes applied to the running deploy will be incorporated in the code base? How can you properly "undo" the changes you've made while you were tinkering with the system. Anyway, a nice and inspiring post.