[swift-evolution] Specify type of a delegate which conforms to a protocol

davesweeris at mac.com davesweeris at mac.com
Tue Feb 9 14:00:31 CST 2016

That might solve Inder’s problem, but strictly speaking it doesn’t actually restrict the type to an enum. This struct meets all of RawRepresentable’s requirements:
struct Foo : RawRepresentable {
    typealias RawValue = String
    init?(rawValue: Foo.RawValue) {
        return nil
    var rawValue: RawValue = "bar"

(Although in practice I can’t think of why that would matter, since you can’t do anything with a RawRepresentable other than get it’s rawValue or call init?(rawValue: String), and neither of those rely on enum features… I’ll stop being pedantic now.)

Anyway, the bigger point is that there’s no way to restrict a generic type to be an enum. I think it’s because there wouldn’t be a way to switch on it (or do other enum-ish things) without knowing all its cases, which requires knowing exactly which type it is, which means it’s no longer a generic type. That’s just a guess, though.

Maybe someone should propose that we allow something like this:
func foobar <T, U where T: (case .foo, case .bar)> (value: T) -> U {
    switch value {
    case .foo: ...
    case .bar: ...

- Dave Sweeris

> On Feb 9, 2016, at 10:47, Alex Hoppen via swift-evolution <swift-evolution at swift.org> wrote:
> As for a new syntax, I was also thinking about this issue a few days ago and it occurred to me that there are even more types that cannot be properly represented in Swift. For example there is currently no way (that I know of) to create a variable that can contain any enum that is backed by a String. However, these kinds of constraints can easily be specified in generic constraints (the ones in angle brackets). Maybe we could add the same syntax for variables/constants as well so that the code would look something like the following for your issue:
> var<T, where T: UIViewController, T: MyProtocol> myVar: T
> or for enums backed by a String:
> var<T: RawRepresentable where T.RawValue == String> myVar: T
> Tell me what you think about it.
> - Alex
>> On 09 Feb 2016, at 15:58, Inder Kumar Rathore . via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> Hi All,
>> I used to do this in Obj-C but I'm unable to do this in swift
>> UIViewController<MyProtocol> *delegate;
>> I posted this question on dev forums but didn't get the solution and finally I'm reporting here so that It can be added as a features in the coming releases.
>> Thanks
>> Rathore
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