Potential solution to Brent's problem

GenericRSSFolder.swift

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

import Cocoa

protocol Feed {
	var url: String {get}
}

protocol Folder {
	typealias FeedType

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

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

extension LocalFeed: Equatable { }

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

class LocalFolder<F: Feed where F: Equatable>: Folder {
	typealias FeedType = F

	var feeds = [FeedType]()

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

let folder = LocalFolder<LocalFeed>()

Not sure if this satisfies all your requirements, but it seems like making Folder generic through an associated type might get you to where you want to be.