scala：FOO不能转换为FOO [英] scala: FOO cannot be cast to FOO

问题描述

在Scala，我非常困惑的这个相当重复的错误消息：

In Scala, I am incredibly confused by this fairly tautological error message:

java.lang.ClassCastException: FOO cannot be cast to FOO

我希望有人总是可以转换成自己的类型。

I would expect that someone can always be cast to its own type.

我试图对scala编译器运行下面的包装器，位于 http： //code.google.com/p/rooscaloo/source/browse/trunk/rooscaloo/src/org/darevay/rooscaloo/Interpreter.scala 不幸的是，Scala正在说 ResultHolder无法投射到ResultHolder 当我执行以下操作：

I am trying to run the following wrapper around the scala compiler, located at http://code.google.com/p/rooscaloo/source/browse/trunk/rooscaloo/src/org/darevay/rooscaloo/Interpreter.scala Unfortunately, Scala is saying ResultHolder cannot be cast to ResultHolder when I do the following:

import org.darevay.rooscaloo._
println(new Interpreter().eval("2"))

c> println 应该接受 Any 。应该这样做的是Interpreter.eval返回 ResultHolder 类型，使得 ResultHolder.value 将等于 2 。

I thought println was supposed to accept Any. What should be going on is that Interpreter.eval returns a ResultHolder type, such that ResultHolder.value would be equal to 2.

此外，尝试打印 .value 不工作的错误：

Additionally, trying to print .value doesn't work with the error:

(fragment of Test.scala):3: error: value value is not a member of Any                                                                                                                                                 
println(new Interpreter().eval("2").value)

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更多详情

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More details

java.lang.reflect.InvocationTargetException                                                                                                                                                                           
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        ...
Caused by: java.lang.ClassCastException: org.darevay.rooscaloo.ResultHolder cannot be cast to org.darevay.rooscaloo.ResultHolder
        at binder0$.set(<script>:1)
        at binder0.set(<script>)
        ... 24 more (unsure how to print them)

< scala.tools.nsc.interpreter 包试图做奇怪的事情。

Though I am a Scala newbie, perhaps the internal mechanism binder of the scala.tools.nsc.interpreter package is trying to do something weird.

我调用脚本为 scala Test.scala 。

我的问题是：这个错误消息的原因是什么，真正的问题是什么，以及如何获取scala.tools.nsc。[interpreter]的示例代码。感谢。

My question is: What is the cause of this error message, what is the real issue, and how does one get working sample code for scala.tools.nsc.[interpreter]? Thanks.

推荐答案

我遇到类似的问题，我已经转换我的代码使用 IMain＃mostRecentVar 在Scala 2.9中引入。以下是 CompilerMatcher 我写道：

I've run into similar issue, and I've converted my code to use IMain#mostRecentVar introduced in Scala 2.9. Here's from CompilerMatcher I wrote:

  val main = new IMain(s)
  main.compileSources(files.map(toSourceFile(_)): _*)
  code map { c => main.interpret(c) match {
    case IR.Error => error("Error interpreting %s" format (c))
    case _ => 
  }}
  val recent = main.mostRecentVar
  val holder = main.valueOfTerm(recent)
  if (holder != Some(expected))
    println("actual: " + holder.map(_.toString).getOrElse{"None"})

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