[swift-evolution] Proposal: Intermediate mutation qualifier for protocol functions on reference-types

[Chris Lattner]

I'm sorry, but this isn't an area that the open source swift team can help with, please check with official apple channels on that.

-Chris

> On Dec 11, 2015, at 11:42 AM, James Campbell via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hey Swift team can you tell the iTunes Connect team their website is down, there is no way to get a hold of them.
> 
>> On Thu, Dec 10, 2015 at 10:35 PM, Josh Avant via swift-evolution <swift-evolution at swift.org> wrote:
>> Currently, when a reference-type adopts a protocol with a function declared as `mutating`, the reference-type's implementation cannot call that function internally. This is because the compiler enforces an immutable `self` pointer value, and the `mutating` qualifier implies that the function implementation may mutate that `self` pointer value.
>> 
>> However, there seems to be a number of fairly reasonable situations where a reference-type implementation of these `mutating` functions may only want to mutate properties owned by `self`, but not the actual `self` pointer value.
>> 
>> Consider this toy example:
>> 
>> ```
>> import Foundation
>> 
>> protocol RandomDataTransformable {
>>     typealias TransformableType
>>     var data: [TransformableType] { get set }
>>     
>>     mutating func addRandomData()
>> }
>> 
>> extension RandomDataTransformable where TransformableType == Int {
>>     mutating func addRandomData() {
>>         let random = Int(arc4random_uniform(6) + 1)
>>         data.append(random)
>>     }
>> }
>> 
>> 
>> /////
>> 
>> // VALID
>> struct NumberSource_Struct : RandomDataTransformable {
>>     typealias TransformableType = Int
>>     var data: [Int] = []
>>     
>>     mutating func addData() {
>>         addRandomData()
>>     }
>> }
>> 
>> 
>> // VALID
>> class NumberSource_ClassDeclaration: NSObject, RandomDataTransformable {
>>     typealias TransformableType = Int
>>     var data: [Int] = []
>> }
>> 
>> var numberSource = NumberSource_ClassDeclaration()
>> numberSource.addRandomData()
>> 
>> 
>> // INVALID
>> class NumberSource_ClassImplementation: NSObject, RandomDataTransformable {
>>     typealias TransformableType = Int
>>     var data: [Int] = []
>>     
>>     func addData() {
>>         self.addRandomData() // Compiler Error: Cannot use mutating member on immutable value: 'self' is immutable
>>     }
>> }
>> ```
>> 
>> Even despite the fact that the default implementation for `addRandomData` does not mutate the `self` pointer value, reference-type implementations are unable to call that function internally, since it is marked as `mutating`.
>> 
>> Perhaps confusingly, `addRandomData` may be called by externally, by objects which own instances of the reference-type (even though, again, it may not called internally by the implementation, itself).
>> 
>> Currently, the only solution to allow reference-type implementations to call the sample `addRandomData` implementation internally is to qualify the whole `RandomDataTransformable` protocol as `class`. The downside here is that this takes an otherwise perfectly reference- and struct-compatible protocol + extension implementation and restricts it to only apply to classes, decreasing overall code reusability.
>> 
>> My proposal would be to introduce an intermediate mutation qualifier that applies when protocols are adopted by reference-types. The qualifier would specify that the `self` pointer value itself may not be mutated, but `self`'s properties may be, as appropriate.
>> 
>> Thoughts, feedback on this?
>>  
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 
> 
> 
> -- 
>  Wizard
> james at supmenow.com
> +44 7523 279 698
> 
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