[swift-evolution] Evaluating the case of an enum with associated values as a bool

[Chris Lattner]

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 <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
> >> 
> >> _______________________________________________
> >> swift-evolution mailing list
> >> 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>>
> >> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
> > 
> > _______________________________________________
> > swift-evolution mailing list
> > swift-evolution at swift.org <https://lists.swift.org/mailman/listinfo/swift-evolution>
> > https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>_______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20171220/60b7007e/attachment-0001.html>