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

[曹剑楠]

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> 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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