Can invariant testing replace unit testing?

Question

As a programmer, I have bought whole-heartedly into the TDD philosophy and take the effort to make extensive unit tests for any nontrivial code I write. Sometimes this road can be painful (behavioral changes causing cascading multiple unit test changes; high amounts of scaffolding necessary), but on the whole I refuse to program without tests that I can run after every change, and my code is much less buggy as a result.

Recently, I've been playing with Haskell, and it's resident testing library, QuickCheck. In a fashion distinctly different from TDD, QuickCheck has an emphasis on testing invariants of the code, that is, certain properties that hold over all (or substantive subsets) of inputs. A quick example: a stable sorting algorithm should give the same answer if we run it twice, should have increasing output, should be a permutation of the input, etc. Then, QuickCheck generates a variety of random data in order to test these invariants.

It seems to me, at least for pure functions (that is, functions without side effects--and if you do mocking correctly you can convert dirty functions into pure ones), that invariant testing could supplant unit testing as a strict superset of those capabilities. Each unit test consists of an input and an output (in imperative programming languages, the "output" is not just the return of the function but also any changed state, but this can be encapsulated). One could conceivably created a random input generator that is good enough to cover all of the unit test inputs that you would have manually created (and then some, because it would it would generate cases that you wouldn't have thought of); if you find a bug in your program due to some boundary condition, you improve your random input generator so that it generates that case too.

The challenge, then, is whether or not it's possible to formulate useful invariants for every problem. I'd say it is: it's a lot simpler once you have an answer to see if it's correct than it is to calculate the answer in the first place. Thinking about invariants also helps clarify the specification of a complex algorithm much better than ad hoc test cases, which encourage a kind of case-by-case thinking of the problem. You could use a previous version of your program as a model implementation, or a version of a program in another language. Etc. Eventually, you could cover all of your former test-cases without having to explicitly code an input or an output.

Have I gone insane, or am I on to something?

Answer 1

A year later, I now think I have an answer to this question: No! In particular, unit tests will always be necessary and useful for regression tests, in which a test is attached to a bug report and lives on in the codebase to prevent that bug from ever coming back.

However, I suspect that any unit test can be replaced with a test whose inputs are randomly generated. Even in the case of imperative code, the “input” is the order of imperative statements you need to make. Of course, whether or not it’s worth creating the random data generator, and whether or not you can make the random data generator have the right distribution is another question. Unit testing is simply a degenerate case where the random generator always gives the same result.