Scala expected exception snippet

Question

I'm trying to simulate the expected exception behavior of common testing frameworks (e.g. JUnit or TestNG).

Here's what I could come up with so far (working):

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)
  }
}

The snippet has a main method that exercises the code. My problem is that the exception thrown enters in the first pattern matching statement:

case expected : T

Although I'm actually saying that the exception has to be of type T, which would be IllegalArgumentException.

Any ideas?

score 5 · Accepted Answer · edited May 23 '17 at 11:55

Compile with -unchecked and you'll see a warning that the type test expected: T will always return true, thanks to type erasure.

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!

Seeing as you have the class passed in you can use Class#isInstance instead. In your code, that would look like:

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

In a self contained example. Here we pass a Manifest[T] implicitly, which is a way to get the compiler to pass an extra parameter to obtain the information that type erasure threw away:

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

Scala expected exception snippet

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