[swift-evolution] Swift Generic Subtype Problem

[Developer]

OK, one type parameter then

struct IO<A> { //... }

struct Observer<A> { 
  let unObserver : A -> IO<()>
}

Point is, if you're going to introduce variance it has to go both ways.  That means you'll need something more in the language than a typing rule that just works in this one small case.

~Robert Widmann

2016/02/16 0:00、Cao Jiannan <frogcjn at 163.com> のメッセージ:

> OK, let narrow the problem.
> 
> if B is subclass of A
> Optional<B> is subclass of Optional<A>
> 
> Not allow multiple type template.
> 
> 
> 
> 
>> 在 2016年2月16日，12:29，Developer <devteam.codafi at gmail.com> 写道：
>> 
>> OK, so you have covariance covered, now what about contravariance?  Surely a structure like this
>> 
>> struct Arrow<A, B> {
>>   let unArrow : A -> B
>> }
>> 
>> should be contravariant in its first argument and covariant in its second.
>> 
>> ~Robert Widmann
>> 
>> 2016/02/15 22:48、Cao Jiannan via swift-evolution <swift-evolution at swift.org> のメッセージ:
>> 
>>> 
>>> 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
>>> 
>>> 
>>> 
>>> 
>>> Thanks!
>>> 
>>> Jiannan, Cao
>>> 
>>> 
>>> 
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 
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