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    What I have experienced, though, is that my first design only worked for some simple cases and I later realized that I need a more general solution. Developing the more general solution required more tests while the original tests for the special cases won't work again. I found it acceptable to (temporarily) remove those tests while I develop the more general solution, adding them back once time is ready. Commented Jan 9, 2017 at 5:23
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    I'm not convinced refactoring is a way to generalize code (outside of the artificial "design patterns" space, of course) or escape local maxima. Refactoring tidies up code, but it won't help you discover a better solution. Commented Jan 9, 2017 at 15:15
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    @Sklivvz Understood, but I don't think it works that way outside toy examples like the ones you posted. Also, it helped you that your function was named "double"; in a way you already knew the answer. TDD definitely helps when you more or less know the answer but want to write it "cleanly". It wouldn't help for discovering algorithms or writing really complex code. This is why Ron Jeffries failed to solve Sudoku this way; you cannot implement an algorithm you're unfamiliar with by TDD'ing it out of obscurity. Commented Jan 9, 2017 at 19:59
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    @VaughnCato Ok, now I'm in the position of either trusting you or being skeptical (which would be rude, so let's not do that). Let's just say, in my experience, it doesn't work like you say. I've never seen a reasonably complex algorithm evolved out of TDD. Maybe my experience is too limited :) Commented Jan 10, 2017 at 17:28
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    @Sklivvz "As long as you can write the appropriate tests" is precisely the point: it sounds like begging the question to me. What I'm saying is that you often cannot. Thinking about an algorithm or a solver is not made easier by writing tests first. You must look at the whole picture first. Trying scenarios is of course necessary, but note TDD is not about writing scenarios: TDD is about test driving the design! You cannot drive the design of a Sudoku solver (or a new solver for a different game) by writing tests first. As anecdotidal evidence (which isn't enough): Jeffries couldn't. Commented Jan 10, 2017 at 18:52