[SR-887] Expose the declaration order of Enum cases with payloads

[masters3d]

Previous ID	SR-887
Radar	None
Original Reporter	@masters3d
Type	Improvement
Status	Reopened
Resolution

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, LanguageFeatureRequest
Assignee	None
Priority	Medium

md5: 8985ba38c2b176524ace48d4f576b55c

Issue Description:

error:
"Enum with raw type cannot have cases with arguments"

In addition to being able to carry a payload, it would be really useful if enums could also have a way to expose their order as an Int or some sort of Index Raw type so they can be ranked by importance.

BEFORE
```
enum HandRank{
    case highCard(PlayingCard)
    case onePair(Rank,card1:Rank, card2:Rank, card3:Rank )
    case twoPair(high:Rank,low:Rank, highCard:PlayingCard)
    case threeOfAKind(three:Rank)
    case straight(high:Rank)
    case flush(Rank, Suit)
    case fullHouse(three:Rank)
    case fourOfAKind(four:Rank)
    case straightFlush(Rank,Suit)
    
    func order()->Int{
        switch self {
        case .highCard(_): return 1
        case .onePair(_, card1: _, card2: _, card3: _): return 2
        case .twoPair(high: _, low: _, highCard: _): return 3
        case threeOfAKind(three:_): return 4
        case straight(high:_): return 5
        case flush(_, _): return 6
        case fullHouse(three:_): return 7
        case fourOfAKind(four:_): return 8
        case straightFlush(_,_): return 9
        }
    }
.........
}

```

AFTER:

```
enum HandRank:Int{
    case highCard(PlayingCard) = 1
    case onePair(Rank,card1:Rank, card2:Rank, card3:Rank )
    case twoPair(high:Rank,low:Rank, highCard:PlayingCard)
    case threeOfAKind(three:Rank)
    case straight(high:Rank)
    case flush(Rank, Suit)
    case fullHouse(three:Rank)
    case fourOfAKind(four:Rank)
    case straightFlush(Rank,Suit)
.....
}
```

[lilyball]

If an enum has a raw type, it doesn't just have a rawValue property that contains the raw value, it also has to respond to init?(rawValue: T) to be created with the raw value. An enum with associated values obviously cannot be created from a raw value, because the associated values would not have a definition.

[masters3d]

I guess it doesn't have to be a raw value. I was just trying to figure out a way to get the order of the cases with payloads with out having to create a helper function.

[lilyball]

Sounds like what you want is some function that takes any enum value and gives you the discriminant, similar to Rust's std::intrinsics::discriminant_value. This would be an API addition and would need to go through swift-evolution.

[belkadan]

"discriminant" would be the wrong name here because (a) Swift doesn't guarantee to use unique discriminants if it can infer the info some other way, and (b) the order Swift allocates discriminants (deliberately) isn't guaranteed to match the order of declarations. (And with normal enums-with-raw-types you can manually set the raw values, but it sounds like you're okay with not doing that.)

I don't think we have anything today that makes sense to expose at the user level, hence the LanguageFeatureRequest tag. This would need to go through the Swift Evolution Process, and I wouldn't call it "raw values" since it's not round-trippable.

[masters3d]

Sounds good. Thanks for the input Kevin and Jordan. I am going to wait a little bit until closer to swift 3-4. There are other things like getting all the cases that also would be nice to get for free.

[masters3d]

One of these proposals may be a requirement for this SR
swiftlang/swift-evolution#199
OR
swiftlang/swift-evolution#114

[masters3d]

allCases solved this for me.

[lilyball]

allCases is not a sufficient solution, for two reasons:

1. That requires an O(N) search to find the index of a given case
2. More importantly, allCases doesn't work for enums with payloads, and this ticket is specifically about enums with payloads.