[swift-evolution] Swift Generic Subtype Problem

[Cao Jiannan]

Also:

union(A,B,C) is subtype of union(A,B,C,D,…)

> 在 2016年2月16日，14:36，Cao Jiannan <frogcjn at 163.com> 写道：
> 
> Hi all,
> 
> I think the best way to solve the either problem is to separate it from generic. 
> Optional and Either shouldn’t work the same way of generic type. 
> It’s just a represent of multiple type in one location.
> 
> Using an old friend, Union in C.
> union {
> 	case firstType
> 	case secondType
> }
> 
> This is the final solution for the sub typing problem of optional.
> 
> 
> A  == union(A,A)
> union(A,B) == union(B,A)
> B == union(B,B)
> 
> B is subtype of union(A,B)
> A is subtype of union(A,B)
> 
> suppose 
> a is subclass of A
> b is subclass of B, then
> 	union(a,B) is subtype of union(A,B)
> 	union(A,b) is subtype of union(A,B)
> 	union(a,b) is subtype of union(a,B)
> 	union(a,b) is subtype of union(A,b)
> 
> union can have as many case as possible. e.g., union(A,B,C,D,…)
> 
> So the Optional<UITableView> should be union(UITableView, None)
> and Optional<MyTableVIew> should be union(MyTableView, None), which is subclass of union(UITableView, None)
> 
> This is a final rational solution. I think.
> 
> -Jiannan
> 
>> 下面是被转发的邮件：
>> 
>> 发件人: Cao Jiannan via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
>> 主题: [swift-evolution] Swift Generic Subtype Problem
>> 日期: 2016年2月16日 GMT+8 11:48:18
>> 收件人: swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>> 回复－收件人: Cao Jiannan <frogcjn at 163.com <mailto:frogcjn at 163.com>>
>> 
>> 
>> Hi all,
>> I want to discuss on a problem  about optional generic sub-typing.
>> 
>> This is my suggesion.
>> 
>> if B is subclass of A
>> Either<B,B> is subclass of Either<A,A>, Either<A,B>, Either<B,A>
>> Either<B,A> is subclass of Either<A,A>
>> Either<A,B> is subclass of Either<A,A>
>> 
>> 
>> Why? Let’s see an example code in a real project:
>> 
>> 
>> 
>> Here is a protocol type for some UIViewController,
>> protocol  SegueHandlerType {  
>>     var tableView: UITableView! { get }  
>> }  
>> 
>> so the UITableViewController can conform to the protocal
>> extension UITableViewController : SegueHandlerType {  
>> }  
>> 
>> It's great!
>> What if the tableView is a subclass UITableView?
>> like:
>> class MyTableView : UITableView {
>> }
>> 
>> MyTableViewController {  
>>       @IBOutlet var tableView: MyTableView!  
>> }  
>> 
>> Then 
>> extension MyTableViewController:SegueHandlerType {  
>>   
>> }  
>> will trigger a compiler error.
>>  
>> So the Optional needs a subclass system.
>> Or to say, that the template system needs a subclass system.
>>  
>> Optional<UITableView> should be the super type of Optional<MyTableView>
>> Array<UITableView> should be the super type of Array<MyTableView>
>> 
>> https://forums.developer.apple.com/message/101646#101646 <https://forums.developer.apple.com/message/101646#101646>
>> 
>> 
>> 
>> Thanks!
>> 
>> Jiannan, Cao
>> 
>> 
>> 
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> 

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