Object-Oriented Meets Functional

Have the best of both worlds. Construct elegant class hierarchies for maximum code reuse and extensibility, implement their behavior using higher-order functions. Or anything in-between.

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Scala began life in 2003, created by Martin Odersky and his research group at EPFL, next to Lake Geneva and the Alps, in Lausanne, Switzerland. Scala has since grown into a mature open source programming language, used by hundreds of thousands of developers, and is developed and maintained by scores of people all over the world.
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Scala in a Nutshell

 click the boxes below to see Scala in action! 

Seamless Java Interop

Scala runs on the JVM, so Java and Scala stacks can be freely mixed for totally seamless integration.

Type Inference

So the type system doesn’t feel so static. Don’t work for the type system. Let the type system work for you!

& Distribution

Use data-parallel operations on collections, use actors for concurrency and distribution, or futures for asynchronous programming.


Combine the flexibility of Java-style interfaces with the power of classes. Think principled multiple-inheritance.

Pattern Matching

Think “switch” on steroids. Match against class hierarchies, sequences, and more.

Higher-Order Functions

Functions are first-class objects. Compose them with guaranteed type safety. Use them anywhere, pass them to anything.

class Author(val firstName: String,
    val lastName: String) extends Comparable[Author] {

  override def compareTo(that: Author) = {
    val lastNameComp = this.lastName compareTo that.lastName
    if (lastNameComp != 0) lastNameComp
    else this.firstName compareTo that.firstName

object Author {
  def loadAuthorsFromFile(file: java.io.File): List[Author] = ???
import static scala.collection.JavaConversions.asJavaCollection;

public class App {
    public List<Author> loadAuthorsFromFile(File file) {
        return new ArrayList<Author>(asJavaCollection(

    public void sortAuthors(List<Author> authors) {

    public void displaySortedAuthors(File file) {
        List<Author> authors = loadAuthorsFromFile(file);
        for (Author author : authors) {
                author.lastName() + ", " + author.firstName());

Combine Scala and Java seamlessly

Scala classes are ultimately JVM classes. You can create Java objects, call their methods and inherit from Java classes transparently from Scala. Similarly, Java code can reference Scala classes and objects.

In this example, the Scala class Author implements the Java interface Comparable<T> and works with Java Files. The Java code uses a method from the companion object Author, and accesses fields of the Author class. It also uses JavaConversions to convert between Scala collections and Java collections.

Type inference
scala> class Person(val name: String, val age: Int) {
     |   override def toString = s"$name ($age)"
     | }
defined class Person

scala> def underagePeopleNames(persons: List[Person]) = {
     |   for (person <- persons; if person.age < 18)
     |     yield person.name
     | }
underagePeopleNames: (persons: List[Person])List[String]

scala> def createRandomPeople() = {
     |   val names = List("Alice", "Bob", "Carol",
     |       "Dave", "Eve", "Frank")
     |   for (name <- names) yield {
     |     val age = (Random.nextGaussian()*8 + 20).toInt
     |     new Person(name, age)
     |   }
     | }
createRandomPeople: ()List[Person]

scala> val people = createRandomPeople()
people: List[Person] = List(Alice (16), Bob (16), Carol (19), Dave (18), Eve (26), Frank (11))

scala> underagePeopleNames(people)
res1: List[String] = List(Alice, Bob, Frank)

Let the compiler figure out the types for you

The Scala compiler is smart about static types. Most of the time, you need not tell it the types of your variables. Instead, its powerful type inference will figure them out for you.

In this interactive REPL session (Read-Eval-Print-Loop), we define a class and two functions. You can observe that the compiler infers the result types of the functions automatically, as well as all the intermediate values.

val x = Future { someExpensiveComputation() }
val y = Future { someOtherExpensiveComputation() }
val z = for (a <- x; b <- y) yield a*b
for (c <- z) println("Result: " + c)
println("Meanwhile, the main thread goes on!")

Go Concurrent or Distributed with Futures & Promises

In Scala, futures and promises can be used to process data asynchronously, making it easier to parallelize or even distribute your application.

In this example, the Future{} construct evaluates its argument asynchronously, and returns a handle to the asynchronous result as a Future[Int]. For-comprehensions can be used to register new callbacks (to post new things to do) when the future is completed, i.e., when the computation is finished. And since all this is executed asynchronously, without blocking, the main program thread can continue doing other work in the meantime.

abstract class Spacecraft {
  def engage(): Unit
trait CommandoBridge extends Spacecraft {
  def engage(): Unit = {
    for (_ <- 1 to 3)
  def speedUp(): Unit
trait PulseEngine extends Spacecraft {
  val maxPulse: Int
  var currentPulse: Int = 0
  def speedUp(): Unit = {
    if (currentPulse < maxPulse)
      currentPulse += 1
class StarCruiser extends Spacecraft
                     with CommandoBridge
                     with PulseEngine {
  val maxPulse = 200

Flexibly Combine Interface & Behavior

In Scala, multiple traits can be mixed into a class to combine their interface and their behavior.

Here, a StarCruiser is a Spacecraft with a CommandoBridge that knows how to engage the ship (provided a means to speed up) and a PulseEngine that specifies how to speed up.

Pattern matching
// Define a set of case classes for representing binary trees.
sealed abstract class Tree
case class Node(elem: Int, left: Tree, right: Tree) extends Tree
case object Leaf extends Tree

// Return the in-order traversal sequence of a given tree.
def inOrder(t: Tree): List[Int] = t match {
  case Node(e, l, r) => inOrder(l) ::: List(e) ::: inOrder(r)
  case Leaf          => List()

Switch on the structure of your data

In Scala, case classes are used to represent structural data types. They implicitly equip the class with meaningful toString, equals and hashCode methods, as well as the ability to be deconstructed with pattern matching.

In this example, we define a small set of case classes that represent binary trees of integers (the generic version is omitted for simplicity here). In inOrder, the match construct chooses the right branch, depending on the type of t, and at the same time deconstructs the arguments of a Node.

Go Functional with Higher-Order Functions

In Scala, functions are values, and can be defined as anonymous functions with a concise syntax.

val people: Array[Person]

// Partition `people` into two arrays `minors` and `adults`.
// Use the higher-order function `(_.age < 18)` as a predicate for partitioning.
val (minors, adults) = people partition (_.age < 18)
List<Person> people;

List<Person> minors = new ArrayList<Person>(people.size());
List<Person> adults = new ArrayList<Person>(people.size());
for (Person person : people) {
    if (person.getAge() < 18)

In the Scala example on the left, the partition method, available on all collection types (including Array), returns two new collections of the same type. Elements from the original collection are partitioned according to a predicate, which is given as a lambda, i.e., an anonymous function. The _ stands for the parameter to the lambda, i.e., the element that is being tested. This particular lambda can also be written as (x => x.age < 18).

The same program is implemented in Java on the right.

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What's New

date icon Friday, May 19, 2017

The Scala Center team is extremely happy to announce that Scastie is out of beta! That means anyone can use Scala in the browser over at scastie.scala-lang.org!

What is Scastie?

Scastie is… Scala + sbt in your browser! You can:

  • use any version of Scala (scalac), or even alternate backends such as Dotty, Scala.js, Scala Native, and Typelevel Scala.
  • use any library published on Maven.
  • save and share Scala programs/builds with anybody

This is all possible because Scastie is based on sbt, allowing us to support newer Scala versions and resolve library dependencies on Maven.

Want to see what Scastie can do? Check out our ray tracer demo using Scala.js on Scastie below!

scastie screenshot

How does Scastie work?

When a user evaluates their code, the browser sends all its input to our server. We run your code in an isolated Java Virtual Machine on our servers. We integrated a scaladex interface to allow you to search the Scala ecosystem and include any published Scala library in your project. You don’t need to remember what the latest version number of a specific library is anymore!

The output will be streamed back to the user. A specific protocol will allow the client to interpret different events such as compiler errors, runtime exceptions, instrumentation, console output, etc.

We also enable a worksheet mode, which feels much like a worksheet in an IDE. This lets a user write code as top-level expressions, without having to put code inside of a class or object with a main method. Worksheet mode gives you two ways to interleave your results; on the one hand, when an expression is evaluated, you can see the value and type of the evaluated expression to the right of the code that you wrote. On the other hand, worksheet mode also makes it possible to do a kind of literate programming; that is, you may interleave code and HTML blocks much like in notebook environments such as iPython notebooks.

New features

So, what’s new since we released our original beta a couple of months ago? Other than a stabler Scastie with fewer bugs, we have a brand new UI, support for sharing snippets, and support for Scala.js!

New UI

We collaborated with 47Deg to provide a fresh UI.


The main purpose of Scastie is to help people learn and communicating with code examples.

In the last few months, we’ve worked hard to provide a new sharing model. Now it’s possible to save, fork, and update snippets! Just like JSFiddle, you can now fork an existing snippet, change it however you like, and then login with GitHub to save it!

When logged in through GitHub, you can now create new snippets or edit existing ones.


Since the intial release of our beta, we also added support for Scala.js! In Scala.js mode, once your code is compiled, your snippet is run in your browser.

HTML DOM elements referred in your code such as images appears in the editor for convenience. This feature is illustrated in this example: such doge!.

Since the execution environment is your browser, you can set breakpoints in your javascript inspector. Source mapping is enabled and allowing you to step into your Scala code. This video illustrates how you can debug Scala.js.

You can try it out yourself: ray tracer (Li Haoyi is the author of this demo)

What’s next

Google Summer of Code

Dmitrii Petukhov was accepted as a Google Summer of Code student to work on Auto-completion and type information. Rory Graves, an ENSIME contributor, is the mentor for this project. ENSIME is an IDE server. It can power text editors such as vim an eclipse with IDE functionnalities such as autocompletion.

Scala Center’s Plan

The Scala Center will continue the development of Scastie. We want to improve the integration with Scala projects. We will add an embedded mode were you can include Scastie in your documentation. We also want to use existing data sources such as StackOverflow or GitHub to have more code examples.

Talk to us!

Thoughts or opinions about Scastie? Join us over on Scala Contributors to contribute to the discussion.

Brought to you by

Scastie is an original idea from Aleh Aleshka (OlegYch).

scalacenter logo Scala Center is covering the hosting cost and the engineering work. 47deg logo 47Deg worked on the UI design.


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