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

[David Sweeris]

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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