A Guide to Scala 3

October 25, 2019 · 8 min read

Muhammad Tabaza

CTO of Pragma

Scala3

Scala 3 comes with many amazing new features. This article attempts to explain the most notable ones, so it is by no means comprehensive. It is, however, a very good introduction to these concepts for beginner to intermediate-level Scala programmers.

At the time of this writing, Scala 3 isn’t actually officially released. However, all the features that would be in Scala 3 are available in Dotty, so we’ll be using it instead.

The examples in this article are on GitHub, and in order to run them, you can clone the repository and run sbt console:

git clone https://github.com/Tabzz98/guide-to-scala3-examples.git
cd guide-to-scala3-examples/
sbt console

note

If you use Visual Studio Code, and you have it on the PATH (i.e. the code command is available globally,) you can run sbt launchIDE to get a better editing experience.

Intersection Types

Consider the following definitions:

trait A  {
  val a: String
}
trait B {
  val b: Int
}

If we think of types in terms of sets, a type would be a collection of elements that satisfy certain properties. For an element to be a member of type A, it has to have the property a of type String. The same goes for elements of type B.

Since types are just sets, what would the intersection of A and B be? Well, it’s the set of elements that satisfy the properties of both A and B. We can denote such a type as A & B (the intersection type of A and B,) and we can use it to specify data types such as the type of the argument c in:

def f(c: A & B) = c.a + " & " + c.b

We can create an instance of A & B by creating a subtype of both A and B:

case class C(a: String, b: Int)
  extends A with B

Which can be passed to f:

scala> f(C("Some String", 42))
val res0: String = Some String & 42

Intersection types are actually the equivalent of compound types, so both A & B and A with B are the same type. However, the with syntax will be deprecated in future versions of Scala.

Union Types

In Scala, we can express that a value can be either of type A or of type B as Either[A, B]. Consider this example:

val a = new A { val a = "Some String" }
val b = new B { val b = 42 }

type E = Either[A, B]
val l: E = Left(a)
val r: E = Right(b)

The Either type has two variants: Left, and Right. In the case of E, Left represents the variant that carries a value of type A, while Right carries a value of type B (notice the order of the type arguments passed to Either.)

Let’s say that we want to define a function g of E. In order to deal safely with values of E, we would need to do some pattern matching:

def g(e: E): String = e match {
  case Left(a) => s"String value a: ${a.a}"
  case Right(b) => s"Int value b: ${b.b}"
}

This is great! We can safely express values that belong to one of two types. But there are a few problems with Either:

It’s not commutative (Either[A, B] is not Either[B, A])
It sucks for working with values that can be of three or more types (I mean, Either[A, Either[B, Either[Float, Boolean]]]? This is a nightmare!)

While still thinking of types as sets, let’s think of Either[A, B] as the union of the sets A and B (the set containing all elements of both A and B.) This union can be expressed as A | B:

type U = A | B

Union types solve the two problems listed above, and they’re actually more pleasant to look at and deal with. Consider the definition of h:

def h(v: A | Double | B | Boolean): String = v match {
  case a: A => s"String value a: ${a.a}"
  case d: Double => s"Double value d: $d"
  case b: B => s"Int value b: ${b.b}"
  case bool: Boolean => s"Boolean value bool: $bool"
}

Which can be applied with simple arguments (no need to wrap in Left or Right.) For example:

scala> h(true)
val res0: String = Boolean value bool: true

scala> h(a)
val res1: String = String value a: Some String

scala> h(42.22)
val res2: String = Double value d: 42.22

Enums

Enums are definitions of types’ values by name. They’re useful when a value of a particular type can be one of a well-defined finite set of elements. For example, the definition of WeekDay:

enum WeekDay {
  case Sunday, Monday, Tuesday, Wednesday,
       Thursday, Friday, Saturday
}

Enum values can be parameterized in order to implement algebraic data types (products, to be specific.) In Scala 2.x, we encode ADTs as case classes (or tuples,) and sealed traits. Let’s consider a definition of logical expressions:

sealed trait VerboseLogicalExpression {
  import VerboseLogicalExpression._
  def eval: Boolean = this match {
    case ConstFactor(c) => c
    ase NotFactor(c) => !c
    case Term(l, Some(r)) => l.eval && r.eval
    case Term(l, None) => l.eval
    case Expr(l, Some(r)) => l.eval || r.eval
    case Expr(l, None) => l.eval
  }
}
object VerboseLogicalExpression {
  sealed trait Factor extends VerboseLogicalExpression
  case class ConstFactor(value: Boolean) extends Factor
  case class NotFactor(value: Boolean) extends Factor
  case class Term(left: Factor, right: Option[Factor])
    extends VerboseLogicalExpression
  case class Expr(left: Term, right: Option[Term])
    extends VerboseLogicalExpression
}

Now we can evaluate the expression:

scala> import VerboseLogicalExpression._

scala> Expr(Term(ConstFactor(true), Some(NotFactor(false))), Some(Term(ConstFactor(false), None))).eval
val res0: Boolean = true

Which is just true ∧ ¬false ∨ true. Notice how we’ve had to define a case class for each variant of VerboseLogicalExpression, and made it extend the trait. We also did the same with Factor just to get the behavior of ConstFactor | NotFactor.

Using enums, we can define logical expressions as:

enum LogicalExpression {
  case ConstFactor(value: Boolean)
  case NotFactor(value: Boolean)
  case Term(
    left: ConstFactor | NotFactor,
    right: Option[ConstFactor | NotFactor]
  )
  case Expr(left: Term, right: Option[Term])
  def eval: Boolean = this match {
    case ConstFactor(c) => c
    case NotFactor(c) => !c
    case Term(l, Some(r)) => l.eval && r.eval
    case Term(l, None) => l.eval
    case Expr(l, Some(r)) => l.eval || r.eval
    case Expr(l, None) => l.eval
  }
}

Much cleaner! Note that using the apply method of an enum’s variant would return a value of the enum type, not the specific case type. We can use new to use the constructor of the specific type. So we would define a value of type LogicalExpression.Expr as:

import LogicalExpression._

val expr = new Expr(
  new Term(new ConstFactor(true), Some(new NotFactor(false))),
  Some(new Term(new ConstFactor(false), None))
)

Givens

Givens are definitions that can be used implicitly. In many ways, they are a more refined version of Scala 2.x’s implicits. Consider this example:

trait Add[T] {
  def add(x: T, y: T): T
}

given Add[C] {
  def add(x: C, y: C) = C(x.a + y.a, x.b + y.b)
}

Here, we define a given instance of an Add[C]. This is a more straightforward way of implementing type classes.

note

Givens do not need to have names, since their type is all that matters in most cases.

We can define functions that have given parameters to summon any defined given instance:

def zipAdd[T](xs: List[T], ys: List[T])(given a: Add[T]): List[T] =
  xs.zip(ys).map(a.add)

scala> zipAdd(List(C("Welcome ", 18), C("Scala ", 0)), 
     |        List(C("to", 22), C("3", 2)))
val res0: List[C] = List(C(Welcome to,40), C(Scala 3,2))

Similarly, we can define given value aliases:

given Int = 42

def addMagicNumber(x: Int)(given magicNumber: Int) = x + magicNumber

scala> addMagicNumber(2)
val res0: Int = 44

Implicit conversions are replaced with the definition of a given instance of Conversion[T, U]. For example, we can define an implicit conversion from String to A as:

given Conversion[String, A] {
  def apply(s: String) = new A { val a = s }
}

scala> h(a) // `a` is converted from A to String
val res0: String = String value a: Some String

scala> :type h
A | Double | B | Boolean => String

Using wildcard syntax in imports will not import any given definitions. Instead, you must use .given syntax, or import the given definitions by name. See here for more information about imports.

Extension Methods

Scala 3 offers a really simple way to add new methods to existing types. Consider the following example:

def (x: T) +[T] (y: T)(given a: Add[T]) = a.add(x, y)

This definition (when in scope,) adds the + method to any type T for which a given instance of Add[T] is defined, such as the type C.

scala> C("Yay ", 2) + C("Simplicity!", 40)
val res0: C = C(Yay Simplicity!,42)

In 2.x, you would have to define an implicit class with the extension method defined on it to achieve this. This is a much simpler approach.

Conclusion

There still are many features not covered here: typeclass derivation, match types, and an entirely new macro system, just to name a few. This article would need to be much longer than this in order to cover all of them. Luckily, the documentation offers a great introduction to these concepts, even though it might be lacking in some regards, but that’ll surely get better with the official release of Scala 3.

Recent posts

note

Intersection Types#

Union Types#

Enums#

Givens#

note

Extension Methods#

Conclusion#

Intersection Types

Union Types

Enums

Givens

Extension Methods

Conclusion