We had similar talks about that topic before. There is a thread somewhere on
this, but it’s mainly focused on value type semantics.
If I remember correctly we might even discussed to remove the class constraint
completely from the language and introduce a protocol solution to this problem.
There are also some words about the class constraint in the GenericsManifesto.md
Generalized class constraints
The class constraint can currently only be used for defining protocols. We
could generalize it to associated type and type parameter declarations, e.g.,
protocol P {
associatedtype A : class
}
func foo<T : class>(t: T) { }
As part of this, the magical AnyObject protocol could be replaced with an
existential with a class bound, so that it becomes a typealias:
typealias AnyObject = protocol<class>
See the “Existentials” section, particularly “Generalized existentials”, for
more information.
Instead of : class we might use : AnyObject as a constraint (or :
AnyReference?).
Instead of : struct/enum we should have something like : value or even better :
AnyValue.
I believe we should focus on existential types first before fixing this gap in
Swift. Any<T> might receive the mentioned constraints and could create
something like this:
typealias AnyObject = Any<class>
typealias AnyValue = Any<value>
--
Adrian Zubarev
Sent with Airmail
Am 21. Oktober 2016 um 08:55:40, Xiaodi Wu via swift-evolution
([email protected]) schrieb:
What's your use case for distinguishing structs and enums?
On Fri, Oct 21, 2016 at 1:40 AM Mike Kasianowicz via swift-evolution
<[email protected]> wrote:
Currently protocols can have the class constraint:
protocol MyProtocol : class {}
It would be (a) intuitive and (b) useful to allow such things as:
protocol Model : struct {} or protocol Event : enum {}
These types of restrictions can help prevent accidental anti-patterns or misuse
of APIs.
Seems simple and non-controversial... right?
[Note: I'd like to see even more heavy-handed protocol restrictions in the
future. For example, a protocol describing an enum with a common case, or a
struct with no reference members. Great stuff for defensively coding APIs.]
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