how to use forAll in scalatest to generate only one object of a generator?

Question

Im working with scalatest and scalacheck, alsso working with FeatureSpec.

I have a generator class that generate object for me that looks something like this:

object InvoiceGen {

  def myObj = for {

    country <- Gen.oneOf(Seq("France", "Germany", "United Kingdom", "Austria"))
    type <- Gen.oneOf(Seq("Communication", "Restaurants", "Parking"))
    amount <- Gen.choose(100, 4999)
    number <- Gen.choose(1, 10000)
    valid <- Arbitrary.arbitrary[Boolean]

  } yield SomeObject(country, type, "1/1/2014", amount,number.toString, 35, "something", documentTypeValid, valid, "")

Now, I have the testing class which works with FeatureSpec and everything that I need to run the tests.

In this class I have scenarios, and in each scenario I want to generate a different object. The thing is from what I understand is that to generate object is better to use forAll func, but for all will not sure to bring you an object so you can add minSuccessful(1) to make sure you get at list 1 obj....

I did it like this and it works:

scenario("some scenario") {
      forAll(MyGen.myObj, minSuccessful(1)) { someObject =>
        Given("A connection to the system")
        loginActions shouldBe 'Connected

        When("something")
        //blabla
        Then("something should happened")
        //blabla
      }
    }

but im not sure exactly what it means. What I want is to generate an invoice each scenario and do some actions on it... im not sure why i care if the generation work or didnt work...i just want a generated object to work with.

Answer 1

TL;DR: To get one object, and only one, use myObj.sample.get. Unless your generator is doing something fancy that's perfectly safe and won't blow up.

I presume that your intention is to run some kind of integration/acceptance test with some randomly generated domain object—in other words (ab-)use scalacheck as a simple data generator—and you hope that minSuccessful(1) would ensure that the test only runs once.

Be aware that this is not the case!. scalacheck will run your test multiple times if it fails, to try and shrink the input data to a minimal counterexample.

If you'd like to ensure that your test runs only once you must use sample.

However, if running the test multiple times is fine, prefer minSuccessful(1) to "succeed fast" but still profit from minimized counterexamples in case the test fails.

---

Gen.sample returns an option because generators can fail:

ScalaCheck generators can fail, for instance if you're adding a filter (listingGen.suchThat(...)), and that failure is modeled with the Option type.

But:

[…] if you're sure that your generator never will fail, you can simply call Option.get like you do in your example above. Or you can use Option.getOrElse to replace None with a default value.

Generally if your generator is simple, i.e. does not use generators that could fail and does not use any filters on its own, it's perfectly safe to just call .get on the option returned by .sample. I've been doing that in the past and never had problems with it. If your generators frequently return None from .sample they'd likely make scalacheck fail to successfully generate values as well.

If all that you want is a single object use Gen.sample.get.

---

minSuccessful has a very different meaning: It's the minimal number of successful tests that scalacheck runs—which by no means implies

• that scalacheck takes only a single value out of the generator, or
• that the test runs only once.

With minSuccessful(1) scalacheck wants one successful test. It'll take samples out of the generator until the test runs at least once—i.e. if you filter the generated values with whenever in your test body scalacheck will take samples as long as whenever discards them.

If the test passes scalacheck is happy and won't run the test a second time.

However if the test fails scalacheck will try and produce a minimal example to fail the test. It'll shrink the input data and run the test as long as it fails and then provides you with the minimized counter example rather than the actual input that triggered the initial failure.

That's an important property of property testing as it helps you to discover bugs: The original data is frequently too large to lend itself for debugging. Minimizing it helps you discover the piece of input data that actually triggers the failure, i.e. corner cases like empty strings that you didn't think of.

Answer 2

I think the way you want to use Scalacheck (generate only one object and execute the test for it) defeats the purpose of property-based testing. Let me explain a bit in detail:

In classical unit-testing, you would generate your system under test, be it an object or a system of dependent objects, with some fixed data. This could e.g. be strings like "foo" and "bar" or, if you needed a name, you would use something like "John Doe". For integers and other data, you can also randomly choose some values.

The main advantage is that these are "plain" values—you can directly see them in the code and correlate them with the output of a failed test. The big disadvantage is that the tests will only ever run with the values you specified, which in turn means that your code is also only tested with these values.

In contrast, property-based testing allows you to just describe how the data should look like (e.g. "a positive integer", "a string of maximum 20 characters"). The testing framework will then—with the help of generators—generate a number of matching objects and execute the test for all of them. This way, you can be more sure that your code will actually be correct for different inputs, which after all is the purpose of testing: to check if your code does what it should for the possible inputs.

I never really worked with Scalacheck, but a colleague explained it to me that it also tries to cover edge-cases, e.g. putting in a 0 and MAX_INT for a positive integer, or an empty string for the aforementioned string with max. 20 characters.

So, to sum it up: Running a property-based test only once for one generic object is the wrong thing to do. Instead, once you have the generator infrastructure in place, embrace the advantage you then have and let your code be checked a lot more times!