You don't even need the "REPL" buffer to do REPL style development. You can just use the "eval" keybinds of your editor to eval the source code in the files.
For any test/scratch code, just create a scratch file outside your project and write the experiments there, eval'ing as you type. That file could even be an "official" unit test you include in the project.
For any test/scratch code, just create a scratch file outside your project and write the experiments there, eval'ing as you type. That file could even be an "official" unit test you include in the project.