I think that your right, I called them "toy apps” but your “simple input output apps” sounds a bit better. I think once you get to functions that are so simple TDD DOES work, you could also do it just that little bit faster by still doing the tests after. Like I said, if you genuinely, honestly DO use TDD on some fun projects like calculators at home, that’s fine. I just wanted those people to stop trying to foist styles on bigger apps by lying to themselves and others about the true strengths and weaknesses. I do think TDD would probably work better with FP now that you mention it.