NetworkClient.swift

func performRequest<A: JSONDecodable>(request: NSURLRequest, callback:(Result<A>)->()) {
    let task = NSURLSession.sharedSession().dataTaskWithRequest(request) { data, urlResponse, error in
        callback(parseResult(data, urlResponse: urlResponse, error:error))
    }

    task.resume()
}

func parseResult<A: JSONDecodable>(data: NSData!, urlResponse: NSURLResponse!, error: NSError!)->Result<A> {
    let responseResult: Result<Response> = Result(error, Response(data: data, urlResponse: urlResponse))

     return responseResult >>> parseResponse
                >>> decodeJSON
                >>> decodeObject
}

func parseResponse(response: Response)->Result<NSData> {
    let successRange = 200..<300
    if !successRange.contains(response.statusCode) {
        return .Error(NSError())
    }
    return Result(nil, response.data)
}