Scala:如果在匹配案例中

Question

有什么方法可以在 Scala 中将 if 放入匹配案例中吗？

像这样:

def createSchedules(listTask: List[TaskOrder], physicalResources: List[Physical], humanResources: List[Human], previousTime: Duration): List[TaskSchedule] = listTask match {
    case TaskOrder(id, time, physicalRes, order) :: t =>
      val taskScheduleHumanResources = setHumanResources(physicalRes, humanResources)
      if (physicalRes != null) 
        new TaskSchedule(
          order, 
          order.getProduct(), 
          Task(id, time, physicalRes), 
          physicalRes, 
          taskScheduleHumanResources, 
          order.quantity, 
          previousTime, 
          previousTime + time) :: createSchedules(t, physicalRes, humanResources, previousTime + time)
    case Nil => Nil
  }

这样做我得到一个错误提示:

type mismatch; found : Unit required: List[Objects.TaskSchedule]

最好的方法是什么？

Answer 1

编译器推断 Unit 的原因是您没有 else 子句，这意味着您不会返回值以防 physicalRes 为空。

您需要在代码中添加一个 else 子句:

case TaskOrder(id, time, physicalRes, order) :: t => 
  val taskScheduleHumanResources = setHumanResources(physicalRes, humanResources)
  if (physicalRes != null) // stuff
  else createSchedules(t, physicalRes, humanResources, previousTime + time)

您可以使用collect 只选择想要的元素:

val res: List[TaskSchedule] = listTask.collect {
   case order if order.physicalRes != null => new TaskSchedule(...)
}

或者你也可以折叠集合，只有当你找到非空的physicalRes时才累加:

val res: List[TaskSchedule] = listTask.foldLeft(List.empty[TaskOrder]) {
  case (acc, order) => 
    if (order.physicalRes != null) {
       new TaskSchedule(...) :: acc
    } else acc
}