Solutions to "Scala with Cats": Chapter 2

April 3, 2023

These are my solutions to the exercises of chapter 2 of Scala with Cats.

Exercise 2.3: The Truth About Monoids
Exercise 2.4: All Set for Monoids
Exercise 2.5.4: Adding All the Things

Exercise 2.3: The Truth About Monoids

For this exercise, rather than defining instances for the proposed types, I defined instances for Cats’ Monoid directly. For that purpose, we need to import cats.Monoid.

For the Boolean type, we can define 4 monoid instances. The first is boolean or, with combine being equal to the application of the || operator and empty being false:

val booleanOrMonoid: Monoid[Boolean] = new Monoid[Boolean] {
  def combine(x: Boolean, y: Boolean): Boolean = x || y
  def empty: Boolean = false
}

The second is boolean and, with combine being equal to the application of the && operator and empty being true:

val booleanAndMonoid: Monoid[Boolean] = new Monoid[Boolean] {
  def combine(x: Boolean, y: Boolean): Boolean = x && y
  def empty: Boolean = true
}

The third is boolean exclusive or, with combine being equal to the application of the ^ operator and empty being false:

val booleanXorMonoid: Monoid[Boolean] = new Monoid[Boolean] {
  def combine(x: Boolean, y: Boolean): Boolean = x ^ y
  def empty: Boolean = false
}

The fourth is boolean exclusive nor (the negation of exclusive or), with combine being equal to the negation of the application of the ^ operator and empty being true:

val booleanXnorMonoid: Monoid[Boolean] = new Monoid[Boolean] {
  def combine(x: Boolean, y: Boolean): Boolean = !(x ^ y)
  def empty: Boolean = true
}

To convince ourselves that the monoid laws hold for the proposed monoids, we can verify them on all instances of Boolean values. Since they’re only 2 (true and false), it’s easy to check them all:

object BooleanMonoidProperties extends App {
  final val BooleanValues = List(true, false)

  def checkAssociativity(monoid: Monoid[Boolean]): Boolean =
    (for {
      a <- BooleanValues
      b <- BooleanValues
      c <- BooleanValues
    } yield monoid.combine(monoid.combine(a, b), c) == monoid.combine(a, monoid.combine(b, c))).forall(identity)

  def checkIdentityElement(monoid: Monoid[Boolean]): Boolean =
    (for { a <- BooleanValues } yield monoid.combine(a, monoid.empty) == a).forall(identity)

  def checkMonoidLaws(monoid: Monoid[Boolean]): Boolean =
    checkAssociativity(monoid) && checkIdentityElement(monoid)

  assert(checkMonoidLaws(booleanOrMonoid))
  assert(checkMonoidLaws(booleanAndMonoid))
  assert(checkMonoidLaws(booleanXorMonoid))
  assert(checkMonoidLaws(booleanXnorMonoid))
}

Exercise 2.4: All Set for Monoids

Set union forms a monoid for sets:

def setUnion[A]: Monoid[Set[A]] = new Monoid[Set[A]] {
  def combine(x: Set[A], y: Set[A]): Set[A] = x.union(y)
  def empty: Set[A] = Set.empty[A]
}

Set intersection only forms a semigroup for sets, since we can’t define an identity element for the general case. In theory, the identity element would be the set including all instances of the type of elements in the set, but in practice we can’t produce that for a generic type A:

def setIntersection[A]: Semigroup[Set[A]] = new Semigroup[Set[A]] {
  def combine(x: Set[A], y: Set[A]): Set[A] = x.intersect(y)
}

The book’s solutions suggest an additional monoid (symmetric difference), which didn’t occur to me at the time:

def setSymdiff[A]: Monoid[Set[A]] = new Monoid[Set[A]] {
  def combine(x: Set[A], y: Set[A]): Set[A] = (x.diff(y)).union(y.diff(x))
  def empty: Set[A] = Set.empty[A]
}

Exercise 2.5.4: Adding All the Things

The exercise is clearly hinting us towards using a monoid, but the first step can be defined in terms of Int only. The description doesn’t tell us what we should do in case of an empty list, but, since we’re in a chapter about monoids, I assume we want to return the identity element:

def add(items: List[Int]): Int =
  items.foldLeft(0)(_ + _)

Changing the code above to also work with Option[Int] and making sure there is no code duplication can be achieved by introducing a dependency on a Monoid instance:

import cats.Monoid

def add[A](items: List[A])(implicit monoid: Monoid[A]): A =
  items.foldLeft(monoid.empty)(monoid.combine)

With the above in place we continue to be able to add Ints, but we’re also now able to add Option[Int]s, provided we have the appropriate Monoid instances in place:

import cats.instances.int._
import cats.instances.option._

add(List(1, 2, 3))
// Returns 6.

add(List(1))
// Returns 1.

add(List.empty[Int])
// Returns 0.

add(List(Some(1), Some(2), Some(3), None))
// Returns Some(6).

add(List(Option.apply(1)))
// Returns Some(1).

add(List.empty[Option[Int]])
// Returns None.

To be able to add Order instances without making any modifications to add, we can define a Monoid instance for Order. In this case, we’re piggybacking on the Monoid instance for Double, but we could’ve implemented the sums and the production of the identity element directly:

case class Order(totalCost: Double, quantity: Double)

object Order {
  implicit val orderMonoid: Monoid[Order] = new Monoid[Order] {
    import cats.instances.double._

    val doubleMonoid = Monoid[Double]

    def combine(x: Order, y: Order): Order =
      Order(
        totalCost = doubleMonoid.combine(x.totalCost, y.totalCost),
        quantity = doubleMonoid.combine(x.quantity, y.quantity)
      )

    def empty: Order =
      Order(
        totalCost = doubleMonoid.empty,
        quantity = doubleMonoid.empty
      )
  }
}

jcazevedo.net

Solutions to "Scala with Cats": Chapter 2

Table of Contents

Exercise 2.3: The Truth About Monoids

Exercise 2.4: All Set for Monoids

Exercise 2.5.4: Adding All the Things