> On Feb 18, 2017, at 2:41 PM, Matthew Johnson via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> There are good reasons a library may not wish to allow users to add
> conformances to a protocol. For example, it may not wish to expose the
> conforming concrete types. While similar behavior could be accomplished with
> an enum if cases could be private, that requires an implementation to use
> switch statements rather than polymorphism.
Uh, what? You can expose protocols without exposing the concrete types that
conform to them. That’s kind of the whole point of protocols.
public protocol P {}
private struct S: P {}
public var p: P
reduces to this interface:
public protocol P {
}
public var p: P
The library can then assign something of type S to the “p” variable, and
clients will be able to use it without having access to its concrete type.
Charles
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