这个下划线是什么意思？

Question

我在看akka-http的源码，这里是akka.http.scaladsl.server.directives.RouteDirectives的源码，以complete方法为例，谁能告诉我StandardRoute(_.complete(m))中的下划线是什么意思？

package akka.http.scaladsl.server
package directives

import akka.http.scaladsl.marshalling.ToResponseMarshallable
import akka.http.scaladsl.model._
import StatusCodes._

/**
 * @groupname route Route directives
 * @groupprio route 200
 */
trait RouteDirectives {

  ....
  ....

  /**
   * Completes the request using the given arguments.
   *
   * @group route
   */
  def complete(m: ⇒ ToResponseMarshallable): StandardRoute =
    StandardRoute(_.complete(m))
}

object RouteDirectives extends RouteDirectives {
  private val _reject = StandardRoute(_.reject())
}

Answer 1

StandardRoute(_.complete(m)) 等同于 StandardRoute(x => x.complete(m))

这里的下划线是指x。如果你想知道 underscore 在 scala 中的用例。请检查 this link (Uses of underscore in Scala)

这是来自 akka http 库的代码

object StandardRoute {
  def apply(route: Route): StandardRoute = route match {
    case x: StandardRoute ⇒ x
    case x                ⇒ new StandardRoute { def apply(ctx: RequestContext) = x(ctx) }
  }

  /**
   * Converts the StandardRoute into a directive that never passes the request to its inner route
   * (and always returns its underlying route).
   */
  implicit def toDirective[L: Tuple](route: StandardRoute): Directive[L] =
    Directive[L] { _ ⇒ route }
}

路由只是功能

type Route = RequestContext ⇒ Future[RouteResult]

解释：

考虑这个对象编号。该对象的 apply 方法接受一个函数。现在看看这个 Number 对象将如何使用。

object Number {
 def apply(f: String => Int): Int = {
  f("123") 
 }
}

用法：

Number(_.toInt)

或

Number(x => x.toInt)

Scala REPL

scala> object Number {
     |      def apply(f: String => Int): Int = {
     |       f("123") 
     |      }
     |     }
defined object Number

scala> Number(_.toInt)
res0: Int = 123

scala> Number(x => x.toInt)
res1: Int = 123

这个下划线是什么意思？

What's meaning of this underscore?

scala

akka-http