[swift-evolution] Type information containing multiple additional protocols

[Jordan Rose]

At one point we talked about co-opting "Any" for this purpose, i.e. replacing the "protocol<…>" syntax with "Any<…>". Like a concrete type, you're not required to provide arguments, so "Any" by itself doesn't change meaning. Like an inheritance clause, the compiler would check if you ever had more than one class there.

Jordan

> On Dec 4, 2015, at 23:38 , Jacob Bandes-Storch <jtbandes at gmail.com> wrote:
> 
> I think this is a very useful feature of Obj-C that I've missed several times while writing Swift code.
> 
> However, calling it "TheClass<Protocol1, Protocol2>" directly conflicts with the syntax for specifying generic type parameters.
> 
> If we use "protocol<TheClass, Protocol1, Protocol2>" for this case, then `protocol` stops making sense. Maybe the keyword `class` could be allowed instead, like "class<TheClass, Protocol1, Protocol2>".
> 
> Or, it might work to omit the keyword entirely, leaving only the angle brackets:
> 
> class C {
>     weak var delegate: <UIViewController, UICollectionViewDelegate>?
> }
> 
> if let task = object as? <NSManagedObject, Task> {
>     task.coreDataRelatedFunction()
>     task.taskRelatedFunction()
> }
> 
> Jacob Bandes-Storch
> 
> On Fri, Dec 4, 2015 at 3:07 PM, Daniel Muhra <daniel.muhra at gmail.com <mailto:daniel.muhra at gmail.com>> wrote:
> Currently it doesn’t seem possible to define a property or variable of a specific class that also implements one or more protocols.
> Take as an example the following definition from Objective C:
> 
> NSManagedObject<NSCoding, Task>* someObject;
> 
> To my knowledge, currently it is only possible to define multiple protocols only:
> 
> var object : protocol<NSCoding, Task>
> 
> However, given the first example, sometimes it is not possible to restrict to protocols only. This becomes especially cumbersome, if you would need to cast down to such a construct within a single method and perform several steps which would require all those type information:
> 
> func doSomething(object: AnyObject) {
>     if let managedObject = object as? NSManagedObject {
>         // Do core data related stuff
>     }
>     
>     if let task = object as? Task {
>         // Do task related stuff
>     }  
> }
> 
> In this case you now have managedObject and task though you operate on the same object. Also it is not (easily) possible to interleave actions of those two.
> 
> My first idea here would be to introduce an Objective C like style of type naming. This also has the benefit, that it is perfectly clear, whether a type name refers to a class or a protocol (without first having to check it).
> The downside, of course, is that protocol type variables would become something like Any<MyProtocol>
> 
> Any other ideas are welcome, but the initial problem still stands and should be addressed.
> 
> 
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