Scala：函数式编程之下划线underscore

ゞ浴缸里的玫瑰 2022-07-15 13:49 207阅读 0赞

http://[blog.csdn.net/pipisorry/article/details/52913548][blog.csdn.net_pipisorry_article_details_52913548]

python参考\[[python函数式编程：apply, map, lambda和偏函数 ][python_apply_ map_ lambda_]\]

## 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)  
  
指代一个集合中的每个元素

例如我们要在一个Array a中筛出偶数，并乘以2：a.filter(\_%2==0).map(2\*\_)。  
又如要对缓冲数组ArrayBuffer b排序，可以这样：val bSorted = b.sorted(\_)  
在元组中，可以用方法\_1, \_2, \_3访问组员。如(1,2).\_2。其中句点可以用空格替代。

Note: 改成python语法的话就要使用lambda表达式了，或者直接使用列表解析。

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.

\[[浅谈 Scala 中下划线的用途][Scala]\]

\[[Scala中的下划线到底有多少种应用场景？][Scala 1]\][  
][Link 1]

[皮皮blog][Link 1]

from: [http://blog.csdn.net/pipisorry/article/details/52913548][blog.csdn.net_pipisorry_article_details_52913548]

ref:

[blog.csdn.net_pipisorry_article_details_52913548]: http://blog.csdn.net/pipisorry/article/details/52913548
[python_apply_ map_ lambda_]: http://blog.csdn.net/pipisorry/article/details/38687763
[Scala]: https://my.oschina.net/leejun2005/blog/405305
[Scala 1]: https://www.zhihu.com/question/21622725
[Link 1]: http://blog.csdn.net/pipisorry