Scala 中可变长度参数列表的类型是什么? [英] What is the type of a variable-length argument list in Scala?

问题描述

假设我声明一个函数如下:

Suppose that I declare a function as follows:

def test(args: String*) = args mkString

args 的类型是什么?

推荐答案

这称为可变数量的参数或简称为可变参数.它的静态类型是 Seq[T]，其中 T 代表 T*.因为 Seq[T] 是一个接口，它不能用作实现，在这种情况下是 scala.collection.mutable.WrappedArray[T].要找出这些东西，使用 REPL 会很有用:

This is called a variable number of arguments or in short varargs. It's static type is Seq[T] where T represents T*. Because Seq[T] is an interface it can't be used as an implementation, which is in this case scala.collection.mutable.WrappedArray[T]. To find out such things it can be useful to use the REPL:

// static type
scala> def test(args: String*) = args
test: (args: String*)Seq[String]

// runtime type
scala> def test(args: String*) = args.getClass.getName
test: (args: String*)String

scala> test("")
res2: String = scala.collection.mutable.WrappedArray$ofRef

Varargs 经常与 _* 符号结合使用，它提示编译器将 Seq[T] 的元素传递给函数序列本身:

Varargs are often used in combination with the _* symbol, which is a hint to the compiler to pass the elements of a Seq[T] to a function instead of the sequence itself:

scala> def test[T](seq: T*) = seq
test: [T](seq: T*)Seq[T]

// result contains the sequence
scala> test(Seq(1,2,3))
res3: Seq[Seq[Int]] = WrappedArray(List(1, 2, 3))

// result contains elements of the sequence
scala> test(Seq(1,2,3): _*)
res4: Seq[Int] = List(1, 2, 3)

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