scala – Future.onComplete：无法理解方法签名

/** When this future is completed,either through an exception,or a value,*  apply the provided function.
   *
   *  If the future has already been completed,*  this will either be applied immediately or be scheduled asynchronously.
   *
   *  $multipleCallbacks
   *  $callbackInContext
   */
  def onComplete[U](func: Try[T] => U)(implicit executor: ExecutionContext): Unit

基本用法似乎是：

result.onComplete({
    case Success(listInt) => {
      //Do something with my list
    }
    case Failure(exception) => {
      //Do something with my error
    }
  })

此函数似乎对产生副作用很有用,因为它返回Unit(如记录完成)
我不明白的是函数返回的这个类型U是什么.我们提供的函数的返回类型是否真的有用？它是如何被Scala使用的？

同样的问题可能适用于onSuccess和onFailure

编辑：为了更清楚,def onComplete [U](func：Try [T] => U)优于def onComplete(func：Try [T] => Unit)有什么好处？

编辑：

Chirlo是对的,U型的功能更灵活,我们可以更容易地传递不返回Unit的现有功能.

type T = String

def onComplete[U](func: Try[T] => U): Unit = { }
def onComplete2(func: Try[T] => Unit): Unit = { }

// Existing function that does not return Unit
def log[T]( t : Try[T]): Int = 0

onComplete(log) // This compiles fine
onComplete2(log) // This does not compile