Sent from my iPad
> On May 25, 2016, at 12:41 PM, Charlie Monroe <[email protected]> > wrote: > >> Got it. You could also say it is safer because you can't have a supertype >> case "swallow" a subtype value accidentally. An "exact type" cast would >> prevent this possibility. > > This still can be an issue since you still need to do the switch in > init(instance:), but it's just one place within the entire module, so it can > be more easily managed... Yes, agree. That's why your enum is safer. I think we do need an exact type cast to prevent this problem. 'isExaclty' and 'asExactly' seem are a bit verbose but are very clear. I can't think of anything I like that is more concise. > >> >>> >>> enum AnimalSubclasses { >>> >>> case Dog >>> case Cat >>> >>> init(instance: Animal) { >>> switch instance { >>> case is Dog: self = .Dog >>> case is Cat: self = .Cat >>> default: fatalError("Unhandled instance \(instance)!") >>> } >>> >>> } >>> >>>> One thing I have considered that might also be worth introducing is an >>>> exact match cast. This would prevent the possibility of putting a >>>> superclass case first and having it “steal” subclasses which were intended >>>> to be covered by a case later in the switch. If we introduce exact match >>>> you would be able to write a switch that must always cover every concrete >>>> type, including all subclasses. >>>> >>>>> >>>>> Charlie >>>>> >>>>> >>>>>> On May 25, 2016, at 4:41 AM, Leonardo Pessoa via swift-evolution >>>>>> <[email protected]> wrote: >>>>>> >>>>>> Limiting the amount of subclasses is not really a good idea as you would >>>>>> need to introduce another mechanism in the language while the proposed >>>>>> feature requires much less. And you're thinking only about the >>>>>> restrictive set (internal and private) and forgetting the more open end >>>>>> (public). Why is it so bad for this proposal to support requiring the >>>>>> default case? If its possible for the compiler to discover you covered >>>>>> all possible cases it would be fine not having default but IMHO in most >>>>>> cases it will find out there are more not explicitly covered. >>>>>> From: David Sweeris >>>>>> Sent: 24/05/2016 11:01 PM >>>>>> To: Austin Zheng >>>>>> Cc: Leonardo Pessoa; swift-evolution >>>>>> Subject: Re: [swift-evolution] [Pitch] Exhaustive pattern matching >>>>>> forprotocols and classes >>>>>> >>>>>> Or if there was a way to declare that a class/protocol can only have a >>>>>> defined set of subclasses/conforming types. >>>>>> >>>>>> Sent from my iPhone >>>>>> >>>>>>> On May 24, 2016, at 15:35, Austin Zheng via swift-evolution >>>>>>> <[email protected]> wrote: >>>>>>> >>>>>>> If you pattern match on a type that is declared internal or private, it >>>>>>> is impossible for the compiler to not have an exhaustive list of >>>>>>> subclasses that it can check against. >>>>>>> >>>>>>> Austin >>>>>>> >>>>>>>> On Tue, May 24, 2016 at 1:29 PM, Leonardo Pessoa <[email protected]> >>>>>>>> wrote: >>>>>>>> I like this but I think it would be a lot hard to ensure you have all >>>>>>>> subclasses covered. Think of frameworks that could provide many >>>>>>>> unsealed classes. You could also have an object that would have to >>>>>>>> handle a large subtree (NSObject?) and the order in which the cases >>>>>>>> are evaluated would matter just as in exception handling in languages >>>>>>>> such as Java (or require some evaluation from the compiler to raise >>>>>>>> warnings). I'm +1 for this but these should be open-ended like strings >>>>>>>> and require the default case. >>>>>>>> >>>>>>>> On 24 May 2016 at 17:08, Austin Zheng via swift-evolution >>>>>>>> <[email protected]> wrote: >>>>>>>> > I have been hoping for the exhaustive pattern matching feature for a >>>>>>>> > while >>>>>>>> > now, and would love to see a proposal. >>>>>>>> > >>>>>>>> > Austin >>>>>>>> > >>>>>>>> > On Tue, May 24, 2016 at 1:01 PM, Matthew Johnson via swift-evolution >>>>>>>> > <[email protected]> wrote: >>>>>>>> >> >>>>>>>> >> Swift currently requires a default pattern matching clause when you >>>>>>>> >> switch >>>>>>>> >> on an existential or a non-final class even if the protocol or >>>>>>>> >> class is >>>>>>>> >> non-public and all cases are covered. It would be really nice if >>>>>>>> >> the >>>>>>>> >> default clause were not necessary in this case. The compiler has >>>>>>>> >> the >>>>>>>> >> necessary information to prove exhaustiveness. >>>>>>>> >> >>>>>>>> >> Related to this is the idea of introducing something like a `sealed` >>>>>>>> >> modifier that could be applied to public protocols and classes. The >>>>>>>> >> protocol or class would be visible when the module is imported, but >>>>>>>> >> conformances or subclasses outside the declaring module would be >>>>>>>> >> prohibited. >>>>>>>> >> Internal and private protocols and classes would implicitly be >>>>>>>> >> sealed since >>>>>>>> >> they are not visible outside the module. Any protocols that >>>>>>>> >> inherit from a >>>>>>>> >> sealed protocol or classes that inherit from a sealed class would >>>>>>>> >> also be >>>>>>>> >> implicitly sealed (if we didn’t do this the sealing of the >>>>>>>> >> superprotocol / >>>>>>>> >> superclass could be violated by conforming to or inheriting from a >>>>>>>> >> subprotocol / subclass). >>>>>>>> >> >>>>>>>> >> Here are examples that I would like to see be valid: >>>>>>>> >> >>>>>>>> >> protocol P {} >>>>>>>> >> // alternatively public sealed protocol P {} >>>>>>>> >> struct P1: P {} >>>>>>>> >> struct P2: P {} >>>>>>>> >> >>>>>>>> >> func p(p: P) -> Int { >>>>>>>> >> switch p { >>>>>>>> >> case is P1: return 1 // alternatively an `as` cast >>>>>>>> >> case is P2: return 2 // alternatively an `as` cast >>>>>>>> >> } >>>>>>>> >> } >>>>>>>> >> >>>>>>>> >> class C {} >>>>>>>> >> // alternatively public sealed class C {} >>>>>>>> >> class C1: C {} >>>>>>>> >> class C2: C {} >>>>>>>> >> >>>>>>>> >> func c(c: C) -> Int { >>>>>>>> >> switch c { >>>>>>>> >> case is C1: return 1 // alternatively an `as` cast >>>>>>>> >> case is C2: return 2 // alternatively an `as` cast >>>>>>>> >> case is C: return 0 // alternatively an `as` cast >>>>>>>> >> } >>>>>>>> >> } >>>>>>>> >> >>>>>>>> >> I am wondering if this is something the community is interested in. >>>>>>>> >> If >>>>>>>> >> so, I am wondering if this is something that might be possible in >>>>>>>> >> the Swift >>>>>>>> >> 3 timeframe (maybe just for private and internal protocols and >>>>>>>> >> classes) or >>>>>>>> >> if it should wait for Swift 4 (this is likely the case). >>>>>>>> >> >>>>>>>> >> -Matthew >>>>>>>> >> _______________________________________________ >>>>>>>> >> 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 >>>>>>>> > >>>>>>> >>>>>>> _______________________________________________ >>>>>>> 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 >>>>> >>>>> _______________________________________________ >>>>> 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
