[swift-evolution] [Pitch] Exhaustive pattern matching forprotocols and classes

Wed May 25 12:41:05 CDT 2016

> 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...

> 
>> 
>> 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 at swift.org <mailto:swift-evolution at 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:davesweeris at mac.com>
>>>>> Sent: ‎24/‎05/‎2016 11:01 PM
>>>>> To: Austin Zheng <mailto:austinzheng at gmail.com>
>>>>> Cc: Leonardo Pessoa <mailto:me at lmpessoa.com>; swift-evolution <mailto:swift-evolution at 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 at swift.org <mailto:swift-evolution at 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 <me at lmpessoa.com <mailto:me at 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 at swift.org <mailto:swift-evolution at 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 at swift.org <mailto:swift-evolution at 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
>>>>>> >> _______________________________________________
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>>>>>> >> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>>> >> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>>> >
>>>>>> >
>>>>>> >
>>>>>> > _______________________________________________
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>>>>>> >
>>>>>> 
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