小编Ale*_*man的帖子

实际上，我有几个关于此代码片段的问题：

1. 做 '？？？' 导致这个异常？
2. 什么可以代替'???' ?
3. '???' 是什么意思 代表？

object InfixTypesHard2 extends App {
  val x0: Int --- String --- Boolean = ???
}
    
class ---[T, V](t: T, v: V)

堆栈跟踪：

Exception in thread "main" scala.NotImplementedError: an implementation is missing
    at scala.Predef$.$qmark$qmark$qmark(Predef.scala:344)
    at section3_OOP_operatorOverloading.InfixTypesHard2$.delayedEndpoint$section3_OOP_operatorOverloading$InfixTypesHard2$1(InfixTypesHard2.scala:5)
    at section3_OOP_operatorOverloading.InfixTypesHard2$delayedInit$body.apply(InfixTypesHard2.scala:3)
    at scala.Function0.apply$mcV$sp(Function0.scala:39)
    at scala.Function0.apply$mcV$sp$(Function0.scala:39)
    at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:17)
    at scala.App.$anonfun$main$1(App.scala:76)
    at scala.App.$anonfun$main$1$adapted(App.scala:76)
    at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:563)
    at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:561)
    at scala.collection.AbstractIterable.foreach(Iterable.scala:919)
    at scala.App.main(App.scala:76)
    at scala.App.main$(App.scala:74)
    at section3_OOP_operatorOverloading.InfixTypesHard2$.main(InfixTypesHard2.scala:3)
    at section3_OOP_operatorOverloading.InfixTypesHard2.main(InfixTypesHard2.scala)