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

[Thorsten Seitz]

Just realized that Matthew did introduce `sealed` exactly to enable this for public types. That's fine with me!

-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
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>>>> >
>>>> >
>>>> >
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>>> 
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