Tidy First?

Chapter 9. Explaining Constants

So you’re reading along, and you see a number you don’t recognize. Or you’re reading along and you see a constant string repeated all over the code. You figure out what the constant means.

Create a symbolic constant. Replace uses of the literal constant with the symbol.

I mean, c’mon. I’ve been seeing this advice since I was a wee little programmer and yet still somehow folks think this is okay:

if response.code = 404
    ...blah blah blah...

Okay, I got blame-y there for a second. We’re not here to judge the person who made the mess (pro tip: it might be us). We’re here to take care of ourselves by tidying first before changing things:

PAGE_NOT_FOUND := 404
if response.code = PAGE_NOT_FOUND
    ...blah blah blah...

Be careful. The same literal can appear in two places and mean something different. It doesn’t help to tidy to:

ONE = 1
...ONE... # everywhere you need unity

You’re reading. You understand. You’re putting that understanding into the code so you don’t have to hold it in your head.

There are a few tidyings downstream of this one about putting constants that change together or need to be understood together in one place and separating them from constants that cluster for other reasons. I’m going to let you figure those out. Coupling, cohesion, just do your thing.