Trait implemented by exception types that can modify their detail message.

This trait facilitates the withClue construct provided by trait Assertions. This construct enables extra information (or "clues") to be included in the detail message of a thrown exception. Although both assert and expect provide a way for a clue to be included directly, assertThrows, intercept, and ScalaTest matcher expressions do not. Here's an example of clues provided directly in assert:

assert(1 + 1 === 3, "this is a clue")

and in expect:

expect(3, "this is a clue") { 1 + 1 }

The exceptions thrown by the previous two statements will include the clue string, "this is a clue", in the exceptions detail message. To get the same clue in the detail message of an exception thrown by a failed assertThrows call requires using withClue:

withClue("this is a clue") {
  assertThrows[IndexOutOfBoundsException] {
    "hi".charAt(-1)
  }
}

Similarly, to get a clue in the exception resulting from an exception arising out of a ScalaTest matcher expression, you need to use withClue. Here's an example:

withClue("this is a clue") {
  1 + 1 should === (3)
}

Exception types that mix in this trait have a modifyMessage method, which returns an exception identical to itself, except with the detail message option replaced with the result of invoking the passed function, supplying the current detail message option as the lone String parameter.