[swift-evolution] [Pitch] Generalized supertype constraints

[David James]

+1

It would be nice to have generic supertype constraints, with syntax along the lines of `where A:B`. 

Not sure if this is the same as what’s being suggested.

Example:
struct ViewWrapper<T:UIView> {
    let views:[T]
    func add<V:UIView>(_ view:V) -> ViewWrapper<T> where V:T { // V must be a subtype of T
        let newViews = views + [view] // pseudo code
        return ViewWrapper(views: newViews)
    }
}
let controls  = ViewWrapper<UIControl>(views:[])
controls.add(UIButton()) // succeeds, UIButton is a UIControl
controls.add(UIView()) // fails, UIView is not a UIControl
David

> On 6 Dec 2017, at 00:34, Rex Fenley via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Huge +1, I've asked for this in the past too.
> 
> Have you also found this limitation frustrating? 
>   - Yes
> 
> In what contexts?
>   - APIs that have this requirement and end up enforcing them through runtime type checking and throws. Shows up in some network data mapping code I have that generalizes over Core Data and Realm (and other databases). The protocol implementer must specify the subtype for the raw mapping of JSON and base type for the DB reading/writing layer. Could see this showing up whenever there's a separation of concerns between what business logic belongs to the base type and subtypes of a more generalized system. I could potentially see the same issue showing up in code generalizing the mapping of data to UI, like UITableView/UITableViewCell.
> 
> Does anyone have reservations about introducing this capability?
>   - I do not
>> One of the most frequent frustrations I encounter when writing generic code in Swift is the requirement that supertype constraints be concrete.  When I mentioned this on Twitter (https://twitter.com/anandabits/status/929958479598534656 <https://twitter.com/anandabits/status/929958479598534656>) Doug Gregor mentioned that this feature is smaller and mostly straightforward to design and implement (https://twitter.com/dgregor79/status/929975472779288576 <https://twitter.com/dgregor79/status/929975472779288576>).
>> 
>> I currently have a PR open to add the high-level description of this feature found below to the generics manifesto (https://github.com/apple/swift/pull/13012 <https://github.com/apple/swift/pull/13012>):
>> 
>> Currently, supertype constraints may only be specified using a concrete class or protocol type.  This prevents us from abstracting over the supertype.
>> 
>> ```swift
>> protocol P {
>>   associatedtype Base
>>   associatedtype Derived: Base
>> }
>> ```
>> 
>> In the above example `Base` may be any type.  `Derived` may be the same as `Base` or may be _any_ subtype of `Base`.  All subtype relationships supported by Swift should be supported in this context including, but not limited to, classes and subclasses, existentials and conforming concrete types or refining existentials, `T?` and  `T`, `((Base) -> Void)` and `((Derived) -> Void)`, etc.
>> 
>> Generalized supertype constraints would be accepted in all syntactic locations where generic constraints are accepted.
>> 
>> I would like to see generalized supertype constraints make it into Swift 5 if possible.  I am not an implementer so I will not be able to bring a proposal forward alone but am interested in collaborating with anyone interested in working on implementation.
>> 
>> I am also interested in hearing general feedback on this feature from the community at large.  Have you also found this limitation frustrating?  In what contexts?  Does anyone have reservations about introducing this capability?  If so, what are they?
>> 
>> Matthew
>> 
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>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
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> -- 
> Rex Fenley  |  IOS DEVELOPER
> 
> 
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David James

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