[swift-evolution] [Proposal] Enums with stored properties

[Mateusz Malczak]

I think, I have used quite unfortunate naming, which is a root of an
misunderstanding here. By saying 'enums with stored properties' what I
was really thinking about, was enumeration type with stored constant,
immutable properties (constants). I don't want to duplicate 'struct'
type here, but instead I would like to make it possible to store a
const values in enumeration cases. So going back to my example once
again:

Lest define an enumeration type `Format` with 3 possible cases. Each
case will be able to carry over some additional information - in this
case a pair of numbers (but in fact Any? should be possible)

enum Format {
    case SMALL(30, 30)
    case MEDIUM(60, 60)
    case LARGE(120, 120)
    var width: Double
    var height: Double
    init(width: Double, height: Double) {
        self.width = width
        self.height = height
    }
}

I'm not sure about 'var' clause in that example as it causes all the confusion.

I can access additional info stored in enum case, but it cannot be
modified. Format.SMALL doesn't change, as well as non of its
properties.

// allowed usage
let format = Format.SMALL
let width = format.width // this would be equal to 30 (const value
assigned to 'width' property on enum case .SMALL)

// not allowed usage
let format = Format.SMALL
format.width = 40 // error, stored values are immutable and can not be modified

We get all advantages of enumeration type, and, assuming all cases are
describing the same possible state, we can store some extra
information in each case. This can be called a third enumeration type
feature, right next to associated values and rawType.

--
| Mateusz Malczak


2016-10-10 13:40 GMT+02:00 Jay Abbott <jay at abbott.me.uk>:
> Thanks for the explanation Mateusz, I think I understand. So the enum still
> only has 3 cases, SMALL, MEDIUM, and LARGE, but an instance also has some
> properties?
>
> So some code to use it might be:
> var aFormat = Format.LARGE
> aFormat.width = 150 // aFormat is still Format.LARGE - this doesn't change
>
> Is that right?
>
> On Mon, 10 Oct 2016 at 09:06 Mateusz Malczak via swift-evolution
> <swift-evolution at swift.org> wrote:
>>
>> Hi,
>> > Perhaps it is a bit ugly, but I don’t know if allowing stored properties
>> > on
>> > enums is the solution: that looks very ugly to me too.
>>
>> That may look ugly, but can be very useful, if only you think
>> rawValue's are useful then you should also agree that stored
>> properties would be useful :)
>>
>> --
>> | Mateusz Malczak
>>
>>
>> 2016-10-10 9:26 GMT+02:00 David Hart via swift-evolution
>> <swift-evolution at swift.org>:
>> > Perhaps it is a bit ugly, but I don’t know if allowing stored properties
>> > on
>> > enums is the solution: that looks very ugly to me too.
>> >
>> > On 10 Oct 2016, at 02:36, Erica Sadun via swift-evolution
>> > <swift-evolution at swift.org> wrote:
>> >
>> > I would love to be able to have stored properties in addition to the
>> > varying
>> > elements.
>> >
>> > Now, I end up creating a secondary struct T and doing case a(T,
>> > whatever),
>> > b(T, whatever), c(T, whatever), etc. where the same associated structure
>> > is
>> > every case, *or* I end up putting the enum into a struct which means the
>> > guiding semantics are the struct and not the enumeration. Both
>> > approaches
>> > are ugly.
>> >
>> > -- E
>> >
>> > On Oct 9, 2016, at 6:03 PM, Jay Abbott via swift-evolution
>> > <swift-evolution at swift.org> wrote:
>> >
>> > Mateusz,
>> >
>> > To me, "Enumeration defines a type with well defined set of possible
>> > values"
>> > seems to contradict the idea of having properties that can have
>> > different
>> > values. What could you do with this special enum - what would the code
>> > that
>> > uses it look like?
>> >
>> >
>> >
>> > On Sun, 9 Oct 2016 at 04:56 Robert Widmann via swift-evolution
>> > <swift-evolution at swift.org> wrote:
>> >>
>> >> I’ve started doing this to try and mimic “Smart Constructors” in
>> >> Haskell
>> >> and I think it works quite well.
>> >>
>> >> struct Format {
>> >>   enum FormatBacking {
>> >>     case SMALL(Int, Int)
>> >>     case MEDIUM(Int, Int)
>> >>     case LARGE(Int, Int)
>> >>   }
>> >
>> >
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>> >
>> >
>> >
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