[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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>> >> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
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>> > 
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