[swift-evolution] Evaluating the case of an enum with associated values as a bool
Chris Lattner
clattner at nondot.org
Wed Dec 20 17:11:22 CST 2017
> On Dec 20, 2017, at 2:12 PM, Ethan Diamond <ethanjdiamond at gmail.com> wrote:
>
> Would that synthesize an isY() even though .Y has an associated value there?
I’m not aware of a concrete design for this idea. The details would definitely need to be figured out, but I don’t see why a double optional is itself a problem.
-Chris
>
> enum E {
> case X
> case Y(Int?)
> }
>
> If I had to run that through getY() -> Int??, it still wouldn't be quite what I was looking for with regards to intent. If you are planning an doing an isY though, that would work for most cases where you're evaluating for a given enum and know what it is beforehand. Even so that wouldn't work for a case, for example, where I'm trying to see if two enums are the same case, and don't necessarily care if they're equal.
>
> let value1 = E.Y(1)
> let value2 = E.Y(2)
>
> value1 == value2 // false
> value1 [is the same case as] value 2 // how do I get this?
>
> This would be useful, say, if I was trying to generate a diff of two arrays of enums, which I occasionally do for table / collection views to figure out inserts/removals/updates.
>
> I don't necessarily know if it's feasible, but it would be really great to have something like a Case metatype, the same way we have type(of: ). It would be great to have a case(of: ) that we can evaluate against the shorthand like we do in switch statements.
>
> Ex:
>
> case(of: value1) == .Y // true
> case(of: value1) == .X // false
> case(of: value1) == case(of: value2) // true
>
>
>
> On Wed, Dec 20, 2017 at 1:31 PM Chris Lattner <clattner at nondot.org <mailto:clattner at nondot.org>> wrote:
> In the past, we’ve discussed synthesizing predicate members onto enums. E.g. given:
>
> enum E {
> case X
> case Y(Int)
> }
>
> you’d get something like:
>
> extension E {
> func isX() -> Bool { return self == .X }
> func getY() -> Int? { … }
> }
>
> which would solve the client side of this nicely.
>
> -Chris
>
>
>
>
>> On Dec 20, 2017, at 11:24 AM, Ethan Diamond via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>
>> Sorry all for attaching the original post to the Non-Exhaustive enums thread. I"m moving it down to it's own thread.
>>
>> My understanding is I'm not allowed to write up a proposal unless I have the time to implement it. Is that still true? This is a major pain point for me to avoid having to write things like this:
>>
>> if case .search = presenter.state { return true } else { return false }
>>
>> Side note: Thanks Kevin, didn't know you could nest enums in switches like that. Super helpful!
>>
>> ------------------------------------------------------
>> I thought I would add another case that isn’t possible with current syntax (so far as I’m aware). You can’t negate the comparison to do something for all cases except a particular case. You have to have an empty if block and use the else block, or have an empty case in a switch statement and use the default.
>>
>> enum Enum {
>> case a(param: String)
>> case b(param: String)
>> case c(param: String)
>> }
>>
>> let enumeration: Enum = .a(param: "Hi")
>>
>> if !(case .a = enumeration) {
>> // Do something
>> }
>>
>> — Charles
>>
>> > On Dec 20, 2017, at 9:55 AM, Kevin Nattinger via swift-evolution <swift-evolution at swift.org <https://lists.swift.org/mailman/listinfo/swift-evolution>> wrote:
>> >
>> > I agree this would be useful. At the moment I have to hack around it with things like `var isFoo: Bool { if case .foo = self …`* with cases I commonly need, but this is definitely a feature that has come up before and I support. It is potentially related to getting the values through an accessor, which has also come up several times.
>> >
>> > Sidenote, your `switch` example is actually trivial with existing syntax:
>> >
>> > switch enumeration {
>> > case .a(.c(let param)): // or just .a(.c) if you don't need the value
>> > print(param)
>> > default:
>> > break
>> > }
>> >
>> > I use this from time to time switching over, e.g., optional enums.
>> >
>> > *: ugliest syntax ever, and it can't even be used as a standalone expression.
>> >
>> >
>> >> On Dec 20, 2017, at 8:44 AM, Ethan Diamond via swift-evolution <swift-evolution at swift.org <https://lists.swift.org/mailman/listinfo/swift-evolution> <mailto:swift-evolution at swift.org <https://lists.swift.org/mailman/listinfo/swift-evolution>>> wrote:
>> >>
>> >> Hello everyone,
>> >>
>> >> One major pain point I've run into with Swift is the inability to evaluate the case of an enum that has associated values in a way that just returns a bool. We've been given the ability in a switch statement:
>> >>
>> >> enum Enum {
>> >> case a(param: String)
>> >> case b(param: String)
>> >> }
>> >>
>> >> let enumeration: Enum = a(param: "Hi")
>> >> switch enumeration {
>> >> case a:
>> >> // Do something
>> >> case b:
>> >> // Do something
>> >> }
>> >>
>> >> We'e been given the ability in the context of an if statement:
>> >>
>> >> enum Enum {
>> >> case a(param: String)
>> >> case b(param: String)
>> >> }
>> >>
>> >> let enumeration: Enum = a(param: "Hi")
>> >>
>> >> if case .a = enumeration {
>> >> // Do something
>> >> }
>> >>
>> >> But without a basic was of getting a bool for if an enum is a given case, here's a list of things I can't do:
>> >>
>> >> Where statements:
>> >>
>> >> enum Enum {
>> >> case a(param: Enum2)
>> >> case b(param: Enum2)
>> >> }
>> >>
>> >> enum Enum2 {
>> >> case c(param: String)
>> >> case d(param: String)
>> >> }
>> >>
>> >> let enumeration: Enum = a(param: "Hi")
>> >> switch enumeration {
>> >> case a(let inner) where [INNER CASE IS .c]
>> >> }
>> >>
>> >> ---------
>> >>
>> >> Filter an array for a certain case:
>> >>
>> >> Expertly explained by Erica Sadun here: http://ericasadun.com/2017/01/31/challenge-filtering-associated-value-enumeration-arrays/ <http://ericasadun.com/2017/01/31/challenge-filtering-associated-value-enumeration-arrays/> <http://ericasadun.com/2017/01/31/challenge-filtering-associated-value-enumeration-arrays/ <http://ericasadun.com/2017/01/31/challenge-filtering-associated-value-enumeration-arrays/>>
>> >>
>> >> ---------
>> >>
>> >> Nicely set a UIButton to hidden if an enum is a certain case:
>> >>
>> >> enum State {
>> >> case `default`
>> >> case searching(results: [Result])
>> >> }
>> >>
>> >> myButton.isHidden = [STATE IS .searching]
>> >>
>> >> ---------
>> >>
>> >> I've run into this issue a ton of times because I tend to represent my views a State enums. I haven't seen anything on the board for plans for solving this issue, thought. Has there been any discussion about addressing it? Ideally I'd be able to do this:
>> >>
>> >> enum Enum {
>> >> case a(param: String)
>> >> case b(param: String)
>> >> }
>> >>
>> >> let enumeration: Enum = a(param: "Hi")
>> >>
>> >> case .a = enumeration // Bool
>> >> case .a(let param) = enumeration // Bool, assigns "Hi" to "param"
>> >>
>> >> Thanks!
>> >> Ethan
>> >>
>> >> _______________________________________________
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>> >
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