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

[Maximilian Hünenberger]

I don't see union and intersection types either in the "goals list" or in the "non-goals list". Either way it should be a separate proposal.

- Maximilian

> Am 11.02.2016 um 09:29 schrieb Thorsten Seitz <tseitz42 at icloud.com>:
> 
> 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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