[swift-evolution] [swift-evolution-announce] [Review] SE-0083: Remove bridging conversion behavior from dynamic casts

[Douglas Gregor]

> On May 23, 2016, at 5:26 PM, Jordan Rose via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I am way late, but I share Brent’s concerns. I don’t think this addresses the very common case of “getting a String out of a heterogeneous dictionary”.
> 
> let name = plist[“name”] as! String
> 
> becomes one of these:
> 
> let name = plist[“name”] as! NSString as String
> let name = String(plist[“name”] as! NSString)
> let name = String(forceBridging: plist[“name”]) // not in the proposal
> 
> none of which I’m particularly happy with. 

I am also way, way late, here, but this ties into a philosophical concern I have. The bridging that we have in place was designed to put Swift’s value types front-and-center in the Swift experience, even when interoperating with Objective-C APIs using reference-semantic types. It was a specific goal that one should not have to juggle between Swift.Array and NSArray—NSArray is bridged away in imported APIs, Swift arrays implicitly convert to AnyObject when working an AnyObject-based API, dynamic bridging conversions would pass through NSArray to get to Swift arrays, etc. So while one can certainly reach for NS(Mutable)Array in Swift, one should not *have* to do so in Swift.

This proposal and SE-0072 are chipping away at that bridging story, making the explicit use of NSArray/NSString/etc. required for interoperability with Objective-C APIs. While it does make the language more explicit and predictable (and dynamic casting more efficient!), it makes the use of these bridged reference-semantic more prevalent, which may lead to more overall confusion about which set of types to use. There might even be a portability argument: the current scheme lets you gloss over Any vs. AnyObject (which is a current difference we see in ObjC Foundation vs. corelibs Foundation).

	- Doug

> 
> Jordan
> 
> 
>> On May 19, 2016, at 02:31, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> 	* What is your evaluation of the proposal?
>> 
>> The review is technically over, but I don't believe a decision has been announced yet, so...
>> 
>> I am generally in favor, but I have a serious concern about the readability of certain conversions with this change. To wit, conversions like these:
>> 
>> 	myArray as! [NSView]
>> 	myDictionary as! [String: NSView]
>> 
>> Are about to become something more like these:
>> 
>> 	[NSView](forcedLazyBridging: myArray)
>> 	[String: NSView](forcedLazyBridging: myDictionary)
>> 	
>> Or these:
>> 
>> 	Array<NSView>(forcedLazyBridging: myArray)
>> 	Dictionary<String, NSView>(forcedLazyBridging: myDictionary)
>> 
>> Either option is a significant regression in code readability compared to the status quo.
>> 
>> It's enough to make me wonder if we shouldn't have special-cased conversion methods for NSArray, NSDictionary, and NSSet:
>> 
>> 	myArray.of(NSView)				// returns [NSView]
>> 	myDictionary.of(NSView, for: String) 	// returns [String: NSView]
>> 	mySet.of(NSView)					// returns Set<NSView>
>> 
>> On the other hand, if you *don't* have to specify an element type, these aren't so bad:
>> 
>> 	Array(forcedLazyBridging: myArray)
>> 	Dictionary(forcedLazyBridging: myDictionary)
>> 
>> And it gets even better if you use something a little saner than `forcedLazyBridging` for the label.
>> 
>>> 	* Is the problem being addressed significant enough to warrant a change to Swift?
>> 
>> Yes. Conversions are a mess, and it'll be nice to clean them up.
>> 
>>> 	* Does this proposal fit well with the feel and direction of Swift?
>> 
>> Yes.
>> 
>>> 	* If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?
>> 
>> Most languages I've used have had much simpler cast systems with nothing particularly close to Swift's bridging casts.
>> 
>>> 	* How much effort did you put into your review? A glance, a quick reading, or an in-depth study?
>> 
>> Quick reading.
>> 
>> -- 
>> Brent Royal-Gordon
>> Architechies
>> 
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