[swift-evolution] [Pitch] Exhaustive pattern matching for protocols and classes

[Matthew Johnson]

Sent from my iPad

> On May 25, 2016, at 11:18 AM, Thorsten Seitz via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Just realized that Matthew did introduce `sealed` exactly to enable this for public types. That's fine with me!

Yeah, and it doesn't require repeating the subclass all in one place which I think is a better fit for Swift.

I'm thinking the "exact type" cast (not in my original post) should also be a part of the solution.  What do you think of that?

> 
> -Thorsten 
> 
>> Am 25.05.2016 um 18:11 schrieb Thorsten Seitz via swift-evolution <swift-evolution at swift.org>:
>> 
>> Ceylon uses the following syntax for stating that a class has a finite set of subclasses:
>> 
>> class C of C1 | C2 {...}
>> 
>> where `|` is the type union operator. Swift could use a simple comma separated list instead after the `or`. The advantage over sealed+private/internal would be thatnthe class or protocol could be public as well.
>> 
>> -Thorsten 
>> 
>>> Am 25.05.2016 um 04:01 schrieb David Sweeris via swift-evolution <swift-evolution at swift.org>:
>>> 
>>> 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> 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> 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> 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> 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 at swift.org
>>>>> >> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>> >
>>>>> >
>>>>> >
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>>>>> >
>>>> 
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