[swift-evolution] Discussion: Enforce usage of @available on enum cases

[Stuart Breckenridge]

Taking the long form approach:

enum Things:String {
    @available (iOS 9.0, tvOS 9.0, watchOS 2.0, *)
    @available (OSX, unavailable, message="Not available on OS X.")
    case MobileThing = "com.example.mobile"
    
    @available (iOS, unavailable, message="Not available on iOS.")
    @available (tvOS, unavailable, message="Not available on tvOS.")
    @available (watchOS, unavailable, message="Not available on watchOS.")
    @available (OSX 10.11, *)
    case DesktopThing = "com.example.desktop"
}

This yields the expected result in a tvOS app:

func that(thing:Things) -> () {
        print(thing)
}
that(.DesktopThing) // 'DesktopThing' is unavailable. Not available on tvOS.

A limited availability attribute does have some merit, perhaps using a different keyword (so you know you are choosing it), for example:

    	@restricted (OSX 10.11, *, message="Only available on OS X.")
    	case DesktopThing = "com.example.desktop"
	
	func that(thing:Things) -> () {
        	print(thing)
	}
	that(.DesktopThing) // 'DesktopThing' is unavailable. Only available on OS X.


...but this is outside the scope of the original topic.

Stuart


> On 26 May 2016, at 11:23, Jordan Rose <jordan_rose at apple.com> wrote:
> 
> This was deliberate, and came from the pain of bringing up tvOS (and watchOS). Clang actually had a hack to treat “available on iOS” as “available on tvOS” for a while because of this. When you port something to a new platform, presumably everything that’s been introduced so far should be available from the start.
> 
> If we introduced a limited availability attribute, I think we’d want to make it more verbose than the normal one, so that you know you’re choosing it.
> 
> Jordan
> 
> 
>> On May 25, 2016, at 20:18, Jacob Bandes-Storch <jtbandes at gmail.com <mailto:jtbandes at gmail.com>> wrote:
>> 
>> Was this an intentional choice, or is it just a temporary limitation? Would a proposal to allow something like "@available(OS X 10.11)" be welcomed?
>> 
>> 
>> On Wed, May 25, 2016 at 8:17 PM, Jordan Rose via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> The “*” in @available means “all other platforms”. If you want to mark something unavailable on a particular platform, you’ll have to use the long form of the @available attribute. There’s currently no way to mark something “only available on these platforms”.
>> 
>> Jordan
>> 
>>> On May 25, 2016, at 18:48, Stuart Breckenridge via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> Presently, you can mark enum cases as @available but it has no impact. Take the following example:
>>> 
>>> enum Things:String {
>>> 	@available (iOS 9.3, *) case MobileThing = "com.example.mobilething"
>>> 	@available (OSX 10.11, *) case DesktopThing = "com.example.desktopthing"
>>> }
>>> 
>>> Despite the above compiling, the following code will run iOS with no warnings:
>>> 
>>> func takesA(thing:Things) {
>>> 	print(thing)
>>> }
>>> 
>>> takesA(.DesktopThing)
>>> 
>>> 
>>> Are there enough compelling use-cases for @available on enum cases to be considered? At the very least, if @available doesn't do anything on cases, it shouldn't be permitted.
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>> 
>> 
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>> 
> 

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