[swift-evolution] [Proposal] Enums with static stored properties for each case

[Brent Royal-Gordon]

>> - Abusing rawValue is just that: an abuse.
> 
> My original proposal does not replace rawValue and is compatible with it.

`rawValue` has a different purpose from how you're using it. It's supposed to allow you to convert your type to some other *equivalent* type, like an equivalent integer or string. Moreover, it's supposed to allow you to *reconstruct* the instance from the raw value—remember, `RawRepresentable` has an `init(rawValue:)` requirement.

It is *not* supposed to be an ancillary bag of information on the side. You're cramming a square peg into a round hole here.

(Also, if you use `rawValue` for an ancillary bag of information, that means you *can't* use it on the same type for its intended purpose. For instance, you would not be able to assign numbers to your Planet enum's cases to help you serialize them or bridge them to Objective-C. That's not good.)

>> - Using `where` just doesn't match the use of `where` elsewhere in the language; everywhere else, it's some kind of condition.
> 
> It is also used in generic type constraints. Plus it reads like human
> language: `case mercury where (mass: 3.303e+23, radius: 2.4397e6)`

But a generic constraint is also a type of condition: it specifies types which are permitted and divides them from types that are not.

This is *not* a condition. It's not anything like a condition. It's simply not consistent with anything else in the language.

>> - Dictionaries are the most straightforward way to handle this with the current language, but their lack of exhaustiveness checking is a problem.
> 
> Dictionaries can be used as workaround, but they cannot (lack of
> exhaustiveness) solve the problem.

I agree that they're a halfway solution.

If `ValuesEnumerable` were to be accepted (and to have a generic requirement for its `allValues` property), you could write a Dictionary-like type which ensured at initialization time that it was exhaustive. That's not as good as compile time, but it's not bad—sort of a three-quarters solution.

	struct ExhaustiveDictionary<Key: Hashable, Value where Key: ValuesEnumerable>: Collection, DictionaryLiteralConvertible {
		private var dictionary: [Key: Value]
		
		init(dictionaryLiteral elements: (Key, Value)...) {
			dictionary = [:]
			for (k, v) in elements {
				dictionary[k] = v
			}
			
			if dictionary.count != Key.allValues.count {
				let missingKeys = Key.allValues.filter { dictionary[$0] == nil }
				preconditionFailure("ExhaustiveDictionary is missing elements from \(Key.self): \(missingKeys)")
			}
		}
		
		var startIndex: Dictionary.Index {
			return dictionary.startIndex
		}
		var endIndex: Dictionary.Index {
			return dictionary.endIndex
		}
		subscript(index: Dictionary.Index) -> (Key, Value) {
			return dictionary[index]
		}
		func index(after i: Dictionary.Index) -> Dictionary.Index {
			return dictionary.index(after: i)
		}		

		subscript(key: Key) -> Value {
			get { return dictionary[key]! }
			set { dictionary[key] = newValue }
		}
	}

>> What I would do is borrow the "accessors" concept from the property behaviors proposal and extend it so that it supported both functions and variables.
> 
> Wouldn't accessor just be a redundant keyword here? Currently enums do
> not support stored properties, so I guess there is no extra need to
> mark properties with any special keyword.

The keyword is mainly to indicate the unusual syntax at the definition site, where you only have to specify the name of the accessor you're defining, not a `func` or `var` keyword, a return type, or even parameter names. (Like `willSet`, there's a default parameter name you can use.) Secondarily, though, I think it's helpful to indicate very explicitly that this is not an ordinary method or property definition, even if the compiler could perhaps sort things out without it. `accessor` is something a user can Google if they've never seen it before.

> Property accessors might work for enums with associated values, but
> not so well without them.

The two have nothing to do with each other. I showed your planets example, which has no associated values but uses accessors just fine.

-- 
Brent Royal-Gordon
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