Tests are facts. Code is theory.

Programmers have turned to science to help resolve the software crisis. But they're doing it wrong.

Science envy

Programmers have science envy. We feel that, unlike much of our code, science works. Scientists have spent hundreds of years honing a methodology that helps them assimilate new knowledge and correct error, while we have spent decades frantically accumulating complexity that we can't handle. Strangely, scientific theories become more accurate over time, whereas software systems often decay.

The software industry has tried to learn from science and engineering's success. We call our programming degrees "Computer Science" and "Software Engineering", though they are neither. "Computer Science" students do almost no experiments. The "Software Engineering" concept of exhaustive up-front design has become so discredited that even those who can't imagine any other way feel obliged to pretend that they "don't do Waterfall".

Of course, science and engineering are just analogies when applied to programming. They are meant to be useful ways of imagining our profession, not to be literally true. But in their naive form, I don't think analogies between programming and science are very useful. If we want to benefit from scientific rigour, we need to be more rigorous in how we appropriate scientific concepts.

Scientific testing

Some software testers have used the scientific method as a way of framing their testing activities. For example, David Saff, Marat Boshernitsan and Michael D. Ernst explicitly cite Karl Popper and the scientific method in their paper on test theories. Test theories are invariant properties possessed by a piece of code which Saff et al attempt to falsify over a wide range of data points with an extension to the JUnit testing framework.

I find the reciprocal of this approach useful when debugging. I start with a defect, form a theory as to its cause, then design a test to try and falsify that theory. If I suspect that the issue is caused by rogue javascript, I'll disable javascript and attempt to reproduce the issue. If I can, I've disproved my theory and I need to find another explanation. This helps me to eliminate false causes and gradually home in on the bug.

The problem with analogies that treat tests as theories and code as a phenomena is that they tell us nothing about how to write code. The software under test is like gravity, a chemical reaction or the weather. It may or may not have an underlying structure and beauty, but any insights we gain during testing are inevitably after-the-fact.

Worse, they are static models. When software changes over time, the knowledge gathered through "scientific testing" may no longer apply. The scope of scientific testing is confined to a specific version of the software. For example, a tested and verified "theory" about the memory profile of an application may become invalid when a programmer makes a small change to a caching policy.

Tests are facts. Code is theory.

Science's strength is its ability to assimilate new discoveries. If we want to share in its success, a scientific model of software development needs to preserve science's adaptability.

We can go some way to achieving this by reversing the roles of testing and coding in the scientific testing model. Tests are facts. Code's role is as a theory that explains those facts as gracefully and simply as possible.

New requirements mean new tests. New tests are newly discovered facts that must be incorporated into the code's model of reality. Software can be seen as a specialised theory that attempts to embody what the stakeholders want the application to do.

How does that help us?

Once we accept that code as a theory, we are then in a position to justify employing the most powerful weapon in science's armoury - Occam's razor. Our role is to write the simplest possible code that is consistent with the facts/tests/requirements. Whenever we have the opportunity to eliminate concepts from our code, we should.

Simple code isn't just cheaper. It's more valuable too, because it's easier to change and extend. We can justify this with reference to scientists' experience that the simplest theory is the most likely to survive subsequent discoveries.

As new requirements arrive and our understanding of the domain deepens, we have the opportunity to refactor. Refactoring isn't rework or throwing away effort. Refactoring is enhancing code's value by incorporating new knowledge on what we want our software to do. This could be by adding functionality, or in reducing complexity. Either makes the software as a whole more valuable.

Science celebrates refactoring. Each new piece of evidence clarifies scientists' understanding of phenomena and helps yield more useful theories. Often these refinements are small, but occasionally Einstein will have an insight that supercedes Newton's laws of motion. Domain driven design founder Eric Evans describes such pivotal moments on software projects as "breakthroughs".

Non-developers often assume an application is invariably more valuable with a feature than without it. Yet the example of special relativity allows us to explain otherwise. Newton's laws of motion are perfectly adequate for ordinary use. Unless we are interested in bodies moving close to the speed of light, it's not worth bothering with the additional complexity Einstein's theories bring.

If stakeholders are willing to accept that the application targets the common case and excludes troublesome edge cases, they will enjoy software that is simpler and therefore cheaper and more valuable. Sometimes, there is value in absent features. Always, there is value in simpler code.

Crave simplicity. Celebrate deletion. If science responded to new information by adding special cases then science would be in as big a mess as the software industry. As you incorporate new requirements, attempt to refine your code so that it remains flexible enough to accomodate tomorrow's requirements. Otherwise, your code will become less and less fit for its purpose, which is to provide business value.

Conclusion

When John Maynard Keynes was attacked for repeatedly revising his economic theories, he said, "When the facts change, I change my mind – what do you do, sir?" Take the same attitude with your code, but treat requirements and tests as your facts. And remember, your code is just your best approximation of what your stakeholders want it to be.