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

[Austin Zheng]

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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> >> swift-evolution at swift.org
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> >
> >
> >
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> >
>
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