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

[Thorsten Seitz]

Ceylon uses & for intersection types (and | for union types) which I find quite intuitive, meaning a type which conforms to this AND that protocol/type (OR in the case of unions).

It works like expected, i.e.
A & B & A == A & B 

or
typealias A = B & C
A & C == B & C
A & D == B & C & D

class A<T> {
        var x: T & B
}

var a: A<C>
a.x has type C & B

var b: A<B>
b.x has type B

etc.

-Thorsten 


> Am 10.02.2016 um 14:59 schrieb Ross O'Brien via swift-evolution <swift-evolution at swift.org>:
> 
> I agree with everything Brent just said.
> My question would then be: how does this extend?
> 
> Continuing the example:
> 'typealias ViewControllerTableViewDataSource = all<UIViewController, UITableViewDataSource>'
> 
> It should then be possible to have a subtype of this which also conforms to UITableViewDelegate.
> So, given the above, this would work:
> 'typealias ViewControllerTableViewPackage = all<UIViewController, UITableViewDataSource, UITableViewDelegate>'.
> Would this also work?
> 'typealias ViewControllerTableViewPackage = all<ViewControllerTableViewDataSource, UITableViewDelegate>'
> Assuming someone defined this typealias: 'typealias TableViewPackage = protocol<UITableViewDataSource, UITableViewDelegate>', would the above also be equivalent to this?:
> 'typealias ViewControllerTableViewPackage = all<UIViewController, TableViewPackage>'
> 
> The TableViewPackage protocol, defined above, is considered by Swift to be a 'non-nominal type'. Thus this is illegal:
> 'extension TableViewPackage'
> (I don't know why. Perhaps a type conforming to the separate protocols has to opt-in to the combination?)
> 
> However, we can have the following:
> 'extension UITableViewDelegate where Self : UITableViewDataSource'
> 'extension UITableViewDelegate where Self : UIViewController'
> 'extension UITableViewDelegate where Self : UIViewController, Self : UITableViewDataSource'
> 
> Similarly we can't currently have this:
> 'extension UIViewController where Self : UITableViewDataSource, UITableViewDelegate'
> because UIViewController is not a generic type (and this is part of the original complaint).
> 
> It would be nice to be able to write this out as: 'extension all<UIViewController, UITableViewDataSource, UITableViewDelegate>', even if it's just syntactic sugar for 'extension UITableViewDelegate where Self : UIViewController, Self : UITableViewDataSource'.
> 
> -- Ross
> 
> 
>> On Wed, Feb 10, 2016 at 1:44 PM, Maximilian Hünenberger <swift-evolution at swift.org> wrote:
>> In the thread "Partially constrained protocols" we have discussed a similar approach using where clauses:
>> 
>>         protocol<MyProtocol where Self: UIViewController>
>> 
>> - Maximilian
>> 
>> Am 10.02.2016 um 14:00 schrieb Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org>:
>> 
>> >> So, I definitely think there is room for improvement here… how about recycling the inheritance syntax?
>> >>
>> >> let controller: (UIViewController, UITableViewDatasource)
>> >
>> > This declares a tuple containing a UIViewController and a UITableViewDataSource.
>> >
>> >> I added the braces because it would be really when you add the question mark for an optional value; an alternative for this case would be
>> >>
>> >> let controller: Optional<UIViewController, UITableViewDatasource>
>> >
>> > This attempts to declare an optional with two generic types, which doesn't work because Optional only has one type parameter. (But other types, like Dictionary, *do* take two type parameters.)
>> >
>> > Swift does already have a syntax for declaring that a type must conform to two (or more!) protocols:
>> >
>> >    let controller: protocol<UITableViewDataSource, UITableViewDelegate>
>> >
>> > I think this could probably be extended to support one class type as well, perhaps with a new name:
>> >
>> >    let controller: all<UIViewController, UITableViewDataSource>
>> >
>> > --
>> > Brent Royal-Gordon
>> > Architechies
>> >
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