> On Nov 7, 2016, at 11:23 AM, Alexis via swift-dev <swift-dev@swift.org> wrote: > > >> On Nov 4, 2016, at 11:55 PM, Dave Abrahams via swift-dev >> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote: >> >> >> on Fri Nov 04 2016, Slava Pestov <swift-dev-AT-swift.org >> <http://swift-dev-at-swift.org/>> wrote: >> >>> If the casts are always in one direction, can you make one protocol >>> refine another? >> >> Yeah, I am shocked if they don't do that already. > > They do; _NSFoo: _NSFooCore > > But the problem is that we have _NSFooCores and want _NSFoos.
I am confused. Then why can't you just do an unconditional checked down cast to _NSFoo from _NSFooCore? That can definitely be expressed at the SIL level? > >> >>> Also note that @objc protocols are self-conforming as long as they >>> don’t contain initializers or static methods, but I’m not sure if that >>> helps. >> >> Doesn't; we're not using these in a generic context; they're just >> existentials. >> >>> >>> >>>> On Nov 4, 2016, at 4:29 PM, Alexis via swift-dev <swift-dev@swift.org >>>> <mailto:swift-dev@swift.org>> wrote: >>>> >>>> The swift standard library has this nasty little pattern/problem in it: >>>> >>>> The types in the core library want to know about several types >>>> defined in foundation: NSString, NSArray, NSDictionary, etc. But >>>> core is imported by Foundation, so it can’t (circular references >>>> between modules). Thankfully, everything in ObjC is pretty opaquely >>>> defined and uniform, and Swift knows how to hook into that uniform >>>> layout. So the core library defines Shadow Protocols which provide >>>> whatever subset of that type’s API is considered interesting. These >>>> protocols are then used in place of the ObjC types. There’s also >>>> some magic compiler hooks so core lib types can subclass those >>>> foundation types. >>>> >>>> However there’s sometimes two Shadow Protocols: one that defines the >>>> APIs the stdlib should provide (_NSFooCore), and one that extends >>>> that with extra APIs the stdlib wants to consume (_NSFoo). This >>>> leads to an awkward situation: as far as the runtime is concerned, >>>> the stdlib’s _NSFooCores don’t conform to _NSFoo! We know they do >>>> because it’s all just a big lie to hook into ObjC message passing >>>> with a bit of type safety, but the runtime doesn’t. So if you try to >>>> do a safe type cast, it will fail. This leads to a situation where >>>> we sprinkle code with unsafeBitCast’s to _NSFoo which is a massive >>>> refactoring hazard. >>>> >>>> For instance, there was a struct-containing-a-class that was being >>>> cast to _NSFoo in HashedCollections. This happened to work (but was >>>> probably still a violation of strict aliasing?) because the struct’s >>>> only field was the class. However the struct was later changed to a >>>> class, which silently made the cast completely incorrect, banishing >>>> the real _NSFoo to the shadow (protocol) realm. >>>> >>>> Can we do anything better here? Note that there’s a few places where >>>> we also cast an AnyObject into an _NSFoo, but there’s some chance >>>> this is all legacy junk that can be updated to at least use >>>> _NSFooCore, if not _NSFoo itself. >>>> _______________________________________________ >>>> swift-dev mailing list >>>> swift-dev@swift.org <mailto:swift-dev@swift.org> >>>> https://lists.swift.org/mailman/listinfo/swift-dev >>> >>> _______________________________________________ >>> swift-dev mailing list >>> swift-dev@swift.org <mailto:swift-dev@swift.org> >>> https://lists.swift.org/mailman/listinfo/swift-dev >>> <https://lists.swift.org/mailman/listinfo/swift-dev> >>> >> >> -- >> -Dave >> >> _______________________________________________ >> swift-dev mailing list >> swift-dev@swift.org <mailto:swift-dev@swift.org> >> https://lists.swift.org/mailman/listinfo/swift-dev >> <https://lists.swift.org/mailman/listinfo/swift-dev> > _______________________________________________ > swift-dev mailing list > swift-dev@swift.org > https://lists.swift.org/mailman/listinfo/swift-dev
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