Justin du Coeur (jducoeur) wrote,
Justin du Coeur
jducoeur

Extensible enumerations

(This one's just for the programmers, but it's not as opaque a topic as usual.)

So in the course of thinking about my last post (about Querki), I happened to look at my desired feature list. And I was reminded of one of those bugbears of most programming languages: enumerations.

Here's the thing: what I want, much of the time, is the ability for subclasses to *extend* enumerations. The most common use case is return codes. I might have a base class that implements the concept of return codes, and includes an enumeration of the basic codes for success, warning, failure -- generic stuff. As I add subclasses with more precise functionality, I want those subclasses to be able to add more-precise return codes, that describe the results in more detail.

But I can't do that, at least not safely, in any language that I'm thinking of offhand. The thing is, I wind up with two possibilities. Either the return code is something like a raw integer under the hood, so it's totally unsafe and not really type-checked -- this allows for risky code. Or the enumeration *is* strongly typed -- in which case, I can't add values. Why not? Because adding values is effectively *extending* the type, rather than *specializing* it, and that's a no-no in any decent OO system. It implies that a subclass could add values that don't exist in the parent, so code written for the parent doesn't necessarily work with the subclass -- a violation of the basic principles of OO.

When I think about it, though, it occurs to me that it's easy to fix this: just redefine my terms. What I really want is the idea of *hierarchical* enumerations. That is, the base class defines "success" and "failure", but no details. But this is precisely what OO is good at: what I want is to think of those success and failure values as *classes*, which I can then subclass. So I want to be able to define a "subclass" of the enumeration that adds specializations to the existing values. So instead of tacking "Null pointer" or "Not authorized" to the end of my enumeration, I should be thinking of them as specialized values of "Failure". That follows good OO principles, and seems like it would work well.

Does anyone know any languages that formally do this? I *can* do it in Scala decently well using case classes, but that's not quite optimal: I'd prefer a syntactic construct that is more explicit about this notion of a hierarchical, extensible enumeration. But I don't think I've ever come across one.

Anyway, I need to chew on this further. It's directly relevant to Querki, because I want to add a formal concept of enumerations there, and it needs to be exceptionally rich in order to cope with the use cases. (Since Querki is going to be used for world-building, I need a lot of schema flexibility.) I may well apply this concept there...

This was originally posted via DreamWidth, at http://jducoeur.dreamwidth.org/975.html. Feel free to comment either here or there.
Tags: programming
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