I imagine some systems may start out by tinkering with definitions in the REPL in the live system, and then as it grows, the best definition of the system is found in the current state of the REPL, rather than any more formal specification of the system – including by source code.

At some point maybe the system state will be captured into source code for longer term maintenance, but I can totally see the source code being secondary to the current state of the system during exploration.

After all, that's how I tend to treat SQL databases early on. The schema evolves in the live server, and only later do I dump it into a schema creation script and start using migrations to change it.