> On May 26, 2016, at 10:13 AM, Thorsten Seitz <tseit...@icloud.com> wrote:
>
>>
>> Am 26.05.2016 um 16:59 schrieb Matthew Johnson <matt...@anandabits.com
>> <mailto:matt...@anandabits.com>>:
>>
>>>
>>> On May 26, 2016, at 9:52 AM, Thorsten Seitz <tseit...@icloud.com
>>> <mailto:tseit...@icloud.com>> wrote:
>>>
>>>>
>>>> Am 26.05.2016 um 15:40 schrieb Matthew Johnson <matt...@anandabits.com
>>>> <mailto:matt...@anandabits.com>>:
>>>>
>>>>
>>>>
>>>> Sent from my iPad
>>>>
>>>> On May 26, 2016, at 8:25 AM, Thorsten Seitz <tseit...@icloud.com
>>>> <mailto: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.
>>>
>>> Why?
>>>
>>> switch a {
>>> case C: …
>>> case D: …
>>> case E: …
>>> }
>>>
>>> is exhaustive because B has to be sealed as well (I would require this to
>>> be declared explicitly).
>>> Abstractness seems not to be necessary for that IMO.
>>> Either you match against B *or* against all its subclasses.
>>
>> In my example `B` is not abstract. `a` might have dynamic type of `B` which
>> would not match any of those cases.
>
> Ah, now I realize why the requirement for abstractness makes sense. Of
> course. Thanks for pointing that out!
>
>
>> You didn’t mention that you have to make all non-leaf classes `abstract` (or
>> at least I didn’t understand that from what you wrote).
>
> The requirement in Ceylon is for classes with enumerated subclasses. B would
> have to declare D and E as enumerated subclasses (like I did for Child2 in
> the Ceylon example). In Ceylon this is not required, B could have normal
> subclasses D and E, but then you could not match against D or E at all
> because these would not be exhaustive (B might have other subclasses). The
> problem therefore is not stemming from disjointness requirements.
> As soon as D and E are declared as disjoint (i.e. if B would be declared
> `sealed`) matching against them would be possible but B would have to be
> abstract.
In my example B is implicitly sealed because it has a sealed ancestor. If it
weren’t you would be able to violate the sealed property of the ancestor by
inheriting from B.
>
> If I remove the enumerated subclasses from Child2 in the example I am allowed
> to make Child2 non-abstract. This of course results in the type error that
> the cases Child1 | Grandchild1 | Grandchild2 are not covering Parent anymore
> and I have to change the cases to match Child1 | Child2, removing the
> grandchildren matches because they are not disjoint with Child2.
Got it. The enumerated subclasses are “partial sealing”. They seal one level
of one branch of the inheritance hierarchy. Interesting. You can get
exhaustive matching without needing to seal the entire hierarchy. I’m not sure
if I like this or not. It is something to consider though, and it is more
flexible.
>
>
>> I don’t like a design that requires non-leaf classes to be abstract. First,
>> it would require introducing abstract classes into Swift, which is a totally
>> separate conversation and something I am not convinced is a good idea.
>
> Well, I think abstract classes are something that is sorely missing in Swift!
> Very sorely missing…
I’ll keep my mind open about it. My concern is that people will gravitate to
the familiar where protocols would be a better design. I haven’t yet missed
abstract classes in Swift.
> But as you say that is a separate conversation and an easy fix would be to
> restrict `sealed` to protocols first.
Why do that? We can have exhaustive switches without requiring abstract.
>
>
>> But it is also overly restrictive. There are valid cases where you might
>> want an exhaustive switch for a sealed hierarchy that has concrete parent
>> classes.
>
> In that case your suggestion of `isExactly` (or something shorter :-) would
> indeed be the solution.
Yep. Any ideas for something more concise?
>
> -Thorsten
>
>
>
>>
>> If you want all non-leaf types to be abstract you should probably consider
>> using protocols in Swift.
>>
>>>
>>> Example in Ceylon:
>>> abstract class Parent() of Child1 | Child2 {}
>>>
>>> class Child1() extends Parent() {}
>>>
>>> abstract class Child2() of Grandchild1 | Grandchild2 extends Parent() {}
>>>
>>> class Grandchild1() extends Child2() {}
>>>
>>> class Grandchild2() extends Child2() {}
>>>
>>> void main() {
>>> Parent foo = Child1();
>>>
>>> switch (foo)
>>> case (is Child1) {
>>> print("Child1");
>>> }
>>> case (is Grandchild1) {
>>> print("Grandchild1");
>>> }
>>> case (is Grandchild2) {
>>> print("Grandchild2");
>>> }
>>> }
>>>
>>> -Thorsten
>>>
>>>
>>>>
>>>> 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 <mailto:swift-evolution@swift.org>>:
>>>>>>
>>>>>>
>>>>>>
>>>>>> Sent from my iPad
>>>>>>
>>>>>> On May 25, 2016, at 12:41 PM, Charlie Monroe <char...@charliemonroe.net
>>>>>> <mailto: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 <mailto: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 <mailto:daveswee...@mac.com>
>>>>>>>>>>>> Sent: 24/05/2016 11:01 PM
>>>>>>>>>>>> To: Austin Zheng <mailto:austinzh...@gmail.com>
>>>>>>>>>>>> Cc: Leonardo Pessoa <mailto:m...@lmpessoa.com>; swift-evolution
>>>>>>>>>>>> <mailto:swift-evolution@swift.org>
>>>>>>>>>>>> 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 <mailto: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 <mailto: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 <mailto: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 <mailto: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 <mailto:swift-evolution@swift.org>
>>>>>>>>>>>>> >> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>>>>>>>> >> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>>>>>>>>>> >
>>>>>>>>>>>>> >
>>>>>>>>>>>>> >
>>>>>>>>>>>>> > _______________________________________________
>>>>>>>>>>>>> > swift-evolution mailing list
>>>>>>>>>>>>> > swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>>>>>>>>> > https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>>>>>>>> > <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>>>>>>>>>> >
>>>>>>>>>>>>>
>>>>>>>>>>>>> _______________________________________________
>>>>>>>>>>>>> swift-evolution mailing list
>>>>>>>>>>>>> swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>>>>>>>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>>>>>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
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>>>>>>>>>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>>>>>>>>
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>>>>>>>>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>>>>
>>>>>> _______________________________________________
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