Code from http://robots.thoughtbot.com/efficient-json-in-swift-with-functional-concepts-and-generics

perform-request

func performRequest<A: Decodable>(request: NSURLRequest, callback: (Result<A>) -> ()) {
  let task = NSURLSession.sharedSession().dataTaskWithRequest(request) { data, urlResponse, error in
    let responseResult = Result(error, Response(data: data, urlResponse: urlResponse))
    let result = responseResult >>> parseResponse
                                >>> decodeJSON
                                >>> A.decode
    callback(result)
  }
  task.resume()
}