scala – 为什么`Future#toString`返回`“List()”`？

可以从REPL中观察到此行为：

scala> import scala.concurrent.Future,scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.ExecutionContext.Implicits.global

scala> Future(1).toString
res0: scala.concurrent.Future[Int] = Success(1)

scala> Future(1).toString
res1: scala.concurrent.Future[Int] = List()

请注意,Scala 2.12.x可能会显式实现Future#toString以返回“Future(< not completed>)”而不是(source).

编辑：证明这不是来自REPL或“某些隐藏隐式”的伪影(-Yno-predef删除所有默认隐含)：

Future.scala：

import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

object Main extends App {
  System.out.println(Future(1).toString)
}

build.sbt：

scalaVersion := "2.11.8"

scalacOptions := Seq(
  "-deprecation","-encoding","UTF-8","-feature","-unchecked","-Yno-predef","-Xfatal-warnings","-Xlint","-Yinline-warnings","-Yno-adapted-args","-Ywarn-dead-code","-Ywarn-unused-import","-Ywarn-numeric-widen","-Ywarn-value-discard","-Xfuture")