Sent from my iPad
> On May 26, 2016, at 8:25 AM, Thorsten Seitz <tseit...@icloud.com> wrote:
>
> Ceylon requires checks whether cases are disjoint, i.e. when one case
> contains a superclass of another case then this will be a type error „cases
> are not disjoint“.
>
> FWIW: Ceylon requires classes with enumerated subclasses to be abstract.
Interesting, thanks for mentioning this. The abstract requirement is what
makes disjointedness at least partly possible (what if a subclass has further
descendants though?). But it still only works for a single level of
inheritance:
sealed abstract class A {}
class B : A {}
class C : A {}
class D : B {}
class E : B {}
With a disjoint requirement I cannot ever match D and E because that would not
be exhaustive and I am prohibited from matching them along side B which would
be exhaustive but isn't disjoint.
I don't think that solution is appropriate to Swift.
>
> -Thorsten
>
>
>> Am 25.05.2016 um 19:49 schrieb Matthew Johnson via swift-evolution
>> <swift-evolution@swift.org>:
>>
>>
>>
>> Sent from my iPad
>>
>>> On May 25, 2016, at 12:41 PM, Charlie Monroe <char...@charliemonroe.net>
>>> 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
>>>>>>>> <swift-evolution@swift.org> 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
>>>>>>>>> <swift-evolution@swift.org> 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 <m...@lmpessoa.com>
>>>>>>>>>> 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
>>>>>>>>>> <swift-evolution@swift.org> 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
>>>>>>>>>> > <swift-evolution@swift.org> 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
>>>>>>>>>> >> swift-evolution@swift.org
>>>>>>>>>> >> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>>>>> >
>>>>>>>>>> >
>>>>>>>>>> >
>>>>>>>>>> > _______________________________________________
>>>>>>>>>> > swift-evolution mailing list
>>>>>>>>>> > swift-evolution@swift.org
>>>>>>>>>> > https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>>>>> >
>>>>>>>>>
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