Scala中下划线的用途-白红宇

存在性类型：

def foo(l: List[Option[_]]) = def f(m: M[_])

高阶类型参数：

case class A[K[_],T](a: K[T])def f[M[_]]

临时变量：

val _ = 5

临时参数：

List(1, 2, 3) foreach {
      _ => println("Hi") }    //List(1, 2, 3) foreach { t => println("Hi") }

通配模式：

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)

通配导入：

// Imports all the classes in the package matchingimport scala.util.matching._// Imports all the members of the object Fun (static import in Java).import com.test.Fun._

隐藏导入：

// Imports all the members of the object Fun but renames Foo to Barimport com.test.Fun.{
      Foo => Bar , _ }// Imports all the members except Foo. To exclude a member rename it to _import com.test.Fun.{
      Foo => _ , _ }

连接字母和标点符号：

def bang_!(x: Int) = 5

占位符：

( (_: 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)

偏函数：

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())

初始化默认值:

var d:Double = _ var i:Int = _

参数序列：

//Range转换为ListList(1 to 5:_*)//Range转换为VectorVector(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: _*)

作为参数名：

//访问mapvar 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)//元组(1,2)._2