let promise: Promise<String> = Promise { (_, fulfill, reject) in
    performSomeAsynchronousTaskWithCompletion() { (resultString: String?, error: ErrorType?) in
        if let string = resultString {
            fulfill(string)
        }
        else {
            reject(error)
        }
    }
}

promise.success { string in
    print("The result is: \(string)")
}.failure { error in
    print("Something went wrong: \(error)")
}

promise.then { (string) -> AnyObject in
    guard let data = string.dataUsingEncoding(NSUTF8StringEncoding) else {
        throw PromiseError.Generic
    }
    return try NSJSONSerialization.JSONObjectWithData(data, options: [])
}.success { (jsonObject) in
    print("Parsed json object: \(jsonObject)")
}.failure { error in
    print("Could not parse json: \(error)")
}