Unit testing didn't fully make sense to me until I played around with quickcheck (and eventually theorem proving in Coq). Unit tests vanish nicely to theorems a

Unit testing didn't fully make sense to me until I played around with quickcheck (and eventually theorem proving in Coq). Unit tests vanish nicely to theorems and (empirical) proofs if your code expresses a succinct API. This is one end of the testing continuum. I use this sort of stuff extensively when doing mathematical computing and statistics because there's usually a clear mathematical boundary. Once you're inside it, it's relatively easy to write down global properties (theorems) of your code's API. The moment you cross that boundary your testing apparatuses have to get more complex and your tested properties less well-defined. Unit tests are hazier than quickcheck properties and integration tests hazier still. This continuum seems to be precisely the same as the code reuse continuum. Highly abstracted, testable code with a shapely API is a highly reusable library whether you like it or not. Maybe it's being called by other code, maybe it's being called by your UI, maybe it's being called by the user themselves.