[swift-evolution] [Proposal] Enums with stored properties
Mateusz Malczak
mateusz at malczak.info
Mon Oct 10 08:45:16 CDT 2016
> Maybe I am missing something, but is this what you want?
It a good illustration of what I would like to be able to define with
my proposed feature.
But instead to creating a class/struct to in switch case I would like
enumeration type to be able to carry that information within its cases
For example : https://swiftlang.ng.bluemix.net/#/repl/57fb99c54f9bcf25fdd415f3
> class Size {
> let width: Double
> let height: Double
> init(width: Double, height: Double) { … }
> }
>
> enum Format {
> case SMALL
> case MEDIUM
> case LARGE
> var size: Size {
> switch self {
> case SMALL: return Size(width: 100, height: 100)
> case MEDIUM: …
> ….
> }
> }
> }
>
> let format = Format.SMALL
> let width = format.size.width
>
> format.size.width = 50 // error, immutable
>
>
>
>> On 10 Oct 2016, at 14:35, Jay Abbott via swift-evolution <swift-evolution at swift.org> wrote:
>>
>> Is this what you're trying to achieve, only using a nicer syntax to represent it?
>> http://swiftlang.ng.bluemix.net/#/repl/57fb8ac27365890cc848f831
>>
>>
>> On Mon, 10 Oct 2016 at 13:04 Mateusz Malczak <mateusz at malczak.info> wrote:
>> 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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>> >> > swift-evolution at swift.org
>> >> > https://lists.swift.org/mailman/listinfo/swift-evolution
>> >> >
>> >> >
>> >> >
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