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。

1、存在性类型：Existential typesdef 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") } match { case List(1,_,_) => " a list with three element and the first element is 1" case List(_*) => " a list with zero or more elements " case Map[_,_] => " matches a map with any key type and any value type " case _ => } 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 // Imports all the members of the object Fun but renames Foo to Bar import com.test.Fun.{ Foo => Bar , _ } // Imports all the members except Foo. To exclude a member rename it to _ import com.test.Fun.{ Foo => _ , _ } 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) _ + _ ( (_: Int) + (_: Int) )(2,3) val nums = List(1,2,3,4,5,6,7,8,9,10) nums map (_ + 2) nums sortWith(_>_) nums filter (_ % 2 == 0) nums reduceLeft(_+_) nums reduce (_ + _) nums reduceLeft(_ max _) nums.exists(_ > 5) nums.takeWhile(_ < 8) 10、偏应用函数：Partially applied functions def fun = { // Some code } val funLike = fun _ List(1, 2, 3) foreach println _ 1 to 5 map (10 * _) //List("foo", "bar", "baz").map(_.toUpperCase()) List("foo", "bar", "baz").map(n => n.toUpperCase()) ******·····用于将方法转换成函数，比如val f=sqrt _，以后直接调用f(250)就能求平方根了 11、初始化默认值：default value var i: Int = _ 12、作为参数名： //访问map var m3 = Map((1,100), (2,200)) for(e<-m3) println(e._1 + ": " + e._2) m3 filter (e=>e._1>1) m3 filterKeys (_>1) m3.map(e=>(e._1*10, e._2)) m3 map (e=>e._2) //访问元组：tuple getters (1,2)._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。 //Range转换为List List(1 to 5:_*) //Range转换为Vector Vector(1 to 5: _*) //可变参数中 def capitalizeAll(args: String*) = { args.map { arg => arg.capitalize } } val arr = Array("what's", "up", "doc?") capitalizeAll(arr: _*)

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：

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

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

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?

[2] Scala punctuation (AKA symbols and operators)

[3] Scala中的下划线到底有多少种应用场景？

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

[5] Scala简明教程

 

转载自https://my.oschina/leejun2005/blog/405305