Property Based Testing

Functional programming in Scala

The following set of sections represent the exercises contained in the book "Functional Programming in Scala", written by Paul Chiusano and Rúnar Bjarnason and published by Manning. This content library is meant to be used in tandem with the book. We use the same numeration for the exercises for you to follow them.

For more information about "Functional Programming in Scala" please visit its official website.

Note: some of the exercises in this chapter are somewhat open-ended, and weren't included in this section. You can always head to the official repository containing hints for all exercises available in the book.

A brief tour of property-based testing

Exercise 8.1

To get used to property testing, let's try to figure out the properties that specify the implementation of a sum: List[Int] => Int function:

* The sum of the empty list is 0.
* The sum of a list whose elements are all equal to `x` is just the list's length multiplied by `x`. We might
  express this as `sum(List.fill(n)(x)) == n * x`
* For any list, `l`, `sum(l) == sum(l.reverse)`, since addition is commutative.
* Given a list, `List(x,y,z,p,q)`, `sum(List(x,y,z,p,q)) == sum(List(x,y)) + sum(List(z,p,q))`, since addition is
  associative. More generally, we can partition a list into two subsequences whose sum is equal to the sum of the
  overall list.
* The sum of 1,2,3...n is `n*(n+1)/2`.

Exercise 8.2

Likewise, these would be the properties that specify a function that finds the maximum of a List[Int]:

* The max of a single element list is equal to that element.
* The max of a list is greater than or equal to all elements of the list.
* The max of a list is an element of that list.
* The max of the empty list is unspecified and should throw an error or return `None`.

The meaning and API of properties

Exercise 8.3

Taking this representation of Prop, let's implement &&:

trait Prop { def check: Boolean }

def &&(p: Prop): Prop = new Prop {
  def check = Prop.this.check && p.check

Let's try it out while testing a couple of properties for Strings. First, a substring of a String containing its last character should be the same that accessing the character directly. Secondly, calling startsWith on a string should yield an affirmative result when compared to the original string.

val genString = Gen.stringN(5)

val propA = forAll(genString)(s => s.substring(res0) == s.charAt(res1).toString)
val propB = forAll(genString)(s => s.startsWith(s) == res2)

// Don't worry about the implementations of run and its types, we'll deal with it later:
val result = (propA && propB).run(100, 100, RNG.Simple(System.currentTimeMillis))
result shouldBe Passed

The meaning and API of generators

Exercise 8.4

Let's implement Gen.choose using the following representation for Gen:

case class Gen[A](sample: State[RNG, A])

choose should generate integers in the range start to stopExclusive:

def choose(start: Int, stopExclusive: Int): Gen[Int] =
  Gen(State(RNG.nonNegativeInt).map(n => start + n % (stopExclusive - start)))

val rng = RNG.Simple(47)
// We use sample on the Gen instance to generate a State containing a RNG and an Int value. We can then run it and
// obtain both the random-generated number and a new generator to keep generating more.
choose(res0, 10) should be >= res1
choose(0, res2) should be < res3

Exercise 8.5

We can implement other functions for Gen. Let's look at them, starting by unit, that always generates the same value:

def unit[A](a: => A): Gen[A] = Gen(State.unit(a))

val rng = RNG.Simple(47)
unit(42) shouldBe res0
unit("foo") shouldBe res1

boolean generates random Boolean values:

val boolean: Gen[Boolean] = Gen(State(RNG.boolean))

We can also implement listOfN, a function that generates lists of length n using the provided generator:

def listOfN[A](n: Int, g: Gen[A]): Gen[List[A]] =

val rng = RNG.Simple(47)
val listOfBooleans = listOfN(10, Gen.boolean)
listOfBooleans.size shouldBe res0

listOfN(3, Gen.unit(42)) shouldBe res1

Generators that depend on generated values

Exercise 8.6

flatMap, lets one generator depend on another. This is its implementation:

def flatMap[B](f: A => Gen[B]): Gen[B] =
  Gen(sample.flatMap(a => f(a).sample))

We can use flatMap to implement a more dynamic version of listOfN:

def listOfN_1[A](size: Gen[Int], g: Gen[A]): Gen[List[A]] =
  size flatMap (n => Gen.listOfN(n, g))

val rng = RNG.Simple(47)
val intGen = choose(0, 10)
val generatedList = listOfN_1(intGen, unit(42))
generatedList.size should be >= res0
generatedList.size should be < res1

listOfN_1(Gen.unit(1), Gen.unit(42)) shouldBe res2

Exercise 8.7

Through the use of flatMap we can implement union, a function to combine two generators of the same type into one, by pulling values from each one with the same likelihood:

def union[A](g1: Gen[A], g2: Gen[A]): Gen[A] =
  boolean.flatMap(b => if (b) g1 else g2)

Exercise 8.8

Following a similar principle we can implement weighted, a version of union accepting a weight for each Gen and generates values from each one with a probability proportional to its weight:

def weighted[A](g1: (Gen[A], Double), g2: (Gen[A], Double)): Gen[A] = {
  // The probability we should pull from `g1`.
  val g1Threshold = g1._2.abs / (g1._2.abs + g2._2.abs)
  Gen(State(RNG.double).flatMap(d => if (d < g1Threshold) g1._1.sample else g2._1.sample))

Exercise 8.9

Let's implement && and || to compose Prop values:

def &&(p: Prop) = Prop {
  (max, n, rng) =>
    run(max, n, rng) match {
      case Passed | Proved =>, n, rng)
      case x => x

def ||(p: Prop) = Prop {
  (max, n, rng) =>
    run(max, n, rng) match {
      // In case of failure, run the other prop.
      case Falsified(msg, _) => p.tag(msg).run(max, n, rng)
      case x => x

Let's try those out:

val genZeroToTen = Gen.choose(0, 10)
val genElevenToTwenty = Gen.choose(11, 20)
val genCombination = Gen.union(genZeroToTen, genElevenToTwenty)

val combinedProp = (forAll(genCombination)(_ < 10) ||
  forAll(genCombination)(_ < 20)) &&
  forAll(genCombination)(_ >= 0)

val result =, 100, RNG.Simple(System.currentTimeMillis))
result shouldBe res0

Test case minimization

Exercise 8.10

Let's implement helper functions to convert Gen into the new SGen type:

case class SGen[+A](forSize: Int => Gen[A])

case class Gen[+A](sample: State[RNG, A]) {
  // ...
  def unsized: SGen[A] = SGen(_ => this)

Exercise 8.11

SGen supports many of the same operations as Gen. Let's define some convenience functions of SGen that just delegate to their counterparts on Gen:

case class SGen[+A](g: Int => Gen[A]) {
  def apply(n: Int): Gen[A] = g(n)

  def map[B](f: A => B): SGen[B] = SGen { g(_) map f }

  def flatMap[B](f: A => SGen[B]): SGen[B] = {
    val g2: Int => Gen[B] = n => {
      g(n) flatMap { f(_).g(n) }

  def **[B](s2: SGen[B]): SGen[(A, B)] =
    SGen(n => apply(n) ** s2(n))

Exercise 8.12

We can also implement a listOf combinator that doesn't need an explicit size. It should return an SGen, and its implementation should generate lists of the requested size. Let's try it out:

def listOf[A](g: Gen[A]): SGen[List[A]] =
  SGen(n => g.listOfN(n))

val gen = Gen.unit(42)
val prop = forAll(listOf(gen))(l => l.forall(_ == res0)), 100, RNG.Simple(System.currentTimeMillis)) shouldBe Passed

Using the library and improving its usability

Exercise 8.13

Let's define a listOf1 function to generate nonempty lists:

def listOf1[A](g: Gen[A]): SGen[List[A]] =
  SGen(n => g.listOfN(n max res0))

val prop = forAll(listOf1(Gen.choose(0, 10)))(l => l.size == 1 && l.forall(_ < 10)), 100, RNG.Simple(System.currentTimeMillis))

Exercise 8.14

Now let's write a property to verify the behavior of List.sorted:

val sortedProp = forAll(listOf(smallInt)) { ns =>
  val nss = ns.sorted
  // We specify that every sorted list is either empty, has one element,
  // or has no two consecutive elements `(a,b)` such that `a` is greater than `b`.
  (nss.isEmpty || nss.tail.isEmpty || ! {
    case (a, b) => a > b
  // Also, the sorted list should have all the elements of the input list,
  && ! ns.exists(!nss.contains(_))
  // and it should have no elements not in the input list.
  && ! nss.exists(!ns.contains(_))

Writing a test suite for parallel computations

Exercise 8.16

We can write a generator for Par[Int], building deeply nested parallel computations. Take a look:

// A `Gen[Par[Int]]` generated from a list summation that spawns a new parallel
// computation for each element of the input list summed to produce the final
// result. This is not the most compelling example, but it provides at least some
// variation in structure to use for testing.

val pint2: Gen[Par[Int]] = choose(-100, 100).listOfN(choose(0, 20)).map(l =>
  l.foldLeft(Par.unit(0))((p, i) =>
    Par.fork { Par.map2(p, Par.unit(i))(_ + _) }))

Exercise 8.17

With pint2 we can express the property about fork from chapter 7 (fork(x) == x):

val forkProp = Prop.forAllPar(pint2)(i => equal(Par.fork(i), i)) tag "fork"

Testing higher-order functions

Exercise 8.18

Let's show how we can test higher-order functions with takeWhile and dropWhile from List:

val prop = forAll(listOf(Gen.choose(0, 20))) { l =>
  val index = Gen.choose(0, 20)
  l.takeWhile(_ < index) ++ l.dropWhile(_ < index) == l
}, 100, RNG.Simple(System.currentTimeMillis)) shouldBe res0