             return self.defaultCallbackExecutionContext
         }
     }
-    
 }
 
 enum State {

        }
    }
    
    func onSuccess(callback: T? -> (), executionContext: ExecutionContext? = nil) {
        self.onComplete({ (value, error) in
            if !error {

        }, executionContext: executionContext)
    }
    
    // TODO: private
    func runCallbacks() {
        q.async {
            for callback in self.callbacks {
