[swift-evolution] [proposal] Treat (case .Foo = bar) as a Boolean expression

Chris Lattner clattner at apple.com
Tue May 10 22:31:49 CDT 2016

> On May 10, 2016, at 4:33 AM, Sam Dods via swift-evolution <swift-evolution at swift.org> wrote:
> I propose that (case .Foo = bar) should be treated as an expression with a Boolean value, so the result can be set to a variable or returned from a method.

I agree that this is an important use case that Swift doesn’t serve well right now, but I don’t think this is the right way to go.  

> Considering the following enumeration:
> enum Bar {
>   case foo(name: String)
>   case notFoo
>   case unknownFoo
> }

One of the things we’ve discussed in the past is that we could have enums automatically “synthesize” instance members for projecting cases as optional values or bools.  For example, the above enum could be compiled into the equivalent of:

extension Bar {
   func getAsFoo() -> String? { … }
   var isNotFoo : Bool { … }
   var isUnknownFoo : Bool { … }

Then you could just use:

  if someBar.isUnknownFoo { … }
  if someBar.isFoo != nil { … }
  if let name = someBar. getAsFoo() {...  }
  someBar. getAsFoo()?.doThing() 

etc.  There is a question of naming, and getting the details right, of course.

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