[swift-evolution] [Draft] open and public protocols

[Adrian Zubarev]

Protocols solve way more problems than this. Sometimes they are used as backdoors to existing APIs as well. It’s simply an implementation artifact that such a protocol might end up public, but there is no point to allow the client to conform to it.

I also showed a concrete example in the discussion thread where I needed a public-but-not-open protocol really badly. ;)



-- 
Adrian Zubarev
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Am 18. Februar 2017 um 22:45:00, Charles Srstka via swift-evolution (swift-evolution at swift.org) schrieb:

On Feb 18, 2017, at 2:41 PM, Matthew Johnson via swift-evolution <swift-evolution at 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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