[swift-evolution] Pitch: really_is and really_as operators

Paul Cantrell cantrell at pobox.com
Mon Sep 12 14:24:08 CDT 2016

> On Aug 29, 2016, at 11:14 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
>> On Aug 24, 2016, at 5:08 PM, Charles Srstka via swift-evolution <swift-evolution at swift.org> wrote:
>> I propose the following operators: really_is, really_as, really_as?, and really_as!. These operators would only return a positive result if the type actually was what was being asked for, instead of something that might be able to bridge to that type.
>> let foo: Any = "Foo"
>> let bar: Any = NSString(string: "Bar")
>> let fooIsString = foo is String                  // true
>> let fooReallyIsString = foo really_is String     // true
>> let fooIsNSString = foo is NSString              // true
>> let fooReallyIsNSString = foo really_is NSString // false
>> let barIsString = bar is String                  // true
>> let barReallyIsString = bar really_is String     // false
>> let barIsNSString = bar is NSString              // true
>> let barReallyIsNSString = bar really_is NSString // true
>> Stick with using an unholy combination of Mirror and unsafeBitCast when you need to know what you’ve actually got.
> It would be helpful to know why you want this. What are you trying to do?

I’ve got one:

Siesta has a notion of “observer ownership,” whose semantics in a nutshell are “free the observer when its owners are all deallocated.” (Details here: http://bustoutsolutions.github.io/siesta/guide/memory/)

Never mind the details. The important thing is that owners must be objects because we care about the fact that they have an allocation lifecycle.

In Swift 2, this declaration ensured that callers used only objects as owners:

	func addObserver(observer: ResourceObserver, owner: AnyObject)

In Swift 3, however, an unsuspecting called can pass a struct as an owner. Swift helpfully wraps it in a _SwiftValue, the _SwiftValue is only weakly referenced so it’s immediately deallocated, and so the observer is immediately discarded. It’s nonsensical. The compiler should be able to detect & prevent this.

What I want is:

	func addObserver(observer: ResourceObserver, owner: AnyObjectForRealNotJustAValueWrapper)

AFAIK, this is impossible in Swift 3. Or is it?

Cheers, P

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