[chriswarbo]

I agree. One nice feature of property-driven testing is that assumptions often end up causing test failures. For example (in ScalaTest):

  "Average of list" should "be within range" in {
    forAll() {
      (l: List[Float]) => {
        val avg = l.average
        assert(avg >= l.min && avg <= l.max)
      }
    }

This test will fail, since it doesn't hold for e.g. empty lists. Requiring non-empty lists will still fail, if we have awkward values like NaNs, etc. The following version has a better chance of passing:

  "Average of list" should "be within range" in {
    forAll() {
      (raw: List[Float]) => {
        val l = raw.filter(n => !n.isNaN && !n.isInfinite)
        whenever (l.nonEmpty) {
          val avg = l.average
          assert(avg >= l.min && avg <= l.max)
        }
      }
    }

Getting this test to pass required us to make those assumptions explicit. Of course, it doesn't spot everything; here's an article which explores this example in more depth (in Python) https://hypothesis.works/articles/calculating-the-mean