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
[email protected]
https://lists.swift.org/mailman/listinfo/swift-evolution
