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 parameterscase 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 Barimport 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)
 
//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: _*)

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.