Scala 3 Highlights

Notes on what's coming in Scala 3.

CASE meetup, Nov 19, 2020

These are the notes for my talk at CASE. Some of these code examples are in my running series of blog posts on Scala 3. Most are adapted from the book with a few “borrowed” from the Dotty documentation.

For More Information

Programming Scala 3rd Edition Cover

General Comments

Some features are transitional; you can mix old with new in 3.0, but 3.1, etc. will start deprecating and warning about older features.

To get started, an EPFL SBT plugin brings Dotty/Scala 3 support to SBT:

New Syntax

Blog post

You can now use significant indentation (“braceless”), like Python or Haskell, rather than curly braces. You can also mix and match, or use a compiler flag to force one or the other (see below).



// With braces
trait Monoid2[A] {
  def add(a1: A, a2: A): A
  def zero: A

// Without braces
trait Monoid3[A]:     // Notice the colon
  def add(a1: A, a2: A): A
  def zero: A



def m2(s: String): String = {
  val result = s.toUpperCase
  println(s"output: $result")

def m3(s: String): String =   // =, while Python uses : (confusing! ;)
  val result = s.toUpperCase
  println(s"output: $result")

Partial Functions


val o2:Option[Int] => Int = {
  case Some(i) => i
  case None => 0

val o3:Option[Int] => Int =
  case Some(i) => i
  case None => 0

Match Expressions


0 match {
  case 0 => "zero"
  case _ => "other value"

0 match
  case 0 => "zero"
  case _ => "other value"

But “Custom Controls” Don’t Work

Create a custom loop “control”:


import scala.annotation.tailrec

@tailrec def loop(whileTrue: => Boolean)(f: => Unit): Unit =
  if (whileTrue) loop(whileTrue)(f)

var i=5
loop(i > 0) {
  i -= 1

var j=5
loop(j > 0):       // ERROR
  j -= 1

New Control Syntax

There are also new options for control syntax, but whether or not you use them is controlled by compiler flags:


for (i <- 0 until 5) println(i)   // Original syntax
for i <- 0 until 5 do println(i)  // New syntax: no () and `yield/do`
for i <- 0 until 5 yield 2*i
for i <- 0 until 10
  if i%2 == 0
  ii = 2*i
yield ii

val i = 10
if (i < 10) println("yes")        // Original syntax
else println("no")
if i < 10 then println("yes")     // New syntax: no () and `then`
else println("no")

Contextual Abstractions

We begin the migration aware from the implicit mechanism to constructs that more clearly indicate the intent.

Extension Methods Instead of Implicit Conversions.

Blog post

Remember the ArrowAssoc implicit conversion??


implicit final class ArrowAssoc[A](private val self: A) extends AnyVal {
  @inline def -> [B](y: B): (A, B) = (self, y)
  @deprecated("Use `->` instead...", "2.13.0")  
  def [B](y: B): (A, B) = ->(y)  // you did know this is deprecated, right??

It’s much simpler and more direct to use an extension method. The following example shows how to define a ~> method on any type A. The @targetName annotation defines the name generated in byte code for the method (but this is only visible to other languages, like Java, not Scala code!).

NOTE: Use alpha instead of targetName for Scala 3.0.0-M1.


import scala.annotation.targetName

extension [A,B] (a: A):
  // @targetName called @alpha before 3.0.0-M2:
  @targetName("arrow2") def ~>(b: B): (A, B) = (a, b) 

Extension methods are part of a new syntax for type classes, which I’ll cover in a moment.

NOTE: If you also defined an arrow2 method, it would collide with the target name given for ~>.

There are still cases where implicit conversions are useful, e.g., allow users to specify Double values that are converted to domain types, like Dollars and Percentage:


import scala.language.implicitConversions

case class Dollars(amount: Double):
  override def toString = f"$$$amount%.2f"
case class Percentage(amount: Double):
  override def toString = f"${(amount*100.0)}%.2f%%" 
case class Salary(gross: Dollars, taxes: Percentage):
  def net: Dollars = Dollars(gross.amount * (1.0 - taxes.amount))
given Conversion[Double,Dollars] = d => Dollars(d)

given d2P as Conversion[Double,Percentage] = d => Percentage(d) 

val salary = Salary(100_000.0, 0.20)   // Add _ to number literals for clarity!
println(s"salary: $salary. Net pay: ${}") 

Note the new given syntax. This replaces implicit val/def, in general.

Note the name that is synthesized for the first given instance if you don’t explicitly provide a name, given_Conversion_Double_Dollars.

Note that as keyword when the second given instance is named.

By the way, the first definition is shorthand for this:

given Conversion[Double,Dollars]:
  def apply(d: Double): Dollars = Dollars(d)

Even when a given is anonymous, you can use summon[Conversion[Double,Dollars]] to bind to it. The new method summon is identical to implicitly; a new name for a newly-branded concept:

scala> summon[Conversion[Double,Dollars]]
val res68: Conversion[Double, Dollars] = <function1>

Maybe you already noticed that Conversion looks shockingly similar to A => B.

New Type Classes

Blog post

The syntax combines traits (to define the abstraction), regular and extension methods, and given instances (for type class instances):


trait Semigroup[T]:
  extension (t: T):
    def combine(other: T): T
    def <+>(other: T): T = t.combine(other)

trait Monoid[T] extends Semigroup[T]:
  def unit: T

Notice which members are extensions and which ones aren’t! The extension methods will be instance members and the others will be the equivalent of companion object members; we only need one unit per type T.

Create two monoid instances:


given StringMonoid as Monoid[String]:
  def unit: String = ""
  extension (s: String) def combine(other: String): String = s + other

given IntMonoid as Monoid[Int]:
  def unit: Int = 0
  extension (i: Int) def combine(other: Int): Int = i + other

Try them out:


"2" <+> ("3" <+> "4")             // "234"  Associativity works.
("2" <+> "3") <+> "4"             // "234"
StringMonoid.unit <+> "2"         // "2"
"2" <+> StringMonoid.unit         // "2"

2 <+> (3 <+> 4)                   // 9
(2 <+> 3) <+> 4                   // 9
IntMonoid.unit <+> 2              // 2
2 <+> IntMonoid.unit              // 2

We can actual define the monoid instance for all T for which Numeric[T] exists:


given NumericMonoid[T](using num: Numeric[T]) as Monoid[T]:
  def unit: T =
  extension (t: T) def combine(other: T): T =, other)

2.2 <+> (3.3 <+> 4.4)             // 9.9
(2.2 <+> 3.3) <+> 4.4             // 9.9

BigDecimal(3.14) <+> NumericMonoid.unit
NumericMonoid[BigDecimal].unit  <+> BigDecimal(3.14)

Now we see our first example of a using clause, the successor to an implicit parameter list.

Using Clauses

Blog post

We just saw a using clause. They can be anonymous, too. Here’s an (unnecessary ;) wrapper around Seq for sorting them:


case class SortableSeq[A](seq: Seq[A]):                              // <1>
  def sortByImplicits[B](transform: A => B)(implicit o: Ordering[B]): SortableSeq[A] =
    new SortableSeq(seq.sortBy(transform)(o))

  def sortBy1a[B](transform: A => B)(using o: Ordering[B]): SortableSeq[A] =
    new SortableSeq(seq.sortBy(transform)(o))

  def sortBy1b[B](transform: A => B)(using Ordering[B]): SortableSeq[A] =
    new SortableSeq(seq.sortBy(transform)(summon[Ordering[B]]))

  def sortBy2[B : Ordering](transform: A => B): SortableSeq[A] =
    new SortableSeq(seq.sortBy(transform)(summon[Ordering[B]]))

I passed the implicit/given values explicitly to Seq.sortBy for illustration purposes, but of course I could have passed them implicitly (usingly?).

Given Imports

Blog post

To allow use of _ for imports, but not pull in all givens when you don’t want them:


object O1:
  val name = "O1"
  def m(s: String) = s"$s, hello from $name"
  class C1
  class C2
  given c1 as C1
  given c2 as C2

import O1._             // Import everything EXCEPT the givens, c1 and c2
import O1.given         // Import ONLY the givens (of type C1 and C2)
import O1.{given, _}    // Import everything, givens and non-givens in O1
import O1.{given C1}    // Import just the given of type C1
import O1.c1            // Import just the given c1 of type C1

In Scala 3.0, _ will still import everything, for backwards compatibility, but Scala 3.1 will begin transitioning to this behavior.

NOTE: “non-givens” should be called takes IMHO… If you grew up with the King James Bible (1611) in your Baptist church like I did, they would be giveth and taketh

Infix Operator Notation

Because people abuse operator notation, Scala is migrating towards disallowing it, by default, unless:

  1. The method name uses only “operator characters”.
  2. The method is annotated with @scala.annotation.infix.
  3. Usage is followed by a curly brace.
  4. Back ticks are used:

In our previous example:

scala> "2" combine ("3" combine "4")
1 |"2" combine ("3" combine "4")
  |                 ^^^^^^^
  |Alphanumeric method combine is not declared @infix; it should not be used as infix operator.
  |The operation can be rewritten automatically to `combine` under -deprecation -rewrite.
  |Or rewrite to method syntax .combine(...) manually.
1 |"2" combine ("3" combine "4")
  |    ^^^^^^^
  |Alphanumeric method combine is not declared @infix; it should not be used as infix operator.
  |The operation can be rewritten automatically to `combine` under -deprecation -rewrite.
  |Or rewrite to method syntax .combine(...) manually.

scala> "2" combine { "3" combine { "4" } }
val res0: String = 234

scala> "2" `combine` ("3" `combine` "4")
val res1: String = 234

Or, annotate combine with @infix, then you can use "2" combine ("3" combine "4"):


import scala.annotation.infix

trait Semigroup[T]:
  extension (t: T):
    @infix def combine(other: T): T    // @infix allows "foo combine bar".
    def <+>(other: T): T = t.combine(other)

trait Monoid[T] extends Semigroup[T]:
  def unit: T

Now redefine the previous monoid instances with this new definition.

NOTE: the addition of @infix for each concrete combine definition is required!

given StringMonoid as Monoid[String]:
  def unit: String = ""
  extension (s: String) @infix def combine(other: String): String = s + other

given IntMonoid as Monoid[Int]:
  def unit: Int = 0
  extension (i: Int) @infix def combine(other: Int): Int = i + other

Now combine can be used with infix notation as an alternative to <+>:

scala> "2" combine ("3" combine "4")
val res2: String = 234

Easier Enums

I can never remember the Scala 2 syntax for enums. Now I have an even easier syntax to forget!

Adapted from Dotty docs:

enum SimpleColor:
  case Red, Green, Blue  // These effectively `extends SimpleColor`

enum FancyColor(val rgb: Int):
  case Red   extends FancyColor(0xFF0000)
  case Green extends FancyColor(0x00FF00)
  case Blue  extends FancyColor(0x0000FF)

A new way to define ADTs!

enum Option[+T]:
  case Some(x: T)
  case None

NOTE: As commented by Seth Tisue during the meeting, Scala 3 uses the Scala 2 library unchanged, so types like these won’t be changed to enums until some future release.

Type Madness!! (Or Sanity…)

Opaque Type Aliases

Much better than value classes (which were prone to unexpected boxing, etc.). Example adapted from the Dotty docs:

object Logarithms:

  opaque type Logarithm = Double

  object Logarithm:
    // These are the two ways to lift to the Logarithm type
    def apply(d: Double): Logarithm = math.log(d)
    def safe(d: Double): Option[Logarithm] =
      if (d > 0.0) Some(math.log(d)) else None

  // Extension methods define the instance methods for opaque types
  extension (x: Logarithm):
    def toDouble: Double = math.exp(x)
    def + (y: Logarithm): Logarithm = Logarithm(math.exp(x) + math.exp(y))
    def * (y: Logarithm): Logarithm = x + y

Open Classes

No more ad-hoc extensions of concrete types (unless you want ‘em):

// File Writer.scala
package p

open class Writer[T]:   // Fully concrete type, but extension is allowed.
  /** Sends to stdout, can be overridden */
  def send(x: T) = println(x)

  /** Sends all arguments using `send` */
  def sendAll(xs: T*) = xs.foreach(send)

// File EncryptedWriter.scala
package p

class EncryptedWriter[T: Encryptable] extends Writer[T]:
  override def send(x: T) = super.send(encrypt(x))

(Wouldn’t make sense for abstract classes and traits, which must be extended to become concrete…)

What if a type isn’t open, but you want to subclass it in a test to stub methods (i.e., make a test double)? Use import scala.language.adhocExtensions in the test file. This is the advantage over declaring the type final, which provides no mechanism for this sort of “exceptional” extension.

Intersection and Union Types

Intersection Types

Intersection types work much like with for trait mixins:

trait Resettable:
  override def toString:String = "Resettable:"+super.toString
  def reset(): Unit

trait Growable[T]:
  override def toString:String = "Growable:"+super.toString
  def add(t: T): Unit

def f(x: Resettable & Growable[String]): String = 

Note how the argument x for f is declared.

I find it a little confusing, but you have to actually instantiate these types using with:

case class RG(var str: String = "") extends Resettable with Growable[String]:
  override def toString:String = s"RG(str=$str):"+super.toString
  def reset(): Unit = str = ""
  def add(s: String): Unit = str = str + s

case class GR(var str: String = "") extends Growable[String] with Resettable:
  override def toString:String = s"GR(str=$str):"+super.toString
  def reset(): Unit = str = ""
  def add(s: String): Unit = str = str + s

I declared two types, one with Resettable & Growable[String] and the other with Growable[String] & Resettable. They are considered the same type by f. In other words, they commute, just like the intersection operation in set theory. In contrast, Scala 2 treated Resettable with Growable[String] and Growable[String] with Resettable as different types.

Let’s try them!

scala> val rg = new RG
     | val gr = new GR
val rg: RG = RG(str=):Growable:Resettable:rs$line$16$RG@173b3581
val gr: GR = GR(str=):Resettable:Growable:rs$line$16$GR@367164f5

scala> f(rg)     // Both can be passed to `f`, showing commutativity.
     | f(gr)     // But toString shows different ordering of the "supers"!
val res0: String = RG(str=firstsecond):Growable:Resettable:rs$line$16$RG@697f3db1
val res1: String = GR(str=firstsecond):Resettable:Growable:rs$line$16$GR@b3cd3f1e

The :rs$line$... comes from calling super.toString on the object wrapping the code in the REPL! Just ignore it…

Linearization is still used to decide which of an overridden method gets called when you use super.method(...). In this example, super.toString returns different results for Resettable & Growable[String] vs. Growable[String] & Resettable, even though those two types are considered equivalent from the type-checking perspective! Note which trait’s toString got called first for each case. (Hint: linearization is basically right to left ordering.)

WARNING: While A & B and B & A are considered equivalent types, the behaviors of overridden methods may be different, due to linearization.

Union Types

Union types could replace Either[A,B], but they aren’t limited to two nested types. Consider this pseudo DB query example:

case class User(name: String, password: String)

def getUser(id: String, dbc: DBConnection): String | User | Seq[User] = 
    val results = dbc.query(s"SELECT * FROM users WHERE id = $id")
    results.size match 
      case 0 => s"No records found for id = $id"
      case 1 =>[User]
      case _ =>[User])
    case dbe: DBException => dbe.getMessage

getUser(dbc) match
  case message: String => error(message)
  case User(name, password) => ...
  case seq: Seq[User] => ...

Note how pattern matching is necessary here. What you give up are the useful operations on Either, like map, flatMap, etc.

Speaking of match expressions…

Match Types

This can be a bit “quirky”, but it’s a cool feature. (Example adapted from the Dotty docs):

type Elem[X] = X match
  case String => Char
  case Array[t] => t
  case IterableOnce[t] => t
  case ? => X

val char: Elem[String] = 'c'
val doub: Elem[List[Double]] = 1.0
val tupl: Elem[Option[(Int,Double)]] = (1, 2.0)

val bad1: Elem[List[Double]] = "1.0"        // ERROR
val bad2: Elem[List[Double]] = (1.0, 2.0)   // ERROR

summon[Elem[String] =:= Char]  // ...: Char =:= Char = generalized constraint
summon[Elem[List[Int]] =:= Int]
summon[Elem[Nil.type] =:= Nothing]
summon[Elem[Array[Float]] =:= Float]
summon[Elem[Option[String]] =:= String]
summon[Elem[Some[String]] =:= String]
summon[Elem[None.type] =:= Nothing]
summon[Elem[Float] =:= Float]
summon[Elem[Option[List[Long]]] =:= Long]   // ERROR


The book’s code examples use the flag -source 3.1 to force deprecation warnings for older constructs. The default, -source 3.0, is more forgiving.

Flags to control syntax preferences:

  • -indent: Allow significant indentation.
  • -noindent: Require classical {…} syntax, indentation is not significant.
  • -new-syntax: Require then in conditional expressions.
  • -old-syntax: Require (...) around conditions.

Flags to help migration:

  • -language:Scala2: Compile Scala 2 code, highlight what needs updating.
  • -migration: Emit warning and location for migration issues from Scala 2.
  • -rewrite: Attempt to fix code automatically.

Things I Didn’t Cover

  • Trait parameters
  • Completely new macro system
  • @mixin traits
  • export clauses
  • type lambdas
  • kind polymorphism
  • dependent function types (we’ve had dependent method types)
  • New types like Tuple
  • Explicit nulls
  • Safe initialization
  • @main methods
  • No more 22-arity limits on tuples and functions
  • … and lots of smaller refinements