scala下划线的作用

https://stackoverflow.com/questions/8000903/what-are-all-the-uses-of-an-underscore-in-scala

Existential types

def foo(l: List[Option[_]]) = ...

Higher kinded type parameters

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

Ignored variables

val _ = 5

Ignored parameters

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

Ignored names of self types

trait MySeq { _: Seq[_] => }

Wildcard patterns

Some(5) match { case Some(_) => println("Yes") }

Wildcard patterns in interpolations

"abc" match { case s"a$_c" => }

Sequence wildcard in patterns

C(1, 2, 3) match { case C(vs @ _*) => vs.foreach(f(_)) }

Wildcard imports

import java.util._

Hiding imports

import java.util.{ArrayList => _, _}

Joining letters to operators

def bang_!(x: Int) = 5

Assignment operators

def foo_=(x: Int) { ... }

Placeholder syntax

List(1, 2, 3) map (_ + 2)

Method values

List(1, 2, 3) foreach println _

Converting call-by-name parameters to functions

def toFunction(callByName: => Int): () => Int = callByName _

Default initializer

var x: String = _   // unloved syntax may be eliminated