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

[Ethan Diamond]

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
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