Scala Lazy Collection 有人可以解释以下行为

Question

我有一个普通列表并在其上定义了两个过滤器。我用第一个过滤器过滤列表，然后用下一个过滤器过滤输出并获取最后一个元素。

1. 我正在对列表对象 lst (NORMAL LIST) 执行上述操作
2. 我正在对 lst.view (LAZY LIST) 进行上述操作

第二个操作似乎需要更多的迭代。似乎在某一时刻重新开始了整个操作。请找到下面的代码。有人可以解释一下原因吗？

object SolutionTest {
  val lst = List(("Mark", 32), ("Bob", 22), ("Jane", 8), ("Jill", 21),("Nick", 50), ("Nancy", 42), ("Mike", 19), ("Sara", 12), ("Paula", 42),("John", 21))
                                              //> lst  : List[(String, Int)] = List((Mark,32), (Bob,22), (Jane,8), (Jill,21), 
                                              //| (Nick,50), (Nancy,42), (Mike,19), (Sara,12), (Paula,42), (John,21))
    lst.size                                  //> res0: Int = 10

  def filter1(tup:(String, Int)):Boolean={
    println("from filter1 "+ tup)
    val (_, age) = tup
    age > 17
  }                                               //> filter1: (tup: (String, Int))Boolean

  def filter2(tup:(String, Int)):Boolean={
    println("from filter2 "+ tup)
    val (name, _) = tup
    name.startsWith("J")
  }                                               //> filter2: (tup: (String, Int))Boolean


//NORMAL LIST                                                  
lst.filter(filter1).filter(filter2).last  //> from filter1 (Mark,32)
                                              //| from filter1 (Bob,22)
                                              //| from filter1 (Jane,8)
                                              //| from filter1 (Jill,21)
                                              //| from filter1 (Nick,50)
                                              //| from filter1 (Nancy,42)
                                              //| from filter1 (Mike,19)
                                              //| from filter1 (Sara,12)
                                              //| from filter1 (Paula,42)
                                              //| from filter1 (John,21)
                                              //| from filter2 (Mark,32)
                                              //| from filter2 (Bob,22)
                                              //| from filter2 (Jill,21)
                                              //| from filter2 (Nick,50)
                                              //| from filter2 (Nancy,42)
                                              //| from filter2 (Mike,19)
                                              //| from filter2 (Paula,42)
                                              //| from filter2 (John,21)
                                              //| res1: (String, Int) = (John,21)

//LAZY LIST
lst.view.filter(filter1).filter(filter2).last
                                              //> from filter1 (Mark,32)
                                              //| from filter2 (Mark,32)
                                              //| from filter1 (Bob,22)
                                              //| from filter2 (Bob,22)
                                              //| from filter1 (Jane,8)
                                              //| from filter1 (Jill,21)
                                              //| from filter2 (Jill,21)
                                              //| from filter1 (Mark,32) RESTARTING THE OPERATION HERE!!!
                                              //| from filter2 (Mark,32)
                                              //| from filter1 (Bob,22)
                                              //| from filter2 (Bob,22)
                                              //| from filter1 (Jane,8)
                                              //| from filter1 (Jill,21)
                                              //| from filter2 (Jill,21)
                                              //| from filter1 (Nick,50)
                                              //| from filter2 (Nick,50)
                                              //| from filter1 (Nancy,42)
                                              //| from filter2 (Nancy,42)
                                              //| from filter1 (Mike,19)
                                              //| from filter2 (Mike,19)
                                              //| from filter1 (Sara,12)
                                              //| from filter1 (Paula,42)
                                              //| from filter2 (Paula,42)
                                              //| from filter1 (John,21)
                                              //| from filter2 (John,21)
                                              //| res2: (String, Int) = (John,21)
}

Answer 1

在您的评论“RESTARTING HERE”之前的第一个操作来自寻找过滤序列中的第一个元素。 Scala 开始过滤，直到找到结果中的第一个元素 (Jill,21)。在此之后，它实际上开始处理整个序列。

发生这种情况是因为 last 是 implemented在 TraversableLike 类中，如下所示:

def last: A = {
  var lst = head
  for (x <- this)
    lst = x
  lst
}

它在真正遍历序列之前调用 head。简单的 view 不使用缓存，所以在稍微过滤找到 head 之后，它必须重新启动。

View 上的其他函数可能无法表现出这种重新启动行为。例如 lst.view.filter(filter1).filter(filter2).lastOption 只运行一次序列。