[swift-evolution] [Review] SE-0156: Class and Subtype existentials

[Karl Wagner]

> On 28 Feb 2017, at 22:53, Jordan Rose via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Nitpick: 'C<T> & P' is just 'C<T>' in this example. You'd need a refinement of 'P' to make it interesting ('C<T> & Q').
> 
> Jordan
> 

Or conformance to P needs to be conditional.

That is my only issue with this proposal: why limit it to class types? I think it needs to be generalised to allow all concrete type + protocol existentials, in order to make conditional conformances useful.

For example, let’s say I have a type “MyStruct” which conditionally conforms to “MyProtocol":

protocol MyProtocol { … }

struct MyStruct<T: Something> { … }

// Conditional conformance to MyProtocol
extension MyStruct: MyProtocol where T.AnAssociatedType == SomeType, T.AnotherAssociatedType: SomeOtherProtocol /* ...etc */ { … }

Now, I want a conforming struct. How am I supposed to do that? I think the most logical way is to write:

var conformingStruct: MyStruct & MyProtocol

… which is the generalisation of what is being talked about here.

Sorry if that’s broadened the scope too much.

- Karl

>> On Feb 28, 2017, at 13:20, Joe Groff via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> Composing a base class and protocol requirements opens up the possibility of using the base class to constrain away a protocol's associated types, e.g.:
>> 
>> protocol P { associatedtype T; func foo(_: T) }
>> class C<T>: P { func foo(_: T) {}
>> 
>> It'd be worth calling out explicitly whether this proposal allows `C<T> & P` to be used as an existential type, since although P has free associated types, the composition does not.
>> 
>> -Joe
>> 
>>> On Feb 28, 2017, at 1:11 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> Hello Swift community,
>>> 
>>> The review of SE-0156 "Class and Subtype existentials" begins now and runs through March 7, 2017. The proposal is available here:
>>> 
>>> https://github.com/apple/swift-evolution/blob/master/proposals/0156-subclass-existentials.md <https://github.com/apple/swift-evolution/blob/master/proposals/0156-subclass-existentials.md>
>>> Reviews are an important part of the Swift evolution process. All reviews should be sent to the swift-evolution mailing list at
>>> 
>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>> or, if you would like to keep your feedback private, directly to the review manager. When replying, please try to keep the proposal link at the top of the message:
>>> 
>>> Proposal link:
>>> 
>>> https://github.com/apple/swift-evolution/blob/master/proposals/0156-subclass-existentials.md
>>> Reply text
>>> 
>>> Other replies
>>> What goes into a review?
>>> 
>>> The goal of the review process is to improve the proposal under review through constructive criticism and, eventually, determine the direction of Swift. When writing your review, here are some questions you might want to answer in your review:
>>> 
>>> 	• What is your evaluation of the proposal?
>>> 	• Is the problem being addressed significant enough to warrant a change to Swift?
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>>> 
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>>> Thank you,
>>> 
>>> -Doug
>>> 
>>> Review Manager
>>> 
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