Seq 上的模式匹配

檢查集合中的精確元素數量

def f(ints: Seq[Int]): String = ints match {
  case Seq() =>
      "The Seq is empty !"
  case Seq(first) =>
      s"The seq has exactly one element : $first"
  case Seq(first, second) =>
      s"The seq has exactly two elements : $first, $second"
  case  s @ Seq(_, _, _) => 
      s"s is a Seq of length three and looks like ${s}"  // Note individual elements are not bound to their own names.
  case s: Seq[Int] if s.length == 4 =>
      s"s is a Seq of Ints of exactly length 4"  // Again, individual elements are not bound to their own names.
  case _ =>
      "No match was found!"
}

現場演示

提取第一個（s）元素並將其餘元素保留為集合：

def f(ints: Seq[Int]): String = ints match {
  case Seq(first, second, tail @ _*) =>
      s"The seq has at least two elements : $first, $second. The rest of the Seq is $tail"
  case Seq(first, tail @ _*) =>
      s"The seq has at least one element : $first. The rest of the Seq is $tail"
  // alternative syntax
  // here of course this one will never match since it checks
  // for the same thing as the one above
  case first +: tail =>
      s"The seq has at least one element : $first. The rest of the Seq is $tail"
  case _ =>
      "The seq didn't match any of the above, so it must be empty"
}

通常，可以使用任何可用於構造序列的形式來對現有序列進行模式匹配。

請注意，雖然使用 Nil 和::將在模式匹配序列時起作用，但它確實將其轉換為 List，並且可能會產生意外結果。限制自己使用 Seq( ...) 和+:來避免這種情況。

請注意，雖然使用::不適用於 WrappedArray，Vector 等，請參閱：

scala> def f(ints:Seq[Int]) = ints match {
     | case h::t => h
     | case _ => "No match"
     | }
f: (ints: Seq[Int])Any

scala> f(Array(1,2))
res0: Any = No match

和+:

scala> def g(ints:Seq[Int]) = ints match {
     | case h+:t => h
     | case _ => "No match"
     | }
g: (ints: Seq[Int])Any

scala> g(Array(1,2).toSeq)
res4: Any = 1