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ScalaCheck User Guide

What is ScalaCheck?

ScalaCheck is a tool for testing Scala and Java programs, based on property specifications and automatic test data generation. The basic idea is that you define a property that specifies the behaviour of a method or some unit of code, and ScalaCheck checks that the property holds. All test data are generated automatically in a random fashion, so you don't have to worry about any missed cases.

A quick example

Fire up the Scala interpreter, with ScalaCheck in the classpath.

$ scala -cp scalacheck.jar

Import the forAll method, which creates universally quantified properties. We will dig into the different property methods later on, but forAll is probably the one you will use the most.

scala> import org.scalacheck.Prop.forAll

Define a property.

scala> val propConcatLists = forAll { (l1: List[Int], l2: List[Int]) =>
  l1.size + l2.size == (l1 ::: l2).size }

Check the property!

scala> propConcatLists.check
+ OK, passed 100 tests.

OK, that seemed alright. Now define another property.

scala> val propSqrt = forAll { (n: Int) => scala.math.sqrt(n*n) == n }

Check it!

scala> propSqrt.check
! Falsified after 2 passed tests.
> ARG_0: -1
> ARG_0_ORIGINAL: -488187735

Not surprisingly, the property doesn't hold. The argument -1 falsifies it. You can also see that the argument -488187735 falsifies the property. That was the first argument ScalaCheck found, and it was then simplified to -1. You'll read more about this later on.



A property is the testable unit in ScalaCheck, and is represented by the org.scalacheck.Prop class. There are several ways to create properties in ScalaCheck, one of them is to use the org.scalacheck.Prop.forAll method like in the example above. That method creates universally quantified properties directly, but it is also possible to create new properties by combining other properties, or to use any of the specialised methods in the org.scalacheck.Prop object.

Universally quantified properties

As mentioned before, org.scalacheck.Prop.forAll creates universally quantified properties. forAll takes a function as parameter, and creates a property out of it that can be tested with the check method. The function should return Boolean or another property, and can take parameters of any types, as long as there exist implicit Arbitrary instances for the types. By default, ScalaCheck has instances for common types like Int, String, List, etc, but it is also possible to define your own Arbitrary instances. This will be described in a later section.

Here are some examples of properties defined with help of the org.scalacheck.Prop.forAll method.

import org.scalacheck.Prop.forAll

val propReverseList = forAll { (l: List[String]) => l.reverse.reverse == l }

val propConcatString = forAll { (s1: String, s2: String) =>
  (s1 + s2).endsWith(s2)

When you run check on the properties, ScalaCheck generates random instances of the function parameters and evaluates the results, reporting any failing cases.

You can also give forAll a specific data generator. See the following example:

import org.scalacheck._

val smallInteger = Gen.choose(0,100)

val propSmallInteger = Prop.forAll(smallInteger) { n =>
  n >= 0 && n <= 100

smallInteger defines a generator that generates integers between 0 and 100, inclusively. Generators will be described closer in a later section. propSmallInteger simply specifies that each integer generated should be in the correct range. This way of using the forAll method is good to use when you want to control the data generation by specifying exactly which generator that should be used, and not rely on a default generator for the given type.

Conditional Properties

Sometimes, a specification takes the form of an implication. In ScalaCheck, you can use the implication operator ==>:

import org.scalacheck.Prop.{forAll, propBoolean}

val propMakeList = forAll { (n: Int) =>
  (n >= 0 && n < 10000) ==> (List.fill(n)("").length == n)

Now ScalaCheck will only care for the cases when n is not negative. We also filter out large numbers, since we don't want to generate huge lists.

If the implication operator is given a condition that is hard or impossible to fulfill, ScalaCheck might not find enough passing test cases to state that the property holds. In the following trivial example, all cases where n is non-zero will be thrown away:

scala> import org.scalacheck.Prop.{forAll, propBoolean}

scala> val propTrivial = forAll { (n: Int) =>
     |  (n == 0) ==> (n == 0)
     | }

scala> propTrivial.check
! Gave up after only 4 passed tests. 500 tests were discarded.

It is possible to tell ScalaCheck to try harder when it generates test cases, but generally you should try to refactor your property specification instead of generating more test cases, if you get this scenario.

Using implications, we realise that a property might not just pass or fail, it could also be undecided if the implication condition doesn't get fulfilled. In the section about test execution, the different results of property evaluations will be described in more detail.

Combining Properties

A third way of creating properties, is to combine existing properties into new ones.

val p1 = forAll(...)

val p2 = forAll(...)

val p3 = p1 && p2

val p4 = p1 || p2

val p5 = p1 == p2

val p6 = all(p1, p2) // same as p1 && p2

val p7 = atLeastOne(p1, p2) // same as p1 || p2

Here, p3 will hold if and only if both p1 and p2 hold, p4 will hold if either p1 or p2 holds, and p5 will hold if p1 holds exactly when p2 holds and vice versa.

Grouping properties

Often you want to specify several related properties, perhaps for all methods in a class. ScalaCheck provides a simple way of doing this, through the Properties trait. Look at the following specifications of some of the methods in the java.lang.String class:

import org.scalacheck._

object StringSpecification extends Properties("String") {
  import Prop.forAll

  property("startsWith") = forAll { (a: String, b: String) =>

  property("endsWith") = forAll { (a: String, b: String) =>

  property("substring") = forAll { (a: String, b: String) =>
    (a+b).substring(a.length) == b

  property("substring") = forAll { (a: String, b: String, c: String) =>
    (a+b+c).substring(a.length, a.length+b.length) == b

The Properties class contains a main method that can be used for simple execution of the property tests. Compile and run the tests in the following way:

$ scalac -cp scalacheck.jar StringSpecification.scala

$ scala -cp scalacheck.jar:. StringSpecification
+ String.startsWith: OK, passed 100 tests.
+ String.endsWith: OK, passed 100 tests.
+ String.substring: OK, passed 100 tests.
+ String.substring: OK, passed 100 tests.

You can also use the check method of the Properties class to check all specified properties, just like for simple Prop instances. In fact, Properties is a subtype of Prop, so you can use it just as if it was a single property. That single property holds if and only if all of the contained properties hold.

There is a Properties.include method you can use if you want to group several different property collections into a single one. You could for example create one property collection for your application that consists of all the property collections of your individual classes:

object MyAppSpecification extends Properties("MyApp") {

Labeling Properties

Sometimes it can be difficult to decide exactly what is wrong when a property fails, especially if the property is complex, with many conditions. In such cases, you can label the different parts of the property, so ScalaCheck can tell you exactly what part is failing. Look at the following example, where the different conditions of the property have been labeled differently:

import org.scalacheck.Prop.{forAll, propBoolean}

val complexProp = forAll { (m: Int, n: Int) =>
  val res = myMagicFunction(n, m)
  (res >= m)    :| "result > #1" &&
  (res >= n)    :| "result > #2" &&
  (res < m + n) :| "result not sum"

We can see the label if we define myMagicFunction incorrectly and then check the property:

scala> def myMagicFunction(n: Int, m: Int) = n + m
myMagicFunction: (Int,Int)Int

scala> complexProp.check
! Falsified after 0 passed tests.
> Label of failing property: "result not sum"
> ARG_0: "0"
> ARG_1: "0"

It is also possible to write the label before the conditions like this:

import org.scalacheck.Prop.{forAll, propBoolean}

val complexProp = forAll { (m: Int, n: Int) =>
  val res = myMagicFunction(n, m)
  ("result > #1"    |: res >= m) &&
  ("result > #2"    |: res >= n) &&
  ("result not sum" |: res < m + n)

The labeling operator can also be used to inspect intermediate values used in the properties, which can be very useful when trying to understand why a property fails. ScalaCheck always presents the generated property arguments (ARG_0, ARG_1, etc), but sometimes you need to quickly see the value of an intermediate calculation. See the following example, which tries to specify multiplication in a somewhat naive way:

import org.scalacheck.Prop.{forAll, propBoolean, all}

val propMul = forAll { (n: Int, m: Int) =>
  val res = n*m
  ("evidence = " + res) |: all(
    "div1" |: m != 0 ==> (res / m == n),
    "div2" |: n != 0 ==> (res / n == m),
    "lt1"  |: res > m,
    "lt2"  |: res > n

Here we have four different conditions, each with its own label. Instead of using the && operator the conditions are combined in an equivalent way by using the Prop.all method. The implication operators are used to protect us from zero-divisions. A fifth label is added to the combined property to record the result of the multiplication. When we check the property, ScalaCheck tells us the following:

scala> propMul.check
! Falsified after 0 passed tests.
> Labels of failing property:
"evidence = 0"
> ARG_0: "0"
> ARG_1: "0"

As you can see, you can add as many labels as you want to your property, ScalaCheck will present them all if the property fails.


Generators are responsible for generating test data in ScalaCheck, and are represented by the org.scalacheck.Gen class. You need to know how to use this class if you want ScalaCheck to generate data of types that are not supported by default, or if you want to use the forAll method mentioned above, to state properties about a specific subset of a type. In the Gen object, there are several methods for creating new and modifying existing generators. We will show how to use some of them in this section. For a more complete reference of what is available, please see the API scaladoc.

A generator can be seen simply as a function that takes some generation parameters, and (maybe) returns a generated value. That is, the type Gen[T] may be thought of as a function of type Gen.Params => Option[T]. However, the Gen class contains additional methods to make it possible to map generators, use them in for-comprehensions and so on. Conceptually, though, you should think of generators simply as functions, and the combinators in the Gen object can be used to create or modify the behaviour of such generator functions.

Let's see how to create a new generator. The best way to do it is to use the generator combinators that exist in the org.scalacheck.Gen module. These can be combined using a for-comprehension. Suppose you need a generator which generates a tuple that contains two random integer values, one of them being at least twice as big as the other. The following definition does this:

val myGen = for {
  n <- Gen.choose(10,20)
  m <- Gen.choose(2*n, 500)
} yield (n,m)

You can create generators that picks one value out of a selection of values. The following generator generates a vowel:

val vowel = Gen.oneOf('A', 'E', 'I', 'O', 'U', 'Y')

The oneOf method creates a generator that randomly picks one of its parameters each time it generates a value. Notice that plain values are implicitly converted to generators (which always generates that value) if needed.

The distribution is uniform, but if you want to control it you can use the frequency combinator:

val vowel = Gen.frequency(
  (3, 'A'),
  (4, 'E'),
  (2, 'I'),
  (3, 'O'),
  (1, 'U'),
  (1, 'Y')

Now, the vowel generator will generate E:s more often than Y:s. Roughly, 4/14 of the values generated will be E:s, and 1/14 of them will be Y:s.

Generating Case Classes

It is very simple to generate random instances of case classes in ScalaCheck. Consider the following example where a binary integer tree is generated:

sealed abstract class Tree
case class Node(left: Tree, right: Tree, v: Int) extends Tree
case object Leaf extends Tree

import org.scalacheck._
import Gen._
import Arbitrary.arbitrary

val genLeaf = const(Leaf)

val genNode = for {
  v <- arbitrary[Int]
  left <- genTree
  right <- genTree
} yield Node(left, right, v)

def genTree: Gen[Tree] = oneOf(genLeaf, lzy(genNode))

We can now generate a sample tree:

scala> genTree.sample
res0: Option[Tree] = Some(Node(Leaf,Node(Node(Node(Node(Node(Node(Leaf,Leaf,-71),Node(Leaf,Leaf,-49),17),Leaf,-20),Leaf,-7),Node(Node(Leaf,Leaf,26),Leaf,-3),49),Leaf,84),-29))

Sized Generators

When ScalaCheck uses a generator to generate a value, it feeds it with some parameters. One of the parameters the generator is given, is a size value, which some generators use to generate their values. If you want to use the size parameter in your own generator, you can use the Gen.sized method:

def matrix[T](g: Gen[T]): Gen[Seq[Seq[T]]] = Gen.sized { size =>
  val side = scala.math.sqrt(size).asInstanceOf[Int]
  Gen.listOfN(side, Gen.listOfN(side, g))

The matrix generator will use a given generator and create a matrix which side is based on the generator size parameter. It uses the Gen.listOfN which creates a sequence of given length filled with values obtained from the given generator.

Conditional Generators

Conditional generators can be defined using Gen.suchThat in the following way:

val smallEvenInteger = Gen.choose(0,200) suchThat (_ % 2 == 0)

Conditional generators work just like conditional properties, in the sense that if the condition is too hard, ScalaCheck might not be able to generate enough values, and it might report a property test as undecided. The smallEvenInteger definition is probably OK, since it will only throw away half of the generated numbers, but one has to be careful when using the suchThat operator.

Note that if a property fails on a value generated through suchThat, and is later shrunk (see test case minimisation below), the value ultimately reported as failing might not satisfy the condition given to suchThat. Although, it doesn't change the fact that there exists a failing case that does. To avoid confusion, the corresponding shrink for the type can use suchThat method too.

Generating Containers

There is a special generator, Gen.containerOf, that generates containers such as lists and arrays. They take another generator as argument, that is responsible for generating the individual items. You can use it in the following way:

val genIntList        = Gen.containerOf[List,Int](Gen.oneOf(1, 3, 5))

val genStringLazyList = Gen.containerOf[LazyList,String](Gen.alphaStr)

val genBoolArray      = Gen.containerOf[Array,Boolean](true)

By default, ScalaCheck supports generation of List, Stream (Scala 2.10 - 2.12, deprecated in 2.13), LazyList (Scala 2.13), Set, Array, and ArrayList (from java.util). You can add support for additional containers by adding implicit Buildable instances. See Buildable.scala for examples.

There is also Gen.nonEmptyContainerOf for generating non-empty containers, and Gen.containerOfN for generating containers of a given size.

To generate a container by picking an arbitrary number of elements use Gen.someOf, or by picking one or more elements with Gen.atLeastOne.

val zeroOrMoreDigits = Gen.someOf(1 to 9)

val oneOrMoreDigits = Gen.atLeastOne(1 to 9)

Here are generators that randomly pick n elements from a container with Gen.pick:

val fiveDice: Gen[Seq[Int]] = Gen.pick(5, 1 to 6)

val threeLetters: Gen[Seq[Char]] = Gen.pick(3, 'A' to 'Z')

Note that Gen.someOf, Gen.atLeastOne, and Gen.pick only randomly select elements. They do not generate permutations of the result with elements in different orders.

To make your generator artificially permute the order of elements, you can run scala.util.Random.shuffle on each of the generated containers with the map method.

import scala.util.Random

val threeLettersPermuted =

The arbitrary Generator

There is a special generator, org.scalacheck.Arbitrary.arbitrary, which generates arbitrary values of any supported type.

val evenInteger = Arbitrary.arbitrary[Int] suchThat (_ % 2 == 0)

val squares = for {
  xs <- Arbitrary.arbitrary[List[Int]]
} yield => x*x)

The arbitrary generator is the generator used by ScalaCheck when it generates values for property parameters. Most of the times, you have to supply the type of the value to arbitrary, like above, since Scala often can't infer the type automatically. You can use arbitrary for any type that has an implicit Arbitrary instance. As mentioned earlier, ScalaCheck has default support for common types, but it is also possible to define your own implicit Arbitrary instances for unsupported types. See the following implicit Arbitrary definition for booleans, that comes from the ScalaCheck implementation.

implicit lazy val arbBool: Arbitrary[Boolean] = Arbitrary(oneOf(true, false))

To get support for your own type T you need to define an implicit def or val of type Arbitrary[T]. Use the factory method Arbitrary(...) to create the Arbitrary instance. This method takes one parameter of type Gen[T] and returns an instance of Arbitrary[T].

Now, let's say you have a custom type Tree[T] that you want to use as a parameter in your properties:

abstract sealed class Tree[T] {
  def merge(t: Tree[T]) = Internal(List(this, t))

  def size: Int = this match {
    case Leaf(_) => 1
    case Internal(children) => (children :\ 0) (_.size + _)

case class Internal[T](children: Seq[Tree[T]]) extends Tree[T]

case class Leaf[T](elem: T) extends Tree[T]

When you specify an implicit generator for your type Tree[T], you also have to assume that there exists an implicit generator for the type T. You do this by specifying an implicit parameter of type Arbitrary[T], so you can use the generator arbitrary[T].

implicit def arbTree[T](implicit a: Arbitrary[T]): Arbitrary[Tree[T]] =
Arbitrary {
  val genLeaf = for(e <- Arbitrary.arbitrary[T]) yield Leaf(e)

  def genInternal(sz: Int): Gen[Tree[T]] = for {
    n <- Gen.choose(sz/3, sz/2)
    c <- Gen.listOfN(n, sizedTree(sz/2))
  } yield Internal(c)

  def sizedTree(sz: Int) =
    if(sz <= 0) genLeaf
    else Gen.frequency((1, genLeaf), (3, genInternal(sz)))

  Gen.sized(sz => sizedTree(sz))

As long as the implicit arbTree function is in scope, you can now write properties like this:

val propMergeTree = forAll( (t1: Tree[Int], t2: Tree[Int]) =>
  t1.size + t2.size == t1.merge(t2).size

Collecting Generated Test Data

It is possible to collect statistics about what kind of test data that has been generated during property evaluation. This is useful if you want to inspect the test case distribution, and make sure your property tests all different kinds of cases, not just trivial ones.

For example, you might have a method that operates on lists, and which behaves differently if the list is sorted or not. Then it is crucial to know if ScalaCheck tests the method with both sorted and unsorted lists. Let us first define an ordered method to help us state the property.

def ordered(l: List[Int]) = l == l.sorted

Now state the property, using Prop.classify to collect interesting information on the generated data. The property itself is not very exciting in this example, we just state that a double reverse should return the original list.

import org.scalacheck.Prop._

val myProp = forAll { (l: List[Int]) =>
  classify(ordered(l), "ordered") {
    classify(l.length > 5, "large", "small") {
      l.reverse.reverse == l

Check the property, and watch the statistics printed by ScalaCheck:

scala> myProp.check
+ OK, passed 100 tests.
> Collected test data:
78% large
16% small, ordered
6% small

Here ScalaCheck tells us that the property hasn't been tested with any large and ordered list (which is no surprise, since the lists are randomised). Maybe we need to use a special generator that generates also large ordered lists, if that is important for testing our method thoroughly. In this particular case, it doesn't matter, since the implementation of reverse probably doesn't care about whether the list is sorted or not.

We can also collect data directly, using the Prop.collect method. In this dummy property, we just want to see if ScalaCheck distributes the generated data evenly:

val dummyProp = forAll(Gen.choose(1,10)) { n =>
  collect(n) {
    n == n

scala> dummyProp.check
+ OK, passed 100 tests.
> Collected test data:
13% 7
13% 5
12% 1
12% 6
11% 2
9% 9
9% 3
8% 10
7% 8
6% 4

As we can see, the frequency for each number is around 10%, which seems reasonable.

Test Execution

As we've seen, we can test properties or property collections by using the check method. In fact, the check method is just a convenient way of running org.scalacheck.Test.check (or Test.checkProperties, for property collections).

The Test module is responsible for all test execution in ScalaCheck. It will generate the arguments and evaluate the properties, repeatedly with larger and larger test data (by increasing the size parameter used by the generators). If it doesn't manage to find a failing test case after a certain number of tests, it reports a property as passed.

The check method looks like this:

def check(params: Test.Parameters, p: Prop): Test.Result

Test.Parameters is a trait that encapsulates testing parameters such as the number of times a property should be tested, the size bounds of the test data, and how many times ScalaCheck should try if it fails to generate arguments. There are also field for callbacks in the Parameters record, if you need to get feedback from the test runner programmatically.

The check method returns an instance of Test.Result which encapsulates the result and some statistics of the property test. Test.Result.status is of the type Test.Status and can have the following values:

/** ScalaCheck found enough cases for which the property holds, so the
 *  property is considered correct. (It is not proved correct, though). */
case object Passed extends Status

/** ScalaCheck managed to prove the property correct */
sealed case class Proved(args: List[Arg]) extends Status

/** The property was proved wrong with the given concrete arguments.  */
sealed case class Failed(args: List[Arg], label: String) extends Status

/** The property test was exhausted, it wasn't possible to generate enough
 *  concrete arguments satisfying the preconditions to get enough passing
 *  property evaluations. */
case object Exhausted extends Status

/** An exception was raised when trying to evaluate the property with the
 *  given concrete arguments. */
sealed case class PropException(args: List[Arg], e: Throwable, label: String) extends Status

/** An exception was raised when trying to generate concrete arguments
 *  for evaluating the property. */
sealed case class GenException(e: Throwable) extends Status

The checkProperties returns test statistics for each property in the tested property collection, as a list. See the API documentation for more details.

It is also possible to test your properties from the command line. Each property and property collection actually has a main method that can parse the test parameters given to it. If you provide the argument -h, you will get a list of possible arguments:

$ scala -cp scalacheck.jar MyProp -h
Incorrect options:
[1.1] failure: option name expected


Available options:
  -workers, -w: Number of threads to execute in parallel for testing
  -minSize, -n: Minimum data generation size
  -verbosity, -v: Verbosity level
  -minSuccessfulTests, -s: Number of tests that must succeed in order to pass a property
  -maxDiscardRatio, -r: The maximum ratio between discarded and succeeded tests allowed before ScalaCheck stops testing a property. At least minSuccessfulTests will always be tested, though.
  -maxSize, -x: Maximum data generation size
  -propFilter, -f: Regular expression to filter properties on

These command line arguments can also be used in SBT to tweak ScalaCheck's testing parameters when you run ScalaCheck tests through SBT. See SBT's documentation for info on how to provide the arguments.

Test Case Minimisation

One interesting feature of ScalaCheck is that if it finds an argument that falsifies a property, it tries to minimise that argument before it is reported. This is done automatically when you use the and Prop.forAll methods to create properties, but not if you use Prop.forAllNoShrink. Let's look at the difference between these methods, by specifying a property that says that no list has duplicate elements in it. This is of course not true, but we want to see the test case minimisation in action!

import org.scalacheck.Arbitrary.arbitrary
import org.scalacheck.Prop.{forAll, forAllNoShrink}

val p1 = forAllNoShrink(arbitrary[List[Int]])(l => l == l.distinct)

val p2 = forAll(arbitrary[List[Int]])(l => l == l.distinct)

val p3 = forAll( (l: List[Int]) => l == l.distinct )

Now, run the tests:

scala> p1.check
! Falsified after 11 passed tests:
> ARG_0 = "List(8, 0, -1, -3, -8, 8, 2, -10, 9, 1, -8)"

scala> p2.check
! Falsified after 4 passed tests:
> ARG_0 = "List(-1, -1)" (2 shrinks)

scala> p3.check
! Falsified after 7 passed tests:
> ARG_0 = "List(-5, -5)" (3 shrinks)

In all cases, ScalaCheck found a list with duplicate elements that falsified the property. However, in the two second cases the list was shrunk into a list with just two identical elements in it, which is the minimal failing test case for the given property. Clearly, it's much easier to find a bug if you are given a simple test case that causes the failure.

Just as you can define implicit Arbitrary generators for your own types, you can also define default shrinking methods. This is done by defining an implicit method that returns a Shrink[T] instance. This is done by using the Shrink(...) factory method, which as its only parameter takes a function and returns an instance of Shrink[T]. In turn, the function should take a value of the given type T, and return a Stream of shrank variants of the given value. As an example, look at the implicit Shrink instance for a tuple as it is defined in ScalaCheck:

/** Shrink instance of 2-tuple */
implicit def shrinkTuple2[T1,T2](implicit s1: Shrink[T1], s2: Shrink[T2]
): Shrink[(T1,T2)] = Shrink { case (t1,t2) =>
  (for(x1 <- shrink(t1)) yield (x1, t2)) append
  (for(x2 <- shrink(t2)) yield (t1, x2))

When implementing a shrinking method, one has to be careful to only return smaller variants of the value, since the shrinking algorithm otherwise could loop. ScalaCheck has implicit shrinking methods for common types such as integers and lists.

If the generator for a type is restricting the range of valid values by construction or using Gen.suchThat, the values that fail tests can still be shrunk without checking the condition, and then ultimately be reported as failing even though they do not satisfy it. To avoid that, use Shrink.suchThat with the condition to be maintained:

/** Generate lists of an even length */
val genEvenList: Gen[List[Int]] = Gen.sized { size =>
  Gen.listOfN(size * 2, Arbitrary.arbitrary[Int])

/** Shrink a list, maintaining even length */
implicit val shrinkEvenList: Shrink[List[Int]] =
  Shrink.shrinkContainer[List,Int].suchThat(_.length % 2 == 0)

Stateful Testing

We have described how ScalaCheck can be used to state properties about isolated units of your code (usually methods), and how such properties can be tested in an automated fashion. However, sometimes you want to not only specify how a method should behave on its own, but also how a collection of methods should behave together, when used as an interface to a larger system. You want to specify how the methods - or commands - affect the system's state throughout time.

An example could be to specify the workflow of an ATM. You'd want to state requirements such as that the user has to enter the correct PIN code before any money could be withdrawn, or that entering an erroneous PIN code three times would make the machine confiscate the credit card.

Formalising such command sequences using ScalaCheck's property combinators is a bit tricky. Instead, there is a small library in org.scalacheck.commands.Commands for modelling commands and specifying conditions about them, which can then be used just as ordinary ScalaCheck properties, and tested with the org.scalacheck.Test module.

Let us now assume we want to test the following trivial counter class:

class Counter {
  private var n = 0
  def inc = n += 1
  def dec = n -= 1
  def get = n
  def reset = n = 0

Counter is the type of our system under test. ScalaCheck supports testing systems in parallel (multiple instances of the system under test), or sequentially (a singleton system). In both cases, the system under test is represented by the abstract type Commands.Sut. In our example, the type Sut will be set to Counter.

We will also need a type encoding the state of the system under test. This is represented by the abstract type Commands.State. The State type should describe enough of the actual system's state for us to be able to define properties about the various commands. In our case, we will set State = Int, and this actually models the real system state exactly. This is a coincidence caused by the simplicity of our Counter implementation. A realistic system would probably have other internal state that we wouldn't need (or even know how) to implement in our abstract representation of the state.

Finally, the commands are modelled by implementing the Commands.Command type. An example will make it easier to understand. We will model the commands inc, dec, get and reset. The scaladoc comments are inlined in the example below to help you out:

import org.scalacheck.commands.Commands
import org.scalacheck.Gen
import org.scalacheck.Prop
import scala.util.{Try, Success}

object CounterSpecification extends Commands {

  type State = Int

  type Sut = Counter

  /** Decides if [[newSut]] should be allowed to be called
   *  with the specified state instance. This can be used to limit the number
   *  of co-existing [[Sut]] instances. The list of existing states represents
   *  the initial states (not the current states) for all [[Sut]] instances
   *  that are active for the moment. If this method is implemented
   *  incorrectly, for example if it returns false even if the list of
   *  existing states is empty, ScalaCheck might hang.
   *  If you want to allow only one [[Sut]] instance to exist at any given time
   *  (a singleton [[Sut]]), implement this method the following way:
   *  {{{
   *  def canCreateNewSut(newState: State, initSuts: Traversable[State]
   *    runningSuts: Traversable[Sut]
   *  ) = {
   *    initSuts.isEmpty && runningSuts.isEmpty
   *  }
   *  }}}
  def canCreateNewSut(newState: State, initSuts: Traversable[State],
    runningSuts: Traversable[Sut]): Boolean = true

  /** The precondition for the initial state, when no commands yet have
   *  run. This is used by ScalaCheck when command sequences are shrinked
   *  and the first state might differ from what is returned from
   *  [[genInitialState]]. */
  def initialPreCondition(state: State): Boolean = {
    // Since the counter implementation doesn't allow initialisation with an
    // arbitrary number, we can only start from zero
    state == 0

  /** Create a new [[Sut]] instance with an internal state that
   *  corresponds to the provided abstract state instance. The provided state
   *  is guaranteed to fulfill [[initialPreCondition]], and
   *  [[newSut]] will never be called if
   *  [[canCreateNewSut]] is not true for the given state. */
  def newSut(state: State): Sut = new Counter

  /** Destroy the system represented by the given [[Sut]]
   *  instance, and release any resources related to it. */
  def destroySut(sut: Sut): Unit = ()

  /** A generator that should produce an initial [[State]] instance that is
   *  usable by [[newSut]] to create a new system under test.
   *  The state returned by this generator is always checked with the
   *  [[initialPreCondition]] method before it is used. */
  def genInitialState: Gen[State] = Gen.const(0)

  /** A generator that, given the current abstract state, should produce
   *  a suitable Command instance. */
  def genCommand(state: State): Gen[Command] = Gen.oneOf(
    Inc, Get, Dec, Reset

  // A UnitCommand is a command that doesn't produce a result
  case object Inc extends UnitCommand {
    def run(sut: Sut): Unit =

    def nextState(state: State): State = state + 1

    // This command has no preconditions
    def preCondition(state: State): Boolean = true

    // This command should always succeed (never throw an exception)
    def postCondition(state: State, success: Boolean): Prop = success

  case object Dec extends UnitCommand {
    def run(sut: Sut): Unit = sut.dec
    def nextState(state: State): State = state - 1
    def preCondition(state: State): Boolean = true
    def postCondition(state: State, success: Boolean): Prop = success

  case object Reset extends UnitCommand {
    def run(sut: Sut): Unit = sut.reset
    def nextState(state: State): State = 0
    def preCondition(state: State): Boolean = true
    def postCondition(state: State, success: Boolean): Prop = success

  case object Get extends Command {

    // The Get command returns an Int
    type Result = Int

    def run(sut: Sut): Result = sut.get

    def nextState(state: State): State = state

    def preCondition(state: State): Boolean = true

    // The post condition verifies that the result we get back from the
    // actual system corresponds to our model of the state
    def postCondition(state: State, result: Try[Result]): Prop = {
      result == Success(state)

Now we can test our Counter implementation. The Commands trait has a method called property that returns a Prop instance that allows us to treat our stateful system specification as an ordinary ScalaCheck property.

+ OK, passed 100 tests.

OK, our implementation seems to work. But let us introduce a bug:

class Counter {
  private var n = 0
  def inc = n += 1
  def dec = if(n > 3) n -= 2 else n -= 1  // Bug!
  def get = n
  def reset = n = 0

Let's test it again:

! Falsified after 64 passed tests.
> Labels of failing property:
initialstate = 0
seqcmds = (Inc; Inc; Inc; Inc; Dec; Get => 2)
> ARG_0: Actions(0,List(Inc, Inc, Inc, Inc, Dec, Get),List())
> ARG_0_ORIGINAL: Actions(0,List(Inc, Dec, Inc, Get, Inc, Inc, Inc, Dec, In
  c, Get, Dec, Dec, Inc, Dec, Dec, Get, Get, Inc, Dec, Reset, Reset, Dec, D
  ec, Dec, Get, Inc, Reset, Dec, Get, Dec, Dec, Get, Dec, Get, Inc, Reset,
  Dec, Inc, Reset, Get, Get, Reset, Inc, Reset, Dec, Inc, Dec, Dec, Get, In
  c, Inc, Reset, Get, Get, Inc, Dec, Dec, Dec, Reset, Get, Inc, Dec, Dec, I

ScalaCheck found a failing command sequence (after testing 64 good ones), and then shrank it down. The resulting command sequence is indeed the minimal failing one! There is no other less complex command sequence that could have discovered the bug. This is a very powerful feature when testing complicated command sequences, where bugs may occur after a very specific sequence of commands that is hard to come up with when doing manual tests.

You can find more examples of stateful specifications in the examples directory in the ScalaCheck repository. The slides from the ScalaDays 2014 presentation about stateful testing in ScalaCheck are also helpful.