Peace of mind

One of my favourite things about unit tests is the peace of mind they afford me. Of course, tests don't guarantee bug-free code, but they definitely contribute to peace of mind. That's especially true when you're altering existing code, whether making functional changes or refactoring.

Peace of mind isn't at all superficial to technical work. It's the whole thing. That which produces it is good work and that which destroys it is bad work. The specs, the measuring instruments, the quality control, the final checkout, these are all means toward the end of satisfying the peace of mind of those responsible for the work. What really counts in the end is their peace of mind, nothing else.

Robert Pirsig, Zen and the Art of Motorcycle Maintenance.

Software is incredibly complex, and it's impossible to hold in your head the entire space of possible behaviour for even a relatively simple program. You certainly can't predict every possible consequence that will result from a change you make. But tests help you with that burden by providing evidence that some behaviour (hopefully the most important parts) have not changed.

Instead of thinking of writing software as a constructive process, sometimes I think it's like corralling an animal. You cut off part of the possible complexity by writing tests, each one reducing the unpredictability of the software by some small factor. Eventually you wind up with a well understood and constrained process, like a ring-fenced animal.

But with software you don't start with a complex process and then constrain it (unless you're working with untested legacy code.) Instead, you're simultaneously constraining and creating the process. And beyond that, as you change the machine, the machine changes you. There's a feedback loop between your mental model of the program and the program itself.

...the nature of the the material at hand determines [the mechanic's] thoughts and motions, which simultaneously change the nature of the material at hand. The material and his thoughts are changing together in a progression of changes until his mind's at rest at the same time the material's right.

Robert Pirsig, Zen and the Art of Motorcycle Maintenance.

That's why tests are so important. You can't have peace of mind without some guarantee of the behaviour of this otherwise wildly unpredictable stuff we know as software. And your peace of mind is  in the software as much as the software itself is in your mind. If you're anxious or scattered when you write software, that is manifested as buggy, unpredictable and ugly code. So:

You should remember that it's peace of mind you're after and not just a fixed machine.

Robert Pirsig, Zen and the Art of Motorcycle Maintenance.