Chaining functions, handling successes and fails through returning structs. Note line111 and line134 are compositions.

callbacks.playground

/**
MARK: All starts here...
*/
func test <T> (isTrue:BooleanType, success: ()->T, fail: ()->T) -> T {
    return isTrue ? success() : fail()
}

/**
MARK: - Callback Data Structures
*/
struct CallbackData <T> {
    let success: Bool = false
    let msg: String = ""
    let data: T? = nil
}
func msgOf <T> (data: CallbackData <T>) -> String {
    return data.msg
}
func dataOf <T> (data: CallbackData <T>) -> T {
    return data.data!
}
func convertCallbackData <T, U> (task: (T?)->U?) -> (CallbackData <T>)->CallbackData <U> {
    return { (data: CallbackData <T>) -> CallbackData <U> in
        let dict_data = task(data.data)
        return CallbackData(success: data.success, msg: data.msg, data: dict_data)
    }
}

/**
MARK: - Location
*/
func currentLocation () -> CallbackData <String> {
    func success () -> CallbackData <String> {
        return CallbackData <String> (
              success: true
            , msg: "Got location"
            , data: "Rob's House"
        )
    }
    
    func fail () -> CallbackData <String> {
        return CallbackData <String> (
              success: false
            , msg: "No location"
            , data: nil
        )
    }
    
    // Write logic here
    let cond = true
    
    return test(cond, success, fail)
}

/**
MARK: Send
*/
func send (data: CallbackData<[String: Any]>) -> CallbackData <[String: Any]> {
    func success () -> CallbackData <[String: Any]> {
        return CallbackData <[String: Any]> (
              success: true
            , msg: "send() recieved: '\(data.msg)'"
            , data: ["json_data": "\(data.data!)"]
        )
    }
    
    func fail () -> CallbackData <[String: Any]> {
        return CallbackData <[String: Any]> (
              success: false
            , msg: "send() nothing recieved"
            , data: nil
        )
    }
    
    // Write logic here
    let cond = data.data != nil
    
    return test(cond, success, fail)
}

/**
MARK: - Scheduler
*/
func scheduler () -> CallbackData <String> {
    func wrapStringInDictionary (str: String?) -> Dictionary <String, Any>? {
        if let u_str = str { return ["data": u_str] }
        return nil
    }
    let dictionaryVersionOf = convertCallbackData(wrapStringInDictionary)
    
    func success <T> (data: CallbackData <T>) -> ()->CallbackData <String> {
        return {
            CallbackData <String> (
                  success: true
                , msg: data.msg
                , data: "\(data.data!)"
            )
        }
    }
    
    func fail <T> (data: CallbackData <T>) -> ()->CallbackData <String> {
        return {
            CallbackData <String> (
                  success: false
                , msg: data.msg
                , data: "\(data.data!)"
            )
        }
    }
    
    let data = send(dictionaryVersionOf(currentLocation()))
    let cond = data.success == true
    
    return test(cond, success(data), fail(data))
}

/**
MARK: Notify
*/
func printOut (str: String) {
    println("Print out:")
    println("  '\(str)'.")
}
func printOut (str: Printable) {
    printOut("\(str)")
}
func printOut <T> (data: CallbackData <T>) {
    printOut(msgOf(data))
}

/**
MARK: ViewController
*/
let viewController = { printOut(dataOf(scheduler())) }
viewController()

//-> Notify:
//     '[json_data: [data: Rob's House]]'.