Solutions to Scala with Cats: Chapter 6

These are my solutions to the exercises of chapter 6 of Scala with Cats.

Table of Contents

• Exercise 6.3.1.1: The Product of Lists
• Exercise 6.4.0.1: Parallel List

Exercise 6.3.1.1: The Product of Lists

The reason product for List produces the Cartesian product is because List forms a Monad, and product is implemented in terms of flatMap. So Semigroupal[List].product(List(1, 2), List(3, 4)) is the same as:

for {
  a <- List(1, 2)
  b <- List(3, 4)
} yield (a, b)

Which results in the Cartesian product.

Exercise 6.4.0.1: Parallel List

List does have a Parallel instance. It zips the lists instead of doing the Cartesian product. This can be exhibited by the following snippet:

import cats.instances.list._
import cats.syntax.parallel._

(List(1, 2), List(3, 4)).parTupled
// Returns List((1, 3), (2, 4)).

(List(1, 2), List(3, 4, 5)).parTupled
// Returns List((1, 3), (2, 4)).