The Scala Programming Language

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!

Concurrency
& Distribution

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

Traits

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.

Learn More

or visit the Scala Documentation

Author.scala

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] = ???
}

App.java

import static scala.collection.JavaConversions.asJavaCollection;

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

    public void sortAuthors(List<Author> authors) {
        Collections.sort(authors);
    }

    public void displaySortedAuthors(File file) {
        List<Author> authors = loadAuthorsFromFile(file);
        sortAuthors(authors);
        for (Author author : authors) {
            System.out.println(
                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.

Concurrent/Distributed

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.

Traits

abstract class Spacecraft {
  def engage(): Unit
}
trait CommandoBridge extends Spacecraft {
  def engage(): Unit = {
    for (_ <- 1 to 3)
      speedUp()
  }
  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.

Scala

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)

Java

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)
        minors.add(person);
    else
        adults.add(person);
}

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.

What's New

blog

August SIP Meeting Results: 2 SIPs Rejected and 2 SIPs Updated

Monday, August 15, 2016

On August 10, the Scala Improvement Process (SIP) Committee held their monthly meeting to discuss and give feedback on four proposals, both new and old. We’re happy that these discussions sparked so much interest and participation in the community. It’s comforting to see the new SIP process becoming fruitful!

Of the four discussed proposals, two were unanimously rejected. The other two progressed and will have a follow-up iteration.

The rejected proposals were:

SIP-12: Uncluttering Scala’s syntax for control structures. Originally proposed in 2011. The proposal suggested syntax changes in if’s, for and while loops, moving Scala’s syntax away from Java and C-like languages. Whereas such changes may be arguably more beautiful, the Committee agreed that would give more problems than benefits. Seth Tisue, the appointed reviewer, fully explains the Committee’s reaction here.
SIP-16: Self-cleaning macros. Originally proposed in 2012. Macros add a whole new dimension to the Scala language. Their experimental implementation was adopted by a lot of Scala libraries and were immensely useful for the creation and evolution of first-class Scala tools. As described by his creator and reviewer, Eugene Burmako, they turned out to be a good experiment, but one that could be improved if major downsides in the design were addressed. With the intention of letting scala.meta eventually replace the old macros, he proposed rejecting SIP-16 to revisit its basic foundations and create a new proposal that would considerably enhance the metaprogramming experience in Scala. In the meantime, the experimental implementation will remain in place. If you’re interested in Eugene’s explanation, follow this.

The two proposals that made it to the next iteration are SIP-23: Literal-based singleton types (reviewer: Adriaan Moors) and SIP-27: Trailing Commas (reviewer: Eugene Burmako). The reviewers and committee members provided more feedback to iterate on. Both proposals already have provisional implementations and will continue to be discussed for inclusion in the language.

Overall, we’re happy to report these results and see Scala continue to move forward! We’re excited to see the Scala community speaking up and collaborating with the process—your vibrant responses will greatly enrich our future deliberations.

Next month, we’ll discuss five more proposals, including SIP-21: Spores and SIP-24: Repeated by-name parameters. We hope to finish off the list of old proposals in the queue and focus ourselves on the most recent proposals and the ones that are to come!

Did you know that each month, we conduct these SIP meetings on-air? You can tune in and ask questions to the SIP committee, and have them answered live. Videos of the meetings are then archived on YouTube. Check the minutes of this meeting in the SIP website.

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News archive or Blog

Object-Oriented Meets Functional

Scala in a Nutshell

Seamless Java Interop

Type Inference

Concurrency
& Distribution

Traits

Pattern Matching

Higher-Order Functions

Combine Scala and Java seamlessly

Let the compiler figure out the types for you

Go Concurrent or Distributed with Futures & Promises

Flexibly Combine Interface & Behavior

Switch on the structure of your data

Go Functional with Higher-Order Functions

Upcoming Events

Upcoming Training

What's New

Recently...

Scala on Twitter

Documentation

Download

Community

Contribute

Scala

Object-Oriented Meets Functional

Scala in a Nutshell

Seamless Java Interop

Type Inference

Concurrency& Distribution

Traits

Pattern Matching

Higher-Order Functions

Combine Scala and Java seamlessly

Let the compiler figure out the types for you

Go Concurrent or Distributed with Futures & Promises

Flexibly Combine Interface & Behavior

Switch on the structure of your data

Go Functional with Higher-Order Functions

Upcoming Events

Upcoming Training

What's New

Recently...

Scala on Twitter

Documentation

Download

Community

Contribute

Scala

Concurrency
& Distribution