[swift-evolution] [Pitch] Reducing the bridging magic in dynamic casts

Joe Groff jgroff at apple.com
Mon May 9 14:38:22 CDT 2016

> On May 6, 2016, at 12:04 AM, Adrian Zubarev via swift-evolution <swift-evolution at swift.org> wrote:
> Definitely a welcome change from me (+1). But this proposal makes me curious about the impact on the `AnyObject` protocol?
> let string = "foo"
> let nsString = string as AnyObject
> nsString.dynamicType // _NSCFConstantString.Type
> NSString().dynamicType // __NSCFConstantString.Type // there are two different types? 
> This sample won’t bridge anymore if SE-0083 will be accepted.

Right, you'd need to do NSString(string) as AnyObject to explicitly bridge.

> Can we also drop the @objc from `AnyObject` protocol and leave it as an implicit protocol for classes? (Personally I’d rename `AnyObject` to `AnyReference` if Swift will introduce other reference types.)

The @objc-ness of AnyObject is more or less an implementation detail. On Darwin platforms at least, AnyObject still has the magic ability to dispatch to all @objc methods, similar to `id` in Objective-C, which vaguely defends its @objc-ness. (If we're going to rename it, my own preference would be to drop the Any and just call it `Object`, since we don't put Any in any other protocol names.)

> This change might allow the replacement of the `class` keyword from protocols with the implicit `AnyObject` protocol, which can be discussed in this thread: Should we rename "class" when referring to	protocol conformance?
> One more thing I’d like to ask: is there any possibility of adding a new `bridge` keyword, which would allow explicit bridging to a different language type (ObjC, etc. if there are any more languages we can bridge to [C or maybe one day C++])?
> T `bridge` U
> T? `bridge` U?

One could write `bridge` as a method in most cases; it doesn't need to be a keyword with special syntax, since you could write `T.bridge(U.self)` (or, if we accept https://github.com/apple/swift-evolution/pull/299, `T.bridge(U)`). Idiomatically, though, we generally use initializers for value-preserving conversions, so U(T) would be more consistent with the rest of the standard library.


> The ugly NSError pattern could be rewritten and migrated to:
> do {
>    try something()
> } catch let error {
>    handle(error `bridge` NSError)
> }
> Is such a change complicated, what do you think?
> -- 
> Adrian Zubarev
> Sent with Airmail
> Am 4. Mai 2016 bei 01:50:54, Joe Groff via swift-evolution (swift-evolution at swift.org) schrieb:
>> Thanks everyone for the initial round of feedback. I've submitted a draft proposal:
>> https://github.com/apple/swift-evolution/pull/289
>> https://github.com/jckarter/swift-evolution/blob/remove-bridging-conversion-dynamic-casts/proposals/XXXX-remove-bridging-from-dynamic-casts.md
>> -Joe
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