[swift-evolution] Pitch: really_is and really_as operators

[Matthew Johnson]

> On Aug 24, 2016, at 9:09 PM, Robert Widmann via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I have 3 qualms with this proposal as it stands:
> 
> - type(of:) will never lie to you. 
> 
> The only question it won’t answer to your satisfaction is the dynamic type of the NSString you boxed up as an Any.
> 
> - No more keywords without significant justification.  
> 
> I don’t buy the performance use case at all - if you were properly concerned about performance you would try to use as many of Swift’s static features as possible.
> 
> - Especially no more keywords that look like they belong in Rust or PHP!
> 
> There is no precedent for the spelling of these operations other than the suffixed punctuation. Given that they’re domain-specific, will definitely be hard to use (given that NSString(string: "Bar”) may not “really” given you an NSString yet that’s what you asked us to check for “really"), and will be obviated by the implementation of SE-0083, I can’t see a reason why we need any of this in the language proper.

One related topic to consider is exhaustive pattern matching for classes.  Now that SE-0117 has been accepted it will be possible to do this for many classes (I would say most if it weren’t for Objective-C classes being so common in Swift and are imported as `open`).  Supporting exhaustive pattern matching well would require some kind of syntax for matching the runtime type exactly.  I have imagined this as being “exact match” cast operators, which is what the `really_*` operators are.  Do you have an alternative in mind for exhaustive pattern matching if we do not introduce exact match cast operators?

> 
> ~Robert Widmann
> 
>> On Aug 24, 2016, at 5:08 PM, Charles Srstka via swift-evolution <swift-evolution at swift.org <mailto: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
>> 
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