允许在析构提取器中对源/目标参数进行不同计数的任何方法

Question

正常成功)案例:正确数量的目标变量进行销毁：

scala> val arr = Array("aa","bb",1,2,3,4,5)
scala> val Array(a,b,one,two,three,four,five) = arr
a: Any = aa
b: Any = bb
one: Any = 1
two: Any = 2
three: Any = 3
four: Any = 4
five: Any = 5

不支持(?)案例

( a)销毁的目标变量较少:目的是将“其他”参数捕获到一个类似序列的变量中：

scala> val arr = Array("aa","bb",1,2,3,4,5)
arr: Array[Any] = Array(aa, bb, 1, 2, 3, 4, 5)
val Array(a,b,one,two, others) = arr
scala.MatchError: [Ljava.lang.Object;@146a8d2 (of class [Ljava.lang.Object;)
        at .<init>(<console>:12)
        at .<clinit>(<console>)
        at .<init>(<console>:7)
        at .<clinit>(<console>)
        at $print(<console>)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:734)

( b)销毁的目标变量多于源变量:意图是有可选的源变量。

scala> val arr = Array("aa","bb",1,2,"optional-1","optional-2")
arr: Array[Any] = Array(aa, bb, 1, 2, optional-1, optional-2)

scala> val Array(a,b,one,two,optional1) = arr
scala.MatchError: [Ljava.lang.Object;@151c590 (of class [Ljava.lang.Object;)
        at .<init>(<console>:12)
        at .<clinit>(<console>)
        at .<init>(<console>:7)
        at .<clinit>(<console>)
        at $print(<console>)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:734)
        at scala.tools.nsc.interpreter.IMain$Request.loadAndRun(IMain.scala:983)

Answer 1

List有一个解构的::操作符，它提供对a)的支持

scala> val l = List("aa","bb",1,2,"optional-1","optional-2")
l: List[Any] = List(aa, bb, 1, 2, optional-1, optional-2)

scala> val a :: b :: one :: two :: others = l
a: Any = aa
b: Any = bb
one: Any = 1
two: Any = 2
others: List[Any] = List(optional-1, optional-2)

在标准库中为其他数据结构提供此操作符的问题是，其他数据结构没有类似的性能特征。对Vector使用类似定义的操作符。

object :#: {
  def unapply[A](a: Vector[A]): Option[(A, Vector[A])] =
    if (a.length > 1) Some(a(0) -> a.drop(1))
    else None
}

scala> val a :#: b :#: one :#: two :#: others = v
a: Any = aa
b: Any = bb
one: Any = 1
two: Any = 2
others: Vector[Any] = Vector(optional-1, optional-2)

我不一定要推广这一点，但是您可以定义一个类似的操作符，它提供对用例b)的支持

object :?: {
  def unapply[A](l: List[A]): Option[(Option[A], List[A])] =
    l match {
      case Nil => Some(None -> Nil)
      case a :: tail => Some(Some(a) -> tail)
    }
}

scala> val l = List("aa","bb")
l: List[String] = List(aa, bb)

scala> val a :?: optional1 :?: optional2 :?: rest = l
a: Option[String] = Some(aa)
optional1: Option[String] = Some(bb)
optional2: Option[String] = None
rest: List[String] = List()