浅谈 Scala 中下划线的用途

Scala 作为一门函数式编程语言，对习惯了指令式编程语言的同学来说，会不大习惯，这里除了思维方式之外，还有语法层面的，比如 underscore（下划线）就会出现在多种场合，令初学者相当疑惑，今天就来总结下 Scala 中下划线的用法。

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1、存在性类型：Existential types
def foo(l: List[Option[_]]) = ...
2、高阶类型参数：Higher kinded type parameters
case class A[K[_],T](a: K[T])
3、临时变量：Ignored variables
val _ = 5
4、临时参数：Ignored parameters
List(1, 2, 3) foreach { _ => println("Hi") }
5、通配模式：Wildcard patterns
Some(5) match { case Some(_) => println("Yes") }
val (a, _) = (1, 2)
for (_ <- 1 to 10)
6、通配导入：Wildcard imports
import java.util._
7、隐藏导入：Hiding imports
import java.util.{ArrayList => _, _}
8、连接字母和标点符号：Joining letters to punctuation
def bang_!(x: Int) = 5
9、占位符语法：Placeholder syntax
List(1, 2, 3) map (_ + 2)
_ + _   
10、偏应用函数：Partially applied functions
List(1, 2, 3) foreach println _
11、初始化默认值：default value
var i: Int = _
12、访问元组：tuple getters
t._2 
13、参数序列：parameters Sequence 
_*作为一个整体，告诉编译器你希望将某个参数当作参数序列处理！例如val s = sum(1 to 5:_*)就是将1 to 5当作参数序列处理。

这里需要注意的是，以下两种写法实现的是完全不一样的功能：

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foo _               // Eta expansion of method into method value
foo(_)              // Partial function application

Example showing why foo(_) and foo _ are different:

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trait PlaceholderExample {
  def process[A](f: A => Unit)
  val set: Set[_ => Unit]
  set.foreach(process _) // Error 
  set.foreach(process(_)) // No Error
}

In the first case, process _ represents a method; Scala takes the polymorphic method and attempts to make it monomorphic by filling in the type parameter, but realizes that there is no type that can be filled in for A that will give the type (_ => Unit) => ? (Existential _ is not a type).

In the second case, process(_) is a lambda; when writing a lambda with no explicit argument type, Scala infers the type from the argument that foreach expects, and _ => Unit is a type (whereas just plain _ isn't), so it can be substituted and inferred.

This may well be the trickiest gotcha in Scala I have ever encountered.

Refer：

[1] What are all the uses of an underscore in Scala?

http://stackoverflow.com/questions/8000903/what-are-all-the-uses-of-an-underscore-in-scala

[2] Scala punctuation (AKA symbols and operators)

http://stackoverflow.com/questions/7888944/scala-punctuation-aka-symbols-and-operators/7890032#7890032

[3] Scala中的下划线到底有多少种应用场景？

http://www.zhihu.com/question/21622725

[4] Strange type mismatch when using member access instead of extractor

http://stackoverflow.com/questions/9610736/strange-type-mismatch-when-using-member-access-instead-of-extractor/9610961

[5] Scala简明教程

http://colobu.com/2015/01/14/Scala-Quick-Start-for-Java-Programmers/