I mean that you can, for example, redefine a function and the new version is used from then on (old references still reference the old function, it is ‘safe’ in that way).
REPL driven development is basically you write code in the REPL, it is put in the system in real-time, any time you save you are saving a ‘dump’ of the system (no source code), and you can pick it up where it left off at any time. This was very popular in business distributing LISP since anyone could just spool up the server from the state and they could introspect the system, but no code distributed.
Easily yep, it just lacks the ability to write out the running ‘state’ of the system.
Yeah the LISP state was utter magic compared to anything else out at the time, even compared to today it is magic. ^.^