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Software engineer at OVO. FP nerd.



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Scala 3 - and what it means to me

Why read this?

There are a whole bunch of changes that come with Scala 3. This is my opinionated selection of the noteworthy changes and how I feel about them! If you (and your team) are anything like me (and mine) then hopefully this will be relevant to you!



First of all I want to say a huge thank you to the folks who have been working hard on Scala 3 and that I’m excited to start enjoying the benefits in my everyday work.

Scala 3 is built on a new foundation called Dotty. The name Dotty comes from Dependent Object Types (DOT) which is the calculus for path-dependent types. The important take-away is that after 8 years of experience refining Scala, the team behind the language have specifically designed Dotty to model a Scala-like language and become a strong new foundation that will enable Scala to be the language they want it to be. DOT is also simpler than the previous foundation, and places emphasis on accessibility and safety.

This post is based on Dotty version 0.22.

For more information checkout the Dotty docs

Dropped and changed features - some "warts" removed

Automatic Eta Expansion

This makes me so happy! πŸ˜„

A fundamental requirement for a functional language is that you can pass around functions in just the same way as data. Now, because scala makes the distinction between methods (which are not objects) and functions (which are), there are certain situations where you need to tell the compiler you want to convert a method to a function so that it can be passed around.

Scala 2:

def foo(a: Int) = a + 1         // This is a method

val fooFunction = foo _         // This is a function

The requirement to add an _ is awkward and confusing for new Scala developers. Thankfully that requirement has gone.

Scala 3:

val fooFunction = foo           // This is a function

You can read more about this wart in Lihaoyi's excellent blog Scala Warts - Weak Eta Expansion

Auto Eta Expansion - detailed reference

Bye-bye package object

Package objects are no longer needed since all kinds of definitions can now be written at the top-level.

package helloworld

def add(x: Iny, y: Iny) = x + y

final case class Person(name: String, age: Int)

val bob = Person("bob", 42)

This also makes me happy. πŸ˜„

The restriction that everything you write needs to live inside an object, class or trait was not needed, and package objects were clearly just a special case of object that attempted to alleviate this problem. This is nice, more warts removed.

πŸ•ŠοΈ To migrate πŸ•ŠοΈ

You will not be required to migrate initially, in Scala 3.0 package objects are only deprecated. That said, it shouldn't be too hard!

package fish

package object nemo {
    val friend = "Dory"


package fish
package nemo

val friend = "Dory"

I expect there will be tools to do this automagically.

Dropped: Package Objects - reference details

Limit 22

For most people reaching this limit has been a rare thing and often easy to workaround - though my team has encountered this problem (in config loading).

This makes me smile. πŸ™‚

In dotty we can go beyond Tuple22 and Function22. So we can all start writing our apps like this

def makeEarth(
    init:           Planet,
    mountains:      Planet => Planet,
    oceans:         Planet => Planet,
    trees:          Planet => Planet,
    grass:          Planet => Planet,
    insects:        Planet => Planet,
    fish:           Planet => Planet,
    bears:          Planet => Planet,
    badgers:        Planet => Planet,
    snakes:         Planet => Planet,
    tigers:         Planet => Planet,
    caves:          Planet => Planet,
    lakes:          Planet => Planet,
    volcanoes:      Planet => Planet,
    geysers:        Planet => Planet,
    deserts:        Planet => Planet,
    crystals:       Planet => Planet,
    buriedTreasure: Planet => Planet,
    coal:           Planet => Planet,
    people:         Planet => Planet,
    food:           Planet => Planet,
    roads:          Planet => Planet,
    swimmingPools:  Planet => Planet,
    sheep:          Planet => Planet,
    dragons:        Planet => Planet,
    pillows:        Planet => Planet
): Planet = ???

Dropped: Limit 22 - reference

Tuples concat!

(1, "thing", 3.14, false) ++ ("more", "and", 2, "more")
> (1, "thing", 3.14, false, "more", "and", 2, "more")

Enough said! ❀️

XML literals

Deprecated, to be replaced with xml string interpolation:

Scala 2:

val people = 
	<person firstName="John" lastName="Smith" age={{john.age}} gender="M" />
	<person firstName="Ada" lastName="Lovelace" age={{ada.age}} gender="F" />

Scala 3:

val people = xml"""
	<person firstName="John" lastName="Smith" age=${john.age} gender="M" />
	<person firstName="Ada" lastName="Lovelace" age=${ada.age} gender="F" />

For those who use XML literals it might come as a blow. It seems the reason to drop this is for simplification - including XML literals in the language places a great burden on the language that some feel is not justified, and instead string interpolation can achieve the same end result, albeit without some of the compile-time safety.

For me and my team, this has no impact, but it is certainly noteworthy. See this discussion on the original proposal.
The TLDR is that it has been shown that the Lift framework still compiles after a re-write and using a third party interpolator. Though there is still some way to go to prove that it all still works correctly.

The official Dotty XML interpolator

No more Auto-Application

This is now an error

def next(): Fish = ...
next // no longer expanded by the compiler to "next()"

I like this. πŸ‘

I like it because code will become more consistent. The parens () should be reserved for side-effecting code. So typically methods that return Unit.
This conforms to the uniform access principle that allows properties to be either fields val x or methods def x without affecting the calling code.
Therefore, side-effects are not properties and should not be permitted to be accessed as properties.

object Thing {
    def iChangeTheWorld(): Unit

Thing.iChangeTheWorld() // Not possible to write: "Thing.iChangeTheWorld" So the parens () tell us that this code has an effect

With this restriction in place the decision to write your methods with or without () is something that will become natural and make code easier to read.

In fact, there are even mistakes in the Scala 2 libraries - in particular def toInt() !

πŸ•ŠοΈ To Migrate πŸ•ŠοΈ

Your code can still be compiled in Dotty under -language:Scala2Compat.

When paired with the -rewrite option, the code will be automatically rewritten to conform to Dotty's stricter checking.

Dropped: Auto-Application - reference

2 New features!

Types - intersection and union

Intersection types

trait HasAge { def age: Int }
trait Named { def name: String }

def introduce(ab: HasAge & Named): String =  {
    s"My name is ${ab.name}, I am ${ab.age} years old"

Well, we sort of already have this. Whenever you write with you create a Compound Type.

trait CooksBread { def cook(bread: Bread): Toast }
trait Pops { def pop: Sound }

trait Toaster extends CooksBread with Pops

But as it stands, Compound Types do not obey some properties, notably the commutativity law.

What this means is that these are not always the same:

trait AB extends A with B
trait BA extends B with A
// AB != BA    :(

It's not typically a deal breaker, but you can appreciate that fixing this is a nice simplification. πŸ‘

The problem comes when A and B share the same member (e.g. both have a def children). Scala 3 solves this by making

  1. the new member's return type the intersection of its parent's types (see children)
  2. forcing you to override conflicting members (see name)
trait A {
  def children: List[A] 
  def name: String = "A"
trait B {
  def children: List[B] 
  def name: String = "B"
class C extends A with B {
  def children: List[A & B] = List.empty
  override def name: String = "C"
val x: A & B = new C
val ys: List[A & B] = x.children
x.name // "C"

Since List is covariant the intersection of List[A] and List[B] is List[A & B]. Pretty nifty

Intersection types - reference

Union Types

This follows a similar story to the intersection types above. In short, we can provide unions with |

val eitherStringOrInt: String | Int = if (condition) "Fish" else 0
val eitherManWomanOrChild: Man | Woman | Child = 
  if (age <  18) Child() else if (mansplaining) Man() else Woman()

eitherManWomanOrChild match {
  case x: Man => x.eat(toast)
  case x: Woman => x.eat(toast)
  case x: Child => x.eat(toast)
} // everyone can eat toast ;) but you have to know who you are to do it Β―\_(ツ)_/Β―

Unions Types are similar to Either or Coproduct in that they allow you to handle more than one distinct possible types. An Either is right-biased and has a Monad, so provides different uses and is more powerful. I'm sure union have their place, however, as yet I am unsure where I would use them - perhaps when following the Principle of Least Power,
when I don't require the added benefits of an Either.


The Enumeration type in Scala 2 is problematic to the point that nobody uses it. Instead we use sealed trait defined ADTs and rely
on libraries like enumeratum to get nice enum behaviour.

Current problems:

  1. Enumerations have the same type after erasure
  2. There’s no exhaustive compile-time matching check
  3. They don’t inter-operate with Java’s enum

Essentially it looks like Scala 3 implements enum in much the same way as enumeratum as a sealed trait, but making this
a first class language feature is a nice win as a developer.

I spend less time writing boilerplate, adding dependencies and explaining to newcomers why we even need to do it in the first place.

Libraries should also provide more consistent support - it becomes less of an "optional" thing to provide support for enumeratum.

enum Color {
  case Red, Green, Blue

We can use enum for ADTs and Generalized Algebraic Data Types (GADTs).

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

Awesome sauce πŸ‘πŸ‘πŸ‘

Enum - reference

Implicits are bad, mkay

So implicits are really a mixed bag. Also known as term inference, the construct is present in other languages like Haskell, Rust and Swift and may be coming to Kotlin, C# and F#. Scala wouldn't be as popular as it is today without them.

So what's the big deal?

Summary of current issues with Scala implicits

  1. Being very powerful, implicits are easily over-used and mis-used
  2. Over-reliance on implicit imports leads to inscrutable type errors
  3. The keyword implicit is used for a large number of language constructs, requiring the reader to decipher what the intent is in each case
  4. Implicit parameters are awkward - e.g. they must have a name, even though in many cases that name is never referenced
  5. Implicits pose challenges for tooling. The set of available implicits depends on context, so command completion has to take context into account.

If you've been writing Scala for a while you may have learnt the common use cases for implicits and are happy with them as they are, but for newcomers, learning Scala can become a mountain to climb.

This section could easily be a whole blog post by itself, for that reason I will only give a brief summary of some of the new syntax.

given and with

A given instance is conceptually the same as an implicit value.

We use the keyword with to specify that we require a given instance.

def max[T](x: T, y: T) with (ord: Ord[T]): T =
  if (ord.compare(x, y) < 0) y else x

This max function can be read as:
"the max of 2 arguments x and y having type T, with an Ordering for T, is found by comparing x and y"

We can define the given instance for the ordering of ints:

given intOrd as Ord[Int] {
  def compare(x: Int, y: Int) =
    if (x < y) -1 else if (x > y) +1 else 0

and use it in either of 2 ways:

package instances.int

given intOrd as Ord[Int] ...
max(2, 3)
max(2, 3).with(intOrd) // less common

importing given instances must be done like so:

import instances.int.{given Ord[Int]} // imports only intOrd (from above) by referencing it's type
import instances.int.{given _} // imports all the given instances in the package

Typically when importing "given instances" it makes more sense to import them by their type than by their name. The name of a given instance is often unimportant, since only the correct type is required for its application. For the same reason, it is not required for given instances to have a name at all.

given Ord[Int] {

πŸ•ŠοΈ To Migrate πŸ•ŠοΈ

Old style implicits can still be imported as they currently are, however they will also be imported through the new given style of imports. Gradually this old style will be deprecated (Scala 3.1) and then in a later version will give a compiler error. This will give libraries and users the chance to migrate smoothly.


Simply summons a given instance.

Scala 2

val mySummonedIntOrd = implicitly[Ord[Int]]

Scala 3

val mySummonedIntOrd = summon[Ord[Int]]

Extension methods - say goodbye to boilerplate

Extension methods allow one to add methods to a type after the type is defined. Example:

case class Person(name: String, age: Int)

def (p: Person).description: String = s"${p.name} is ${p.age} years old"

// usage
val bob = Person("Bob", 40)
bob.description // "Bob is 40 years old"

If you are familiar with Typeclasses (which are an important mechanism for libraries such as Cats), you will know that we often need to define Syntax classes which serve to make writing code easier by converting ordinary values to a partially applied typeclass instance.

Extension methods make this easier

Extension method to add compare as an infix method. Requires a given instance of Ord, and is generic on type T

object syntax {
    @infix def [T](x: T) compare (y: T) with (ord: Ord[T]): Int = ord.compare(x, y)

import syntax._
import instances.int.{given _}

1 compare 5 // -1

In a nutshell this has become much easier to do. πŸ‘πŸ‘


If you're not sure what a typeclass is, don't be put off. This is actually a fairly simple pattern to define behaviour for a single type. We've already seen it in previous examples.

Scala 2

// typeclass
trait Ord[T] {
    def compare(x: T, y: T): Int
// a typeclass instance
implicit val intOrd: Ord[Int] = new Ord[Int] {
  def compare(x: Int, y: Int) =
    if (x < y) -1 else if (x > y) +1 else 0

Scala 3

// typeclass
trait Ord[T] {
    def compare(x: T, y: T): Int
// a typeclass instance
given Ord[Int] {
  def compare(x: Int, y: Int) =
    if (x < y) -1 else if (x > y) +1 else 0

Essentially there is not a lot of change here. Traits already did a good job of representing a typeclass.

Declaring typeclass instances involves less boilerplate now. πŸ‘


Scala 3 introduces a new way to declare an entry point for a program with the @main annotation.

@main def programEntyPoint(): Unit = {
  println("Hello World")

Optional Braces using with

You may have heard that braces are becoming optional in Scala 3. As a result the keyword with has a new role so that curly braces are not required by class bodies and similar constructs.

Scala 2

class NoBraces() {
  def delete(braces: String): String

Scala 3

class NoBraces() with
  def delete(braces: String): String

Personally I like this. πŸ™‚

What I don't like is now I have a new problem - which should we use in our team's production code and will everyone agree?


Scala 3.0 is coming!

Dotty became Feature Complete December 2019, this paves the way for Scala 3.0. I have struggled to find conclusive indication of when that will be, one source says this Autumn. First of all scala 2.14 will be released with the intention to enable smoother migration to Scala 3.0.

You can start by trying out Dotty now, it's super simple to download and have a play in the REPL - and there are example Dotty projects to check out too.

Migration looks to be simple, with a focus on providing auto-re-write tooling for the few breaking changes.

Developer experience will improve, the language will be more streamlined and less quirky. I expect certain features will open the door to better libraries too.

I for one will try to do what I can to help the community, which has given so much to me, without expecting anything in return.

Software engineer at OVO. FP nerd.

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