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

[Kevin Nattinger]

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> 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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