41

为什么 Scala 同时具有unapplyunapplySeq?两者有什么区别?我什么时候应该更喜欢其中一个?

4

3 回答 3

39

无需赘述和简化:

对于常规参数apply构造和unapply解构:

object S {
  def apply(a: A):S = ... // makes a S from an A
  def unapply(s: S): Option[A] = ... // retrieve the A from the S
}
val s = S(a)
s match { case S(a) => a } 

对于重复的参数、apply构造和unapplySeq解构:

object M {
  def apply(a: A*): M = ......... // makes a M from an As.
  def unapplySeq(m: M): Option[Seq[A]] = ... // retrieve the As from the M
}
val m = M(a1, a2, a3)
m match { case M(a1, a2, a3) => ... } 
m match { case M(a, as @ _*) => ... } 

请注意,在第二种情况下,重复参数被视为 aSeq以及 和 之间的相似A*_*

因此,如果您想对自然包含各种单一值的事物进行解构,请使用unapply. 如果要对包含 a 的内容进行解构Seq,请使用unapplySeq.

于 2011-11-27T01:09:59.247 回答
18

固定数量与可变数量。 Scala 中的模式匹配 (pdf)通过镜像示例很好地解释了它。我在这个答案中也有镜像示例。

简要地:

object Sorted {
  def unapply(xs: Seq[Int]) =
    if (xs == xs.sortWith(_ < _)) Some(xs) else None
}

object SortedSeq {
  def unapplySeq(xs: Seq[Int]) =
    if (xs == xs.sortWith(_ < _)) Some(xs) else None
}

scala> List(1,2,3,4) match { case Sorted(xs) => xs }
res0: Seq[Int] = List(1, 2, 3, 4)
scala> List(1,2,3,4) match { case SortedSeq(a, b, c, d) => List(a, b, c, d) }
res1: List[Int] = List(1, 2, 3, 4)
scala> List(1) match { case SortedSeq(a) => a }
res2: Int = 1

那么,您认为以下示例中展示了哪些内容?

scala> List(1) match { case List(x) => x }
res3: Int = 1
于 2011-11-26T23:43:31.110 回答
0

一些例子:

scala> val fruit = List("apples", "oranges", "pears")
fruit: List[String] = List(apples, oranges, pears)

scala> val List(a, b, c) = fruit
a: String = apples
b: String = oranges
c: String = pears

scala> val List(a, b, _*) = fruit
a: String = apples
b: String = oranges

scala> val List(a, _*) = fruit
a: String = apples

scala> val List(a,rest @ _*) = fruit
a: String = apples
rest: Seq[String] = List(oranges, pears)

scala> val a::b::c::Nil = fruit
a: String = apples
b: String = oranges
c: String = pears

scala> val a::b::rest = fruit
a: String = apples
b: String = oranges
rest: List[String] = List(pears)

scala> val a::rest = fruit
a: String = apples
rest: List[String] = List(oranges, pears)

于 2019-04-27T03:01:20.340 回答