Generating functions in Scalacheck

Posted on November 23, 2018

Scalacheck properties are a great way to test your code. In general, you use Scalacheck generators (Gen) to fabricate data that you use to test supposedly-invariant properties of your code.

Sometimes you even want to generate functions. Scalacheck also supports that. This article addresses how Scalacheck does that.

Gen monad

Gen is defined as a monad to allow composition of more complex generators from component generators. Its essence is something like this:

  final case class Seed()

  abstract case class Gen[A](run: Seed => (A, Seed)) {
    def map[B](f: A => B): Gen[B]
    def flatMap[B](f: A => Gen[B]): Gen[B]

A Gen[A] generates A values.

Consuming values

To be able to implement a function, we need to be able to consume values as well as producing them. That is where Cogen[A] comes in.

A Cogen[A] consumes A values.


Cogen’s essence is something like this:

  abstract case class Cogen[A](perturb: (A, Seed) => Seed) {
    def contramap[S](f: S => A): Cogen[S]

Cogen is really just Gen with the arrows reversed, which is a pretty standard trick from category theory – reverse the arrows and stick “Co” in front of the name. Like with monad and comonad.

Also, Cogen is a contravariant functor, hence contramap. This means if you have a Cogen for any type A, you can also get a Cogen for any type S so long as you can convert a S into an A. That is contramap’s signature is trying to say.

Scalacheck provides some Cogen instances for us, defined in exactly this way:

  object Cogen {
    def apply[A](implicit F: Cogen[A]): Cogen[A] = F

  implicit lazy val cogenLong: Cogen[Long] = ...

  implicit lazy val cogenBoolean: Cogen[Boolean] =
      .contramap(b => if (b) 1L else 0L)

  implicit lazy val cogenByte: Cogen[Byte] =

  implicit lazy val cogenShort: Cogen[Short] =

  // :  you get the idea

Generating a function

Armed with the means of generating and consuming values, we should be able to create a function. Consider this starting point:

  def combine(seed0: Seed, cogen: Cogen[Long], gen: Gen[Boolean], n: Long): Boolean = {
    val seed1 = cogen.perturb(n, seed0)

It combines a Gen and a Cogen, converting a Long value n to a Boolean result. If we convert the n parameter to curried form, we get the equivalent code:

  def combineCurried(seed0: Seed, cogen: Cogen[Long], gen: Gen[Boolean]): Long => Boolean =
    n => {
      val seed1 = cogen.perturb(n, seed0)

But now this function returns a function – one from Long to Boolean.

Making this polymorphic:

  def createFunction[A, B](seed0: Seed, cogen: Cogen[A], gen: Gen[B]): A => B =
    n => {
      val seed1 = cogen.perturb(n, seed0)

Scalacheck internally uses a more baroque version of this mechanism to satisfy a generator for a function, ie Gen[A => B], when one is summoned like this:

  val intPredicate: Gen[Int => Boolean] =
    arbitrary[Int => Boolean] // ie Gen.function1(arbitrary[Boolean])(implicitly[Cogen[Int]])