I am going through the coursera functional programming and have an assignment where the scalatest is written using FunSuite and Checkers.
This test framework is new to me but I have some basic idea of using assertion, as I have developed pigunit for an user defined function using assert.
As google didn't give me clear usage of Checkers and how it is different from assert, could anyone clarify where Checkers can be used and why not assert be used.
Thanks
As you know, an assertion is a way of testing that a certain condition holds. These are pretty simple in ScalaTest, as you only need to use assert. For example:
assert(List(1, 2, 3).length == 3)
"Checkers," or, as they are more often called, properties, are a bit different. They are a way to assert that a condition holds for all possible inputs instead of for a single case. For example, here is a property that tests that a list always has a nonnegative length:
check((ls: List[Int]) => ls.length >= 0)
At this point, ScalaTest defers to ScalaCheck to do the heavy lifting. ScalaCheck generates random values for ls in an effort to find one that fails the test. This concept is called property-based testing. You can read more about how to use it in ScalaTest here.
Related
I'm just starting out using ZIO in Scala. I've written some Scalacheck-style tests using ZIO's Gen type, and they appear to work, but I'd like to manually test the generators in the REPL to ensure that they're actually producing the data I expect them to.
The problem: everything in ZIO is wrapped in the ZIO monad, and I need to pass the right data into this monad to unwrap it and view the results. And there's no documentation explaining how to do this in the REPL.
I think I understand how to do it for a basic program with no environment dependencies: call zio.Runtime.default.unsafeRun. But the Gen objects expect an environment of type Random with Sized, and I don't know how to produce an instance of this.
Given a Gen[Random with Sized, T], what is the quickest way to execute it on the REPL and get a List[T] of generated values?
I think I've found a partial solution, but I'm not completely satisfied with it.
For just printing some samples from a Gen on the REPL, this works:
zio.Runtime.default.unsafeRun(
yourGenerator
.runCollectN(50)
.provideLayer(zio.random.Random.live +!+ zio.test.Sized.live(100))
) foreach println
But I don't think this is how it's supposed to be done. provideLayer doesn't typecheck unless I provide both Random and Sized, even though Random should be part of zio.Runtime.default. I think there's something about ZLayer that I still don't understand.
I'm trying to understand what is the correct way to use GivenWhenThen with ScalaCheck generators. If I put the GivenWhenThen statements inside the forAll loop, the test report will print out each GivenWhenThen statement as many times as there are trials of that test. However, if I put them outside (i.e. all of them are immediately before the forAll), then the code loses the advantage that GivenWhenThen provides, namely of having easily readable descriptions of expected behavior immediately next to where that behavior is tested.
Is there a better way to do this?
I'm new to BDD, even to whole testing world.
I'm trying to take BDD practices when writing a simple linear algebra library in swift. So there would be many value object types like Matrix, Vector etc. When writing code, I suppose I still need to stick to the TDD principle (am I right?):
Not writing any single line of code without a failing test
To implement a value object type, I need to make it conform to Equatable protocol and implement its == operator. This is adding code, so I need a failing test. How to write spec for this kinda scenarios ?
One may suggest some approach like:
describe("Matrix") {
it("should be value object") {
let aMatrix = Matrix<Double>(rows: 3, cols:2)
let sameMatrix = Matrix<Double>(rows: 3, cols:2)
expect(sameMatrix) == aMatrix
let differentMatrix = Matrix<Double>(rows: 4, cols: 2)
expect(differentMatrix) != aMatrix
}
}
This would be an ugly boilerplate for two reasons:
There may be plenty of value object types and I need to repeat it for all of them
There may be plenty of cases that would cause two objects being not equal. Taking the spec above for example, an implementation of == like return lhs.rows == rhs.rows would pass the test. In order to reveal this "bug", I need to add another expectation like expect(matrixWithDifferentColmunCount) != aMatrix. And again, this kinda repetition happens for all value object types.
So, how should I test this "isEqual" ( or operator== ) method elegantly ? or shouldn't I test it at all ?
I'm using swift and Quick for testing framework. Quick provides a mechanism called SharedExample to reduce boilerplates. But since swift is a static typing language and Quick's shared example doesn't support generics, I can't directly use a shared example to test value objects.
I came up with a workaround but don't consider it as an elegant one.
Elegance is the enemy of test suites. Test suites are boring. Test suites are repetitive. Test suites are not "DRY." The more clever you make your test suites, the more you try to avoid boilerplate, the more you are testing your test infrastructure instead of your code.
The rule of BDD is to write the test before you write the code. It's a small amount of test code because it's testing a small amount of live code; you just write it.
Yes, that can be taken too far, and I'm not saying you never use a helper function. But when you find yourself refactoring your test suite, you need to ask yourself what the test suite was for.
As a side note, your test doesn't test what it says it does. You're testing that identical constructors create Equal objects and non-identical constructors create non-Equal objects (which in principle is two tests). This doesn't test at all that it's a value object (though it's a perfectly fine thing to test). This isn't a huge deal. Don't let testing philosophy get in the way of useful testing; but it's good to have your titles match your test intent.
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.
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.
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!
I'm using Scalacheck and want to generate collection of a given size. There is a special function for that in scalaCheck, Gen.listOfN(size, Gen[T]).
When in forAll method I print the size of the generated collection it does not always have the defined size. Actually it only has the given size on the first attempt. For example, size 6 --> first attempt the size of collection is 6, second attempt size is only 3. What am I doing wrong?
It sounds like you might be using an old (pre-1.11.0) version of ScalaCheck. In these versions, the generator boundaries weren't always respected.
When ScalaCheck finds a failing test case for your property, it tries to simplify that test case (make it "smaller"). Nowadays (version >= 1.11.0), ScalaCheck tries to respect for example listOfN when doing this simplification, and not test lists with fewer than n items. However, in some cases it is still not possible for ScalaCheck to know what boundaries a generator had from the start, for example when you use the Gen.map method.
For more information on the cases when ScalaCheck still might simplify test cases in unexpected ways (and what you can do to mitigate it), see: Scalacheck won't properly report the failing case