+1 from me; I’ve been dealing with a lot of conversion and yet it’s still pretty confusing largely because of the implicit conversions, it also goes against (pure) Swift’s elegant yet strictly typed checking system.
> On 19 Apr 2016, at 04:21, Joe Pamer via swift-evolution > <[email protected]> wrote: > > Hi everyone, > > Prior to Swift 1.2, conversions between bridged Swift value types and their > associated Objective-C types could be implicitly inferred in both directions. > For example, you could pass an NSString object to a function expecting a > String value, and vice versa. > > In time we found this model to be less than perfect for a variety of reasons: > Allowing implicit conversions between types that lack a subtype relationship > felt wrong in the context of our type system. > Importing Foundation would lead to subtle changes in how seemingly simple > bodies of code were type checked. > The specific rules implemented by the compiler to support implicit bridging > conversions were complex and ad-hoc. > Looking at the Swift code that had been written up until 1.2, these kinds of > implicit conversions did not appear terribly common. (And where they were > present, it wasn’t clear if users actually knew they were taking place.) > > In short, these conversions generally lead to a more confusing and > unpredictable user model. So, for Swift 1.2, we sought to eliminate implicit > bridging conversions entirely, and instead direct users to use explicit > bridging casts in their place. (E.g., “nsStrObj as String”.) > > Unfortunately, when it came time to roll out these changes, we noticed that > some native Objective-C APIs were now more difficult to work with in Swift > 1.2. Specifically, because global Objective-C NSString* constants are > imported into Swift as having type String, APIs that relied on > string-constant lookups into dictionaries imported as [NSObject : AnyObject] > failed to compile. E.g. > > var s : NSAttributedString > let SomeNSFontAttributeName : String // As per the importer. > > let attrs = s.attributesAtIndex(0, effectiveRange:nil) // In Swift 2, ‘attrs’ > has type [NSObject : AnyObject] > let fontName = attrs[SomeNSFontAttributeName] // This will fail to compile > without an implicit conversion from String to NSString. > > For this reason, we decided to make a compromise. We would require explicit > bridging casts when converting from a bridged Objective-C type to its > associated Swift value type (E.g., NSString -> String), but not the other way > around. This would improve the status quo somewhat, and would also avoid > breaking user code in a needless/painful fashion until we could get better > API annotations in place. > > With the introduction of Objective-C generics last year, along with all of > the awesome improvements to API importing happening for Swift 3, I think it’s > time that we take another look at completing this work. Taking a look back at > last year’s “problematic” APIs, all of them now surface richer type > information when imported into Swift 3. As a result, the remaining implicit > bridging conversions now feel far less necessary, since Objective-C APIs are > now more commonly exposed in terms of their appropriate bridged Swift value > types. (For instance, in Swift 3, the above reference to attrs will import as > [String : AnyObject].) > > I propose that we fully eliminate implicit bridging conversions in Swift 3. > This would mean that some users might have to introduce introduce a few more > ‘as’ casts in their code, but we would remove another special case from > Swift's type system and be able to further simplify the compiler. If anyone > is curious and would like to take this model for a spin, I’ve pushed an > experimental branch that implements this proposed change, > inhibit-implicit-conversions. > > Thoughts? > > Thanks! > - Joe > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution
_______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
