[swift-evolution] Pitch: really_is and really_as operators

[Xiaodi Wu]

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> 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
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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