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

Karl Wagner razielim at gmail.com
Tue Feb 28 21:19:06 CST 2017

> 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
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>>> Proposal link:
>>> https://github.com/apple/swift-evolution/blob/master/proposals/0156-subclass-existentials.md
>>> Reply text
>>> Other replies
>>> What goes into a review?
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>>> 	• What is your evaluation of the proposal?
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