[swift-evolution] [Pitch] Fully eliminate implicit bridging conversions in Swift 3

Vladimir.S svabox at gmail.com
Tue Apr 19 07:23:06 CDT 2016

I fully support this proposal. IMO we should be moving forward and improve 
Swift, separate it from ObjC and explicitly bridge when needed.

On 19.04.2016 6:21, Joe Pamer via swift-evolution 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
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