Scala期望异常片段

Question

我正在尝试模拟常见测试框架(例如JUnit或TestNG)的预期异常行为.

到目前为止,这是我能想到的(工作):

trait ExpectAsserts
{
  self : {
    def fail (message : String)
    def success (message : String)
  } =>

  def expect[T](exceptionClass : Class[T])(test : => Unit)
  {
    try
    {
      test
      fail("exception not thrown")
    }
    catch
    {
      case expected : T => success("got exception " + expected)
      case other : Exception => fail("expected "+ exceptionClass + " but " + other + " thrown instead.")
    }
  }
}

object Main extends ExpectAsserts
{
  def main (args : Array[String])
  {
    expect(classOf[ArithmeticException])
    {
      throw new IllegalArgumentException // this should print an error message.
    }
  }

  def fail (s : String)
  {
    System.err.println(s)
  }

  def success(s : String)
  {
    System.out.println(s)
  }
}

该代码段有一个main练习代码的方法.我的问题是抛出的异常进入第一个模式匹配语句:

case expected : T

虽然我实际上是说异常必须属于类型T,但这样做IllegalArgumentException.

有任何想法吗？

Answer 1

编译,-unchecked你会看到一个警告,由于类型擦除,类型测试expected: T将始终返回true.

scala> def foo[T](a: Any) = a match { 
     |    case _: T => "always will match!"
     | }
<console>:22: warning: abstract type T in type pattern T is unchecked since it is eliminated by erasure
          case _: T => "always will match!"
                  ^
foo: [T](a: Any)java.lang.String

scala> foo[String](0)
res3: java.lang.String = always will match!

看到您传入的课程,您可以使用Class#isInstance.在您的代码中,它看起来像:

case expected if clazz.isInstance(expected) => success("got exception " + expected)

在一个自包含的例子中.这里我们Manifest[T]隐式传递,这是一种让编译器传递额外参数以获取类型擦除丢弃的信息的方法:

scala> def foo[T: ClassManifest](a: Any) = manifest[T].erasure.isInstance(a)
foo: [T](a: Any)(implicit evidence$1: Manifest[T])Boolean

scala> foo[String](new {})  // compiler passes Manifest[String] here
res4: Boolean = false

scala> foo[String]("")
res5: Boolean = true

进一步阅读:

• 如何在Scala上绕过类型擦除？或者,为什么我不能获得我的集合的类型参数？
• /sf/ask/93280211/#1338119
• 什么是Scala中的清单,什么时候需要它？
• Specs2 Exception Matchers