> On Nov 4, 2016, at 11:55 PM, Dave Abrahams via swift-dev > <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. > >> 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> >>> 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 >>> 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>
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