如何清理“类型被推断为`Any`”警告？答案

【问题标题】：How to clean up "a type was inferred to be `Any`" warning?如何清理“类型被推断为`Any`”警告？
【发布时间】：2014-06-10 22:30:47
【问题描述】：

我有以下代码：

class TestActor() extends RootsActor() {

  // Receive is a type resolving to PartialFunction[Any, Unit]
  def rec2 : Actor.Receive = {   
    case "ping" => println("Ping received!!!")
  }

  def recAll = List(super.receive, rec2)

  // Compose parent class' receive behavior with this class' receive
  override def receive = recAll.reduceLeft { (a,b) => a orElse b }
}

这在运行时可以正常运行，但会产生以下警告：

[warn] /Users/me/git/proj/roots/src/multi-jvm/scala/stuff/TestActor.scala:18: a type was inferred to be `Any`; this may indicate a programming error.
[warn]  override def receive = recAll.reduceLeft { (a,b) => a orElse b }
[warn]                                                   ^

如何更改此代码以清除警告？

【问题讨论】：

标签： scala warnings typing

【解决方案1】：

编译代码时必须有-Xlint 编译器标志。看起来这个警告确实是在 2.11.x 中添加的。这是 scalac (https://issues.scala-lang.org/browse/SI-9211) 中的一个错误，就好像你删除类型别名（从 orElse 参数）它工作正常

$ scala -Xlint
Welcome to Scala version 2.11.6 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_80).
Type in expressions to have them evaluated.
Type :help for more information.

scala> type Problem = PartialFunction[Any,Unit]
defined type alias Problem

scala> def moreProblems(problem1: Problem, problem2: Problem) = problem1 orElse problem2
<console>:8: warning: a type was inferred to be `Any`; this may indicate a programming error.
       def moreProblems(problem1: Problem, problem2: Problem) = problem1 orElse problem2
                                                                                ^
moreProblems: (problem1: Problem, problem2: Problem)PartialFunction[Any,Unit]

scala> def moreProblems(problem1: PartialFunction[Any, Unit], problem2: Problem) = problem1 orElse problem2
<console>:8: warning: a type was inferred to be `Any`; this may indicate a programming error.
       def moreProblems(problem1: PartialFunction[Any, Unit], problem2: Problem) = problem1 orElse problem2
                                                                                                   ^
moreProblems: (problem1: PartialFunction[Any,Unit], problem2: Problem)PartialFunction[Any,Unit]

scala> def moreProblems(problem1: Problem, problem2: PartialFunction[Any, Unit]) = problem1 orElse problem2
moreProblems: (problem1: Problem, problem2: PartialFunction[Any,Unit])PartialFunction[Any,Unit]

【讨论】：

【解决方案2】：

我没有收到您的代码的警告。如果你使用 orElse 而不使用 reduce 会怎样？

scala> import akka.actor._
import akka.actor._

scala> class RootActor extends Actor { def receive = { case _ => println("bang") }}
defined class RootActor

scala> class TestActor extends RootActor {
     |   def rec2: Actor.Receive = { case "ping" => println("ping") }
     |   override def receive = super.receive orElse rec2
     | }
defined class TestActor

scala>

【讨论】：

【解决方案3】：

我认为这是因为 Akka 的默认 Actors 是无类型的。这意味着您传递的每条消息都是 Any 类型，并且通过模式匹配进行匹配。因此，要么输入 reduceLeft 参数，要么忽略警告。

【讨论】：

【解决方案4】：

这个新警告是在 Scala 2.11 中引入的

【讨论】：