"Never use booleans"
One of my first interactions with a Principal 1 at Amazon was a design review for a design owned by a team that was not mine, merely adjacent to mine. Because I was 21 and a dumbass, I thought that this older guy would hew closer to a PM or a manager and say some sort of irrelevant/ignorant set of remarks rather than a bunch of clever, reasonable, and forward-looking things, and of course the latter is what he did.
(One of the major drawbacks of spending three years getting ‘technical knowledge’ from Hacker News and Reddit was that I was conditioned to mildly antagonize anyone who wasn’t a programmer in their twenties or thirties. Unlearning that vile lesson was a huge part of actually joining the industry.)
Anyway, I was walking back to our floor with the Principal after the meeting, and we engaged in the genre of small talk that I find particularly valuable, which arises when I begin discussions with “so, that was my first one of those, what else should I know?” 2
The Principal chuckled, as if about to share a secret, and said, “honestly, these API reviews are pretty easy. Everyone runs into the same mistakes after a while, and you can give fairly canned responses that are boring but actionable and correct.”
I asked him for an example, and he obliged:
“Never, ever make anything a boolean. Always make it an enum instead. There’s going to end up being a third value, and you’re going to regret it.”
This seemed like such a simple yet weird statement, and over the coming weeks I would try and poke holes in it — surely is_deleted is a reasonable flag!, surely should_expose_rss is a reasonable flag! — and it’s stuck in my head ever since.
Anyway, I’m working on a refactor for Buttondown, and of course I just had to migrate a boolean on the Email model to an enum: Email.is_imported is now Email.source (where source can be app, import, or api, to reflect the fact that now you can create emails through the API)
Was the Principal’s advice good? It’s probably not correct 100% of the time. Does it need to be? Isn’t advice sufficiently good if it’s “mostly correct”?
I’m not sure. But this wasn’t the first time I’ve migrated a boolean to an enum and it won’t be the last. (I seem to remember the anecdote more vividly than the advice itself, which is one of my many vices.)