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http://jetbrains.com/kotlin 3
What is Kotlin?
• Statically typed
• JVM-targeted
• general-purpose programming language
• developed by JetBrains
➡ intended for industrial use
• Docs available today
• Public beta in a few months
http://jetbrains.com/kotlin 4
Outline
• Motivation
• Feature Overview
• Basic Syntax
• Types and Classes
• Higher-order Functions
• Type-safe Groovy-style Builders
http://jetbrains.com/kotlin 5
Motivation
• Started in mid 2010
• Huge Java codebase (>50k classes, 10 years old)
• Building tools for many nice languages
• Want a modern, expressive language
• …that’s easy to introduce in the existing environment
• Need efficient tooling
• None of the existing languages was a good fit
http://jetbrains.com/kotlin 6
Design Goals
• Full Java interoperability
• Compiles as fast as Java
• More concise than Java
• Prevents more kinds of errors than Java
• Way simpler than Scala
http://jetbrains.com/kotlin 7
Tooling
• IntelliJ IDEA plugin developed in parallel with compiler
• Both compiler and plugin will be released under Apache 2 license
• Basic Eclipse plugin will be provided
http://jetbrains.com/kotlin 8
Innovation
• Not a research project, don’t plan to publish papers
• No ground-breaking language features
• Focus on common sense and best practices established in other languages
➡ Groovy, Scala, C#, Gosu…
• Feature selection tradeoff: expressive yet easy to learn
http://jetbrains.com/kotlin 9
Outline
• Motivation
• Feature Overview
• Basic Syntax
• Types and Classes
• Higher-order Functions
• Type-safe Groovy-style Builders
http://jetbrains.com/kotlin 10
Features: Traditional
• Properties
• First-class functions (closures)
• Pattern matching
• Operator overloading
• Local type inference
http://jetbrains.com/kotlin 11
Features: Distinguishing
• Traits (multiple inheritance of behavior)
• Extension functions
• Static null-safety guarantees
• Implicit casts after type checks
• Inline functions (zero-overhead closures)
• Reified generics
• First-class delegation
• Build infrastructure (modules) as part of the language
http://jetbrains.com/kotlin 12
Outline
• Motivation
• Feature Overview
• Basic Syntax
• Types and Classes
• Higher-order Functions
• Type-safe Groovy-style Builders
http://jetbrains.com/kotlin 13
Hello, world!namespace hello // optional semicolons
// namespace-level functions// types on the right// no special syntax for arraysfun main(args : Array<String>) : Unit { println("Hello, world!")}
fun println(message : String) { // System.out is a nullable field System.out?.println(message)}
14
Hello, <names>!fun main(args : Array<String>) { // type inference for local variables var names = ""
for (i in args.indices) { names += args[i] if (i + 1 < args.size) names += ", " } println("Hello, $names!") // string templates}
val Array<*>.indices : Iterable<Int> get() = IntRange<Int>(0, size - 1)
15
Hello, <names>! (Faster version)
fun main(args : Array<String>) { // ‘val’ means final val names = StringBuilder() // no ‘new’
for (i in args.indices) { names += args[i] if (i + 1 < args.size) names += ", " } println("Hello, $names!")}
// operator overload as an extension functionfun StringBuilder.plusAssign(s : String) { this.append(s)}
16
Hello, <names>! (Realistic version)
fun main(args : Array<String>) { val names = args.join(", ") println("Hello, $names!")}
17
join() and forit()fun <T> Iterable<T>.join(separator : String) : String { val names = StringBuilder() forit (this) { names += it.next() if (it.hasNext()) names += separator } return names.toString()}
fun <T> forit(col : Iterable<T>, f : fun(Iterator<T>) : Unit) { val it = col.iterator() while (it.hasNext()) { f(it) }}
http://jetbrains.com/kotlin 18
Outline
• Motivation
• Feature Overview
• Basic Syntax
• Types and Classes
• Higher-order Functions
• Type-safe Groovy-style Builders
http://jetbrains.com/kotlin 19
Mapping to Java typesKotlin Java Kotlin
Any Object Any?
Unit void Unit
Int int Int
Int? Integer Int?
String String String?
Foo Foo Foo?
Array<Foo> Foo[] Array<Foo?>?
IntArray int[] IntArray?
GEN LOAD
http://jetbrains.com/kotlin 20
Mapping to Java typesKotlin Java Kotlin
Any Object Any?
Unit void Unit
Int int Int
Int? Integer Int?
String String String?
Foo Foo Foo?
Array<Foo> Foo[] Array<Foo?>?
IntArray int[] IntArray?
GEN LOAD
http://jetbrains.com/kotlin 21
Mapping to Java typesKotlin Java Kotlin
Any Object Any?
Unit void Unit
Int int Int
Int? Integer Int?
String String String?
Foo Foo Foo?
Array<Foo> Foo[] Array<Foo?>?
IntArray int[] IntArray?
GEN LOAD
http://jetbrains.com/kotlin 22
Mapping to Java typesKotlin Java Kotlin
Any Object Any?
Unit void Unit
Int int Int
Int? Integer Int?
String String String?
Foo Foo Foo?
Array<Foo> Foo[] Array<Foo?>?
IntArray int[] IntArray?
GEN LOAD
http://jetbrains.com/kotlin 23
Mapping to Java typesKotlin Java Kotlin
Any Object Any?
Unit void Unit
Int int Int
Int? Integer Int?
String String String?
Foo Foo Foo?
Array<Foo> Foo[] Array<Foo?>?
IntArray int[] IntArray?
GEN LOAD
http://jetbrains.com/kotlin 24
Null-safetyfun parseInt(s : String) : Int? { try { return Integer.parseInt(s) } catch (e : NumberFormatException) { return null }}
fun main(args : Array<String>) { val x = parseInt("123") val y = parseInt("Hello") print(x?.times(2)) // can’t say: print(x * 2)
if (x != null) { print(x * 2) // allowed after explicit check }}
http://jetbrains.com/kotlin 25
Implicit casts and Whenfun foo(obj : Any?) { if (obj is String) { obj.substring(2) } when (obj) { is String => obj[0] is Int => obj + 1 !is Boolean => null else => ... }}
http://jetbrains.com/kotlin 26
More on when-expressionsfun bar(x : Int) { when (x) { 1, 2, 3 => "1, 2 or 3" -x => "Zero" in 10..100 => "In range" !in 100..1000 => "Out of range" }}
http://jetbrains.com/kotlin 27
Classesopen class Parent(p : Bar) { open fun foo() {} fun bar() {}}
class Child(p : Bar) : Parent(p) { override fun foo() {} }
•Primary constructor outside of class body
•Constructors must initialize supertypes
•Final by default, explicit open annotations
http://jetbrains.com/kotlin 28
Traitstrait T1 : Class1, OtherTrait { // No state fun foo() : Int = 1 // open by default fun bar() : Int // abstract by default}
class Foo(p : Bar) : Class1(p), T1, T2 { override fun bar() : Int = foo() + 1 }
http://jetbrains.com/kotlin 29
Disambiguationtrait A { fun foo() : Int = 1}
open class B() { open fun foo() : Int = 2}
class C() : B(), A { override fun foo() = super<A>.foo()}
http://jetbrains.com/kotlin 30
First-class Delegationtrait List<T> { fun add(t : T) fun get(index : Int) : T}
class ListDecorator<T>(p : List<T>) : List<T> by p { override fun add(t : T) { log.message("Added $t") super.add(t) }
// override fun get(index : Int) : T = super.get()}
31
Class objects (I)
• Classes have no static members
• Each class may have associated class object:
class Example() { class object { fun create() = Example() }}
val e = Example.create()
32
Class objects (II)
Class objects can have supertypes:
class Example() { class object : Factory<Example> { override fun create() = Example() }}
val factory : Factory<Example> = Exampleval e : Example = factory.create()
33
Class objects (III)
Generic constraints for class objects:
class Lazy<T>() where class object T : Factory<T> { private var store : T? = null public val value : T get() { if (store == null) { store = T.create() } return store }}
http://jetbrains.com/kotlin 34
Reified generics
• Type information is retained at runtime
➡ foo is List<T>
➡ Array<T>(3)
➡ T.create()
• Java types are still erased
➡ foo is java.util.List<*>
http://jetbrains.com/kotlin 35
Outline
• Motivation
• Feature Overview
• Basic Syntax
• Types and Classes
• Higher-order Functions
• Type-safe Groovy-style Builders
http://jetbrains.com/kotlin 36
First-class functions
• Functions
➡ fun f(p : Int) : String
• Function types
➡ fun (p : Int) : String
➡ fun (Int) : String
• Function literals
➡ {p => p.toString()}
➡ {(p : Int) => p.toString()}
➡ {(p : Int) : String => p.toString()}
http://jetbrains.com/kotlin 37
Higher-order functionsfun <T> filter( c : Iterable<T>, f : fun(T) : Boolean) : Iterable<T>
filter(list, {s => s.length < 3})
http://jetbrains.com/kotlin 38
Syntax sugar
• Last function literal argument
filter(list) {s => s.length < 3}
• One-parameter function literal
filter(list) { it.length < 3 }
• Infix calls
a contains b // same as a.contains(b)
http://jetbrains.com/kotlin 39
“LINQ” style syntaxusers .filter { it hasPrivilege WRITE } .map { it => it.fullName } .orderBy { lastName }
http://jetbrains.com/kotlin 40
Lock example (I)myLock.lock()
try {
// Do something
}
finally {
myLock.unlock()
}
http://jetbrains.com/kotlin 41
Lock example (II)
lock(myLock) {
// Do something
}
fun lock(l : Lock, body : fun () : Unit)
http://jetbrains.com/kotlin 42
Lock example (III)inline fun lock(l : Lock, body : fun () : Unit) {
l.lock()
try {
body()
}
finally {
l.unlock()
}
}
http://jetbrains.com/kotlin 43
Extension functions• Functions
➡ fun Foo.f(p : Int) : String
• Function types
➡ fun Foo.(p : Int) : String
➡ fun Foo.(Int) : String
• Function literals
➡ {Foo.(p : Int) => this.toString()}
➡ {Foo.(p : Int) : String => this.toString()}
➡ {Foo.(p : Int) : String => toString()}
http://jetbrains.com/kotlin 44
Outline
• Motivation
• Feature Overview
• Basic Syntax
• Types and Classes
• Higher-order Functions
• Type-safe Groovy-style Builders
http://jetbrains.com/kotlin 45
Builders in Groovyhtml { head { title "XML encoding with Groovy" } body { h1 "XML encoding with Groovy" p "this format can be used as an alternative to XML" /* an element with attributes and text content */ ahref:'http://groovy.codehaus.org' ["Groovy"] }}
http://jetbrains.com/kotlin 46
Builders in Kotlinhtml { head { title { +"XML encoding with Kotlin" } } body { h1 { +"XML encoding with Kotlin" } p { +"this format is now type-safe" } /* an element with attributes and text content */ a(href="http://jetbrains.com/kotlin") { +"Kotlin" } }}
http://jetbrains.com/kotlin 47
Builders: Implementation (I)• Function definition
fun html(init : fun HTML.() : Unit) : HTML { val html = HTML() html.init() return html}
• Usage
html { this.head { ... }}
http://jetbrains.com/kotlin 48
Builders: Implementation (II)• Function definition
fun html(init : fun HTML.() : Unit) : HTML { val html = HTML() html.init() return html}
• Usage
html { head { ... }}
http://jetbrains.com/kotlin 49
Builders: Implementation (III)abstract class Tag(val name : String) : Element { val children = ArrayList<Element>() val attributes = HashMap<String, String>() }
abstract class TagWithText(name : String) : Tag(name) { fun String.plus() { children.add(TextElement(this)) }}
class HTML() : Tag("html") { fun head(init : fun Head.() : Unit) { } fun body(init : fun Body.() : Unit) { }}
http://jetbrains.com/kotlin 50
Builders in Kotlinhtml { head { title { +"XML encoding with Kotlin" } } body { h1 { +"XML encoding with Kotlin" } p { +"this format is now type-safe" } /* an element with attributes and text content */ a(href="http://jetbrains.com/kotlin") { +"Kotlin" } }}
http://jetbrains.com/kotlin 51
Resources
• Documentation: http://jetbrains.com/kotlin
• Blog: http://blog.jetbrains.com/kotlin
• Issue tracker: http://youtrack.jetbrains.net/issues/KT
• Twitter:
@project_kotlin
@intelliyole