Equatable protocols in Swift. A reply to Brent Simmons’s post "Secret Projects Diary #2: Swift 2.0 Protocols" (http://inessential.com/2015/07/19/secret_projects_diary_2_swift_2_0_prot).

RSSReaderExample.swift

//: Playground - noun: a place where people can play

import Cocoa

protocol Feed : Equatable {
    var url: String {get}
}

protocol Folder {
    // This is new:
    typealias FeedType

    var feeds: [FeedType] {get}
    func addFeeds(feedsToAdd: [FeedType])
}

func ==<T: Feed>(lhs: T, rhs: T) -> Bool {
    return lhs.url == rhs.url
}

class LocalFeed: Feed {
    var url: String
    init(url: String) {
        self.url = url
    }
}

class LocalFolder: Folder {
    // Notice I’m using concrete types (LocalFeed) for the protocol's interface here.
    // You can't just specify Feed because the compiler must be able to make sure that all
    // places where the protocol specifies Feed have the same concrete type.
    var feeds = [LocalFeed]()

    func addFeeds(feedsToAdd: [LocalFeed]) {
        for oneFeed in feedsToAdd {
            if !feeds.contains(oneFeed) {
                feeds += [oneFeed]
            }
        }
    }
}

Edit: I renamed the typealias Feed to typealias FeedType to avoid confusion with the Feed protocol.