REPLs are a great tool for exploring your ideas, and poking your software to test it "right now" but once you're happy with a small piece of code, nothing beats

REPLs are a great tool for exploring your ideas, and poking your software to test it "right now" but once you're happy with a small piece of code, nothing beats a suite of unit tests to help you refactor with confidence as your implementation gets a bit too hairy and you need to rejigger it in order to progress in a direction you didn't see coming.