scala - 是否可以在不修改返回类型的情况下在收集期间打印值？

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），而不向结果集合添加任何元素。如果我取消注释注释行，Scala 将表达式的值推断为Seq[Any]，这是完全可以理解的。

有人有任何提示如何做到这一点吗？提前致谢！

Answer 1

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 2

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 3

使用 collect，无需将内容包装在 Option 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 }
    }

  }
})