Why do we change code?

In his book, Working Effectively with Legacy Code, Michael Feathers offers four reasons:

1. Adding a feature
2. Fixing a bug
3. Improving the design
4. Optimizing resource usage

I’ve yet to think of a 5th reason.

So what?

These 4 reasons are the basis for a personal rule when writing code:

Each pull request must address only one of these reasons.

Mixing concerns in a single PR can lead to all sorts of problems:

• It makes the PR much harder to understand and review
• It makes deploying the PR more dangerous (if a bug is found, is it due to the new feature, or the refactoring?)
• It makes reverting the PR dangerous (you can’t revert a broken feature without also reverting useful bug fixes)