That works in Lisp, too though, but it's easier and faster to run the tests in the REPL, so you get the best of both worlds. When I write Lisp I usually start

That works in Lisp, too though, but it's easier and faster to run the tests in the REPL, so you get the best of both worlds. When I write Lisp I usually start testing in the REPL, iterate until the test code gets to be a few lines long, and then copy it over to the package's unit tests. Then I can keep iterating on both the test code and the code being tested. I'll do something like this in the REPL: (ql:quickload :mylibrary.test) (mylibrary.test:my-test-function) "mylibrary.test" will depend on "mylibrary", so quickload will load both packages when they've been changed, and I can re-run the test quickly using Alt-P to call up the previous REPL input. And the REPL is also really great for "what if" testing that you may not want to keep forever, like, "What happens if I call this function with an invalid parameter?" Hopefully there's a test for that, but it's often a lot faster to just call the function and see what happens rather than go dig through the test code and lose your focus.