[swift-evolution] Promote "primitive" types to enums in extensions

[James Campbell]

I would rather have a syntax that mirrors the way Protocol does it.

struct Card {
        suit:enum<CardSuit>
        value:Int
}

or we could change it so this only excepts the enum itself unless
you explicitly cast from a Int or another enum:

struct Card {
        suit:CardSuit
        value:Int
}

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On Thu, Mar 24, 2016 at 5:41 PM, Carlos Rodríguez Domínguez <
swift-evolution at swift.org> wrote:

> It is a common practice in C to assign to integer (int, int16, int64,
> etc.) typed variables “constant" values declared in enums. In swift, it is
> in fact possible to do that by using enums' “rawValue” property. When
> importing structs from C into swift, we even get some fields declared with
> an integer type, but expecting the assignment of a “constant” declared
> inside an enum. Of course, this is error prone, it is “old-style”
> programming and very confusing for newcomers. To solve this issue, my
> proposal is to be able to create extensions that promote certain fields
> within a class or struct to enums.
>
> For instance, let’s take these sample C struct and enum:
>
> struct Card {
>         int suit;
>         int rank;
> };
>
> typedef enum {HEARTS, DIAMONDS, CLUBS, SPADES} CardSuit;
>
> (Note: I understand that above code follows a bad programming practice,
> yet it is widely common)
>
> It should be imported into swift as follows:
>
> struct Card {
>         suit:Int
>         value:Int
> }
>
> enum CardSuit : Int {
>         case Hearts, Diamonds, Clubs, Spades
> }
>
> Now, I propose to be able to create an extension as follows:
>
> extension Card {
>         #enumvalue(suit:CardSuit)
> }
>
> From this moment on, the suit field should only receive CardSuit values,
> thus not requiring the use of raw values for assignments.
>
> These extensions should also be of great interest for people using
> CoreData, since it is not possible to declare enums in models. Therefore,
> to declare enums, it is necessary to declare integer values, and then use
> the “unsafe”, “unexpressive" approach explained before.
>
> Note that the proposal intends to only support promotions from integer
> values to enum values, but, for example, it could also be extended to
> string values.
>
> Finally, it could be appropriate to extend this proposal to redeclare
> func’s signatures, in order to promote certain parameters to enum values.
>
> Best,
>
> Carlos.
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> swift-evolution at swift.org
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>
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