Solutions to Scala with Cats: Chapter 1

March 25, 2023

These are my solutions to the exercises of chapter 1 of Scala with Cats.

Setting Up the Scala Project
Exercise 1.3: Printable Library
Exercise 1.4.6: Cat Show
Exercise 1.5.5: Equality, Liberty, and Felinity

Setting Up the Scala Project

I solved the exercises in a sandbox Scala project that has Cats as a dependency. The book recommends using a Giter8 template, so that’s what I used:

$ sbt new scalawithcats/cast-seed.g8

The above command generates (at the time of writing) a minimal project with the following build.sbt file:

name := "scala-with-cats"
version := "0.0.1-SNAPSHOT"

scalaVersion := "2.13.8"

libraryDependencies += "org.typelevel" %% "cats-core" % "2.8.0"

// scalac options come from the sbt-tpolecat plugin so need to set any here

addCompilerPlugin("org.typelevel" %% "kind-projector" % "0.13.2" cross CrossVersion.full)

The above differs a bit from what the book lists, since there are both new Scala 2.13 and Cats versions out already, but I followed along using these settings with minimal issues.

Exercise 1.3: Printable Library

The definition of the Printable type class can be as follows:

trait Printable[A] {
  def format(value: A): String
}

In terms of defining the Printable instances for Scala types, I’d probably prefer to include those in the companion object of Printable so that they were readily available in the implicit scope, but the exercise asks us explicitly to create a PrintableInstances object:

object PrintableInstances {
  implicit val stringPrintable: Printable[String] =
    new Printable[String] {
      def format(value: String): String = value
    }

  implicit val intPrintable: Printable[Int] =
    new Printable[Int] {
      def format(value: Int): String = value.toString
    }
}

The interface methods in the companion object of Printable can be defined as follows:

object Printable {
  def format[A](value: A)(implicit p: Printable[A]): String =
    p.format(value)

  def print[A](value: A)(implicit p: Printable[A]): Unit =
    println(p.format(value))
}

On the above, the print method could have relied on the format method directly, but I opted to not have the unnecessary call.

For the Cat example, we can define a Printable instance for that data type directly in its companion object:

final case class Cat(name: String, age: Int, color: String)

object Cat {
  implicit val printable: Printable[Cat] =
    new Printable[Cat] {
      import PrintableInstances._

      val sp = implicitly[Printable[String]]
      val ip = implicitly[Printable[Int]]

      def format(value: Cat): String = {
        val name = sp.format(value.name)
        val age = ip.format(value.age)
        val color = sp.format(value.color)
        s"$name is a $age year-old $color cat."
      }
    }
}

This allows us to use the Printable instance without explicit imports:

val garfield = Cat("Garfield", 41, "ginger and black")
Printable.print(garfield)
// Prints "Garfield is a 41 year-old ginger and black cat.".

For the extension methods, we can define the PrintableSyntax object as follows:

object PrintableSyntax {
  implicit class PrintableOps[A](val value: A) extends AnyVal {
    def format(implicit p: Printable[A]): String =
      p.format(value)

    def print(implicit p: Printable[A]): Unit =
      println(p.format(value))
  }
}

I have opted to use a value class for performance reasons, but for the purpose of this exercise it was likely unnecessary.

By importing PrintableSyntax._ we can now call print directly on our Cat instance:

import PrintableSyntax.__

val garfield = Cat("Garfield", 41, "ginger and black")
garfield.print
// Prints "Garfield is a 41 year-old ginger and black cat.".

Exercise 1.4.6: Cat Show

To implement the previous example using Show instead of Printable, we need to define an instance of Show for Cat. Similar to the approach taken before, we’re defining the instance directly in the companion object of Cat:

import cats.Show

final case class Cat(name: String, age: Int, color: String)

object Cat {
  implicit val show: Show[Cat] =
    new Show[Cat] {
      val stringShow = Show[String]
      val intShow = Show[Int]

      def show(t: Cat): String = {
        val name = stringShow.show(t.name)
        val age = intShow.show(t.age)
        val color = stringShow.show(t.color)
        s"$name is a $age year-old $color cat."
      }
    }
}

Cats implements summoners for the Show type class, so we no longer need to use implicitly.

This can be used as follows:

import cats.implicits._

val garfield = Cat("Garfield", 41, "ginger and black")
println(garfield.show)
// Prints "Garfield is a 41 year-old ginger and black cat.".

Cats doesn’t have an extension method to directly print an instance using its Show instance, so we’re using println with the value returned by the show call.

Exercise 1.5.5: Equality, Liberty, and Felinity

A possible Eq instance for Cat can be implemented as follows. Similar to the above, I’ve opted to include it in the companion object of Cat.

object Cat {
  implicit val eq: Eq[Cat] =
    new Eq[Cat] {
      val stringEq = Eq[String]
      val intEq = Eq[Int]

      def eqv(x: Cat, y: Cat): Boolean =
        stringEq.eqv(x.name, y.name) && intEq.eqv(x.age, y.age) && stringEq.eqv(x.color, y.color)
    }
}

We can now use it to compare Cat instances:

import cats.implicits._

val cat1 = Cat("Garfield", 38, "orange and black")
val cat2 = Cat("Heathcliff", 33, "orange and black")
val optionCat1 = Option(cat1)
val optionCat2 = Option.empty[Cat]

cat1 === cat2
// Returns false.

cat1 === cat1
// Returns true.

optionCat1 === optionCat2
// Returns false.

jcazevedo.net

Solutions to Scala with Cats: Chapter 1

Table of Contents

Setting Up the Scala Project

Exercise 1.3: Printable Library

Exercise 1.4.6: Cat Show

Exercise 1.5.5: Equality, Liberty, and Felinity