`flatMap`

Question

我有一个类似这样的代码段：

def test() : Seq[Int] = 
  List("A", "B", "C") collect {
    case "A" => 1
    case "B" => 2
    //case _ => println(_)
  }

现在，我想在输出上打印特定值（仅用于调试），而不向生成的集合添加任何元素。如果我取消注释行的注释，Scala会将表达式的值推断为Seq[Any]，这是完全可以理解的。

有人知道怎么做吗？提前感谢！

Answer 1

`flatMap`

List("A", "B", "C") flatMap {
    case "A" => List(1)
    case "B" => List(2)
    case x => println(x); Nil
}

`collect`/`flatten`

List("A", "B", "C").collect {
    case "A" => Some(1)
    case "B" => Some(2)
    case x => println(x); None
}.flatten

Answer 2

def skipped: Nothing = throw new Exception("Oops, not skipped after all!")

List("A", "B", "C") collect {
  case "A" => 1
  case "B" => 2
  case x if { println(x); false } => skipped
}

object PrintSkip {
  def unapply(a: Any): Option[Any] = {
    println(a)
    None
  }
}

List(Some("A"), None, Some("C")) collect {
  case Some("A") => 1
  case None => 2
  case Some(PrintSkip(_)) => skipped
}

Answer 3

有了收藏，就不需要把东西包装在选项中。

List("A", "B", "C").collect(new PartialFunction[String, Int] {
  def apply(s:String):Int = s match {
    case "A" => 1
    case "B" => 2
  }

  def isDefinedAt(s:String) = {
    try {
      apply(s)
      true
    } catch {
      case e:MatchError => { println(s); false }
    }

  }
})

flatMap

collect/flatten

友情链接

`flatMap`

`collect`/`flatten`