Scala 中下划线的用途

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

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。

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") }
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())

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 value

foo(_)              // 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?

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/