Possibly just showing my lack of knowledge here but are open-ended enumerations a common thing? I always thought the whole point of an enum is that it is closed-ended?
For instance, we had an enum that represented a sport that we supported. Initially we supported some sports (say FOOTBALL and ICE_HOCKEY), and over time we added support for other sports, so the enum had to be expanded.
Unfortunately this always required the entire estate to be redeployed. Thankfully this didn’t happen often.
At great expense, we eventually converted this and other enums to “open-ended” enums (essentially Strings with a bit more structure around them, so that you could operate on them as if they were “real” enums). This made upgrades significantly easier.
Now, whether those things should have been enums in the first place is open for debate. But that decision had been made long before I joined the team.
Another example is gender. Initially an enum might represent MALE, FEMALE, UNKNOWN. But over time you might decide you have need for other values: PREFER_NOT_TO_SAY, OTHER, etc.
I prefer to interpret those as an optional/nullable _closed_ enum (or, situationally, a parse error) if I have to switch on them and let ordinary language conventions guide my code rather than having to understand some sort of pseudo-null without language support.
In something like A/B tests it's not uncommon to have something that's effectively runtime reflection on enum fields too. Your code has one or more enums of experiments you support. The UI for scaling up and down is aware of all of those. Those two executables have to be kept in sync somehow. A common solution is for the UI to treat everything as strings with weights attached and for the parsers/serializers in your application code to handle that via some scheme or another (usually handling it poorly when people scale up experiments that no longer exist in your code). The UI though is definitely open-ended as it interprets that enum data, and the only question is how it's represented internally.
It is Data Model design, of which API design a subset.
You can only ever avoid having an other if 1) your schema is fixed and 2) if it is total over the universe of values.
enum WidgetFlavor
{
Vanilla,
Chocolate,
Strawberry,
NumWidgetFlavors
};
As far as programming languages go, all enums are explicitly open-ended in C, C++, and C#, at least, because casting an integer (of the underlying type) to enum is a valid operation.