[swift-evolution] Evaluating the case of an enum with associated values as a bool
Dave Abrahams
dabrahams at apple.com
Thu Dec 21 09:14:06 CST 2017
IIRC what we discussed was synthesizing members of type Optional<Payload> which could then be checked against nil.
if _ = x.failure { ... }
if x.failure != nil { ... }
if let r = x.success {...}
IMO synthesizing predicates would be a huge missed opportunity by comparison
Sent from my iPhone
> On Dec 20, 2017, at 1:31 PM, Chris Lattner via swift-evolution <swift-evolution at swift.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> 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> 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 <mailto:swift-evolution at swift.org>> 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/>
>> >>
>> >> ---------
>> >>
>> >> 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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