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

[Djura Retired Hunter]

Many styles of programming can take advantage of if/else and switch/case being actual expressions (actually, these are all special cases of the very general concept of "folding"). We don't have this in Swift, and I have occasion to be bothered by this almost on a daily basis in my work, especially when I try to be "concise", something for which Swift should be champion.

The best possible outcome would be for Swift to have pattern matching as a proper evaluated expression, but I would accept a new operator that takes advantage of some custom compiler magic, because the use cases are so many and the convenience would be so great.

As Colin Barret said, this is a very popular thing in many modern languages.


Elviro

> Il giorno 20 dic 2017, alle ore 19:32, Colin Barrett via swift-evolution <swift-evolution at swift.org> ha scritto:
> 
> This would be easily solved if pattern matching was available as an expression, such as in Haskell, OCaml / Standard ML, and Scala / Kotlin. :-)
> 
>> On Dec 20, 2017, at 11:44 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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