How to show custom failure messages in ScalaTest?

Does anyone know how to show a custom failure message in ScalaTest?

For example:

NumberOfElements() should equal (5)

Shows the following message when it fails:

10 did not equal 5

But i want more descriptive message like:

NumberOfElements should be 5.

You're the first to ask for such a feature. One way to achieve this is with withClue. Something like:

withClue("NumberOfElements: ") { NumberOfElements() should be (5) }

That should get you this error message:

NumberOfElements: 10 was not equal to 5

If you want to control the message completely you can write a custom matcher. Or you could use an assertion, like this:

assert(NumberOfElements() == 5, "NumberOfElements should be 5")

Can you elaborate on what your use case is? Why is it that 10 did not equal 5 is not up to snuff, and how often have you had this need?

Here's the kind of thing you're requesting:

scala> import org.scalatest.matchers.ShouldMatchers._
import org.scalatest.matchers.ShouldMatchers._

scala> withClue ("Hi:") { 1 + 1 should equal (3) }
org.scalatest.TestFailedException: Hi: 2 did not equal 3
at org.scalatest.matchers.Matchers$class.newTestFailedException(Matchers.scala:150)
at org.scalatest.matchers.ShouldMatchers$.newTestFailedException(ShouldMatchers.scala:2331)


scala> class AssertionHolder(f: => Any) {
     |   def withMessage(s: String) {
     |     withClue(s) { f }
     |   }
     | }
defined class AssertionHolder

scala> implicit def convertAssertion(f: => Any) = new AssertionHolder(f)
convertAssertion: (f: => Any)AssertionHolder

scala> { 1 + 1 should equal (3) } withMessage ("Ho:")
org.scalatest.TestFailedException: Ho: 2 did not equal 3
at org.scalatest.matchers.Matchers$class.newTestFailedException(Matchers.scala:150)
at org.scalatest.matchers.ShouldMatchers$.newTestFailedException(ShouldMatchers.scala:2331)

So this way you can write:

{ NumberOfElements() should be (5) } withMessage ("NumberOfElements:")