And yes, REPL is still helpful even if the compilation is fast; amusingly most languages that have REPL features fast compilation (at least those not based on JVM). The benefit of REPL is that it preserves state which one can experiment on, and sometimes that state can be cumbersome/take long to acquire.
Most languages that have "traditionally" shipped with a REPL don't feature compilation at all. So yeah-- I guess that's fast? (Before you start, I'm not interested in pedantic arguments about how any language can be compiled. I know.)
If it's "cumbersome" to acquire the state you need for testing, then that sounds like a design problem that you should fix. The best kind of tests are reproducible and part of a test suite anyway.
One of the languages that originally featured REPL, Lisp, was always compiled, both to machine code or byte code. Ditto smalltalk. I am not even sure which "traditional" languages you are talking about.
When I mentioned state, I did not mean testing. Sure, a properly designed system will have all it's computaional parts abstracted in a functional way so it will be trivially tested compiled or not. However, REPL is incredibly useful during the development process when the abstractions required are not yet clear, so it allows one to easily explore design possibilities without committing signficant effort of implementing a correct compilable module.
