> the complexity of the problems demands so much code that the REPL cannot keep up.

Now we disagree, at least in part.

REPL-driven development is in part about incrementalism. You wouldn't run all the complex code every time you want to examine some state or try out an experiment. You would run it once at the start of your session, then compile just the definition you're editing with your editor's equivalent of `compile-defun`.

Of course that breaks if you're making changes that touch a bunch of different areas of your program and require rebuilding all the state, but it's usually a mistake to design programs in a way that would make such an issue frequent.