[swift-evolution] Pitch: really_is and really_as operators

[Jaden Geller]

Though this performs an *extra* check over just `x as! U`, it avoids bridging conversions. I don't think this extra check matters though because you wouldn't be using `Any` in code that's performance critical enough for this to matter, so I'm currently -1 on this proposal.

> On Aug 24, 2016, at 6:09 PM, Jaden Geller <jaden.geller at gmail.com> wrote:
> 
> Scratch that last message. Determining what's stored in `Any` was Charles's original goal, and what I said made no sense. This should work:
> 
> func unbridgedCast<T, U>(_ x: T, to: U.Type) -> U? {
>     guard type(of: x) is U.Type else { return nil }
>     return x as! U
> }
> 
> if let x = unbridgedCast(x, to: String.self) { ... }
> 
>> On Aug 24, 2016, at 6:06 PM, Jaden Geller <jaden.geller at gmail.com <mailto:jaden.geller at gmail.com>> wrote:
>> 
>> Actually, the code I proposed does not work. First of all, it doesn't work for subtype relations. This can easily be fixed though by changing `== U.self` to `is U`. More importantly though, it fails when `T` is an existential since `type(of:)` looks within the existential to determine the type. It would be useful if there were a way to determine the *actual* runtime representation of the value.
>> 
>>> On Aug 24, 2016, at 6:00 PM, Jaden Geller <jaden.geller at gmail.com <mailto:jaden.geller at gmail.com>> wrote:
>>> 
>>> Taking that suggestion a step further, it's pretty easy to define a function that performs this sort of casting without bridging.
>>> 
>>> func unbridgedCast<T, U>(_ x: T, to: U.Type) -> U? {
>>>     guard type(of: x) == U.self else { return nil }
>>>     return unsafeBitCast(x, to: U.self)
>>> }
>>> 
>>>> On Aug 24, 2016, at 5:42 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> import Foundation
>>>> 
>>>> let foo: Any = "Hello"
>>>> type(of: foo) == String.self // true
>>>> type(of: foo) == NSString.self // false
>>>> 
>>>> let bar: Any = "Hello" as NSString
>>>> type(of: bar) == String.self // false
>>>> type(of: bar) == NSString.self // true
>>>> 
>>>> Why not this?
>>>> 
>>>> 
>>>> On Wed, Aug 24, 2016 at 19:09 Charles Srstka via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> MOTIVATION:
>>>> 
>>>> SE-0083 appears to be dead in the water, having been deferred until later in Swift 3 back in May and not having been heard from since then, with the Swift 3 release looming closer and closer. However, the predictability gains that would have been provided by this change remain desirable for cases where one needs to know the actual dynamic type of an entity before any bridging magic is involved. Additionally, performance-critical code may desire the ability to check something’s type quickly without incurring the overhead of Objective-C bridging code.
>>>> 
>>>> PROPOSED SOLUTION:
>>>> 
>>>> 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.
>>>> 
>>>> DETAILED DESIGN:
>>>> 
>>>> 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
>>>> 
>>>> ALTERNATIVES CONSIDERED:
>>>> 
>>>> Stick with using an unholy combination of Mirror and unsafeBitCast when you need to know what you’ve actually got.
>>>> 
>>>> Charles
>>>> 
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>> 
>> 
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160824/5fd7e13f/attachment.html>