I think people should in fact test data, not code. Looking at it from purely functional point of view, functions should be either proved that they do what they

I think people should in fact test data, not code. Looking at it from purely functional point of view, functions should be either proved that they do what they should, or they should be asserted and QuickCheck-ed. But what really needs to be tested is that the input parameters (i.e. data) conform to some "hidden" assumptions that we had when we wrote the functions. Because as we modify the program, or even why we modify it, is that these assumptions have changed.