[swift-evolution] [Pitch] Allow use associated type outside of its protocol

[曹剑楠]

Sorry about my typo:

public struct Album {
    public let sectionedMediaItemInfos: [Sectioned<MediaItemInfo>]?}


public struct Sectioned<Item> : SectionProtocol {
    public let title: String?
    public let items: [Item]
    
    public init() {
        items = []
        title = nil
    }
}

public extension Array where Element == Sectioned {
    
    var itemsCount: Int {
        return reduce(0) { (result, section) in result + section.items.count }
    }
    
    var items: [Element.Item] {
        return self.flatMap { $0.items }
    }
}

Allow extension Array with strict its Element with a struct type (may be generic) is highly wanted feature.

> 在 2017年2月20日，上午7:28，曹剑楠 <frogcjn at 163.com> 写道：
> 
> 
> OK, my fault. That solved the problem.
> 
>> var items: [Element.SectionItemType]
> 
> I should use this.
> 
> How about to allow using generic type Section directly instead of declare a protocol
> 
> ```Swift
> public extension Array where Element == Section {
>     
>     var itemsCount: Int {
>         return reduce(0) { (result, section) in result + section.items.count }
>     }
>     
>     var items: [Element.SectionItemType] {
>         return self.flatMap { $0.items }
>     }
> }
> ```
> 
>> 在 2017年2月20日，上午7:14，Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> 写道：
>> 
>> On Sun, Feb 19, 2017 at 5:04 PM, 曹剑楠 via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> Hi All,
>> 
>> I’m coding for section like data structure. 
>> For example:
>> 
>> I have an Album struct, which has many MediaItems.
>> It has a sectioned property called "sectionedMediaItemInfos", which is an array of sections.
>> Each section represents for a disc, and has an "items" array contains all MedaItems in that disc.
>> 
>> The define code is like:
>> 
>> ```Swift
>> public struct Album {
>>     public let sectionedMediaItemInfos: [Sectioned<MediaItemInfo>]?
>> }
>> 
>> public struct Sectioned<Item> : SectionProtocol {
>>     public let title: String?
>>     public let items: [Item]
>>     
>>     public init() {
>>         items = []
>>         title = nil
>>     }
>> }
>> 
>> public protocol SectionProtocol {
>>     associatedtype SectionItemType
>>     var items: [SectionItemType] { get }
>> }
>> ```
>> 
>> Now I want to define some extra properties for sections array, like
>> "sectionMediaItemInfos"."itemsCount" that count all items in each sections.
>> So I can write that extension:
>> 
>> ```Swift
>> public extension Array where Element : SectionProtocol {
>>     
>>     var itemsCount: Int {
>>         return reduce(0) { (result, section) in result + section.items.count }
>>     }
>> 
>> }
>> ```
>> 
>> So I can get my itemsCount with code like:
>> 
>> ```Swift
>> album.sectionedMediaItemInfos.itemsCount
>> ```
>> 
>> That looks good.
>> 
>> Then I want to define code to return all items in this sectioned property.
>> 
>> 
>> ```Swift
>> public extension Array where Element : SectionProtocol {
>>     var items: [SectionProtocol.SectionItemType] {
>>         return .flatMap { $0.items }
>>     }
>> }
>> ```
>>  
>> Sorry, I'm reading this quickly, but I'm confused as to why you're not writing `var items: [Element.SectionItemType]`. That seems to be what you want, no? `SectionProtocol.SectionItemType` has no constraints and, even if the grammar allowed you to write it, would have to be equivalent to `Any`.
>> 
>> This doesn’t work. It reported as "Cannot use associated type 'SectionItemType' outside of its protocol"
>> 
>> The only way to achieve my goals is to untyped the extended "items" property:
>> 
>> ```Swift
>> public extension Array where Element : SectionProtocol {
>>     var items: [Any] {
>>         return self.flatMap { $0.items }
>>     }
>> }
>> ```
>> 
>> Which is not perfect for this case.
>> 
>> 
>> So in this special case, I think allow use associated type outside of its protocol is necessary.
>> 
>> And we may allow define protocol with generic type. That would be more convenient.
>> 
>> 
>> ```Swift
>> public struct Album {
>>     public let sectionedMediaItemInfos: [Sectioned<MediaItemInfo>]?
>> }
>> 
>> 
>> public struct Sectioned<Item> : SectionProtocol {
>>     public let title: String?
>>     public let items: [Item]
>>     
>>     public init() {
>>         items = []
>>         title = nil
>>     }
>> }
>> 
>> public protocol SectionProtocol<SectionItemType> {
>>     var items: [SectionItemType] { get }
>> }
>> 
>> 
>> public extension Array where Element : SectionProtocol {
>>     var items: [Element.SectionItemType] {
>>         return self.flatMap { $0.items }
>>     }
>> }
>> 
>> ```
>> 
>> Thank all!
>> 
>> Jiannan
>> 
>> 
>> 
>> 
>> _______________________________________________
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>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
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>> 
>> 
> 

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