Experimented with Swift generics after reading David Owens' post about protocols (http://owensd.io/2015/08/06/protocols.html)

GenericPrintableResponses.swift

import Foundation

public protocol UniversalResourceType {
    var locator: NSURL { get }
}

public protocol HTTPResponseType : UniversalResourceType, Printable { }

public class HTTPResponse<T: Printable> : UniversalResourceType, Printable {
    public var response: T
    public var locator: NSURL
    init(response: T, locator: NSURL) {
        self.response = response
        self.locator = locator
    }
}

extension String : Printable {
    public var description: String { get { return "\(self)" } }
}

extension HTTPResponse : Printable {
    public var description: String {
        get {
            return "url: \(locator) response: \(response.description)"
        }
    }
}

typealias HTTPJSONResponse = HTTPResponse<NSObject>
typealias HTTPStringResponse = HTTPResponse<String>

let exampleJSON: NSObject =
    NSJSONSerialization.JSONObjectWithData(
        "{ \"foo\": \"bar\" }".dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)!,
        options: NSJSONReadingOptions.allZeros,
        error: nil) as! NSObject

let jsonResponse  = HTTPJSONResponse(
                        response: exampleJSON,
                        locator: NSURL(string: "https://foo.bar/biz")!)

let stringResponse = HTTPStringResponse(response: "foo", locator: NSURL(string: "https://foo.bar/baz")!)

let responses: [protocol<UniversalResourceType, Printable>] = [jsonResponse, stringResponse]

let responseDescriptions = responses.map() { "desc: \($0.description) url: \($0.locator)" }
println(responseDescriptions)
/*
 [desc: url: https://foo.bar/biz response: {
     foo = bar;
 } url: https://foo.bar/biz, desc: url: https://foo.bar/baz response: foo url: https://foo.bar/baz]
 */