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Scala 3 — Book

Implementing Type Classes

Language

A type class is an abstract, parameterized type that lets you add new behavior to any closed data type without using sub-typing. This is useful in multiple use-cases, for example:

  • Expressing how a type you don’t own—from the standard library or a third-party library—conforms to such behavior
  • Expressing such a behavior for multiple types without involving sub-typing relationships between those types

In Scala 3, type classes are just traits with one or more parameters whose implementations are provided by given instances.

Example

For example, Show is a well-known type class in Haskell, and the following code shows one way to implement it in Scala 3. If you imagine that Scala classes don’t have a toString method, you can define a Show type class to add this behavior to any class that you want to be able to convert to a custom string.

The type class

The first step in creating a type class is to declare a parameterized trait that has one or more abstract methods. Because Showable only has one method named show, it’s written like this:

// a type class
trait Showable[A]:
  extension(a: A) def show: String

This is the Scala 3 way of saying that any type that implements this trait must define how the show method works. Notice that the syntax is very close to a normal trait:

// a trait
trait Show:
  def show: String

There are a few important things to point out:

  1. Type-classes like Showable take a type parameter A to say which type we provide the implementation of show for; in contrast, normal traits like Show do not.
  2. To add the show functionality to a certain type A, the normal trait requires that A extends Show, while for type-classes we require to have an implementation of Showable[A].
  3. To allow the same method calling syntax in both Showable that mimics the one of Show, we define Showable.show as an extension method.

Implement concrete instances

The next step is to determine what classes in your application Showable should work for, and then implement that behavior for them. For instance, to implement Showable for this Person class:

case class Person(firstName: String, lastName: String)

you’ll define a given value for Showable[Person]. This code provides a concrete instance of Showable for the Person class:

given Showable[Person] with
  extension(p: Person) def show: String =
    s"${p.firstName} ${p.lastName}"

As shown, this is defined as an extension method on the Person class, and it uses the reference p inside the body of the show method.

Using the type class

Now you can use this type class like this:

val person = Person("John", "Doe")
println(person.show)

Again, if Scala didn’t have a toString method available to every class, you could use this technique to add Showable behavior to any class that you want to be able to convert to a String.

Writing methods that use the type class

As with inheritance, you can define methods that use Showable as a type parameter:

def showAll[S: Showable](xs: List[S]): Unit =
  xs.foreach(x => println(x.show))

showAll(List(Person("Jane"), Person("Mary")))

A type class with multiple methods

Note that if you want to create a type class that has multiple methods, the initial syntax looks like this:

trait HasLegs[A]:
  extension (a: A)
    def walk(): Unit
    def run(): Unit

A real-world example

For a real-world example of how type classes are used in Scala 3, see the CanEqual discussion in the Multiversal Equality section.

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