Just Kotlin

What is Kotlin?
A language from JetBrains
100% Interop with Java
Compile to JVM, Android, and JavaScript

Why is Kotlin interesting
A pragmatic approach, basically a better Java -> easier to migrate
It compiles to (among others) Java 6 bytecode -> sweet spot: Android
First class IDE support with conversion tool

Concise Syntax: Type Inference
val a: Int = 1
val b = 1 // `Int` type is inferred
val c: Int // Type required when no initializer is provided
var x = 5 // Mutable, `Int` type is inferred

Concise Syntax: Data Class
public class Person {
final String firstName;
final String lastName;
public Person(...) {
...
}
// Getters
...
// Hashcode / equals
...
// Tostring
...
// Egh...
}
data class Person(
val firstName: String,
val lastName: String
)

Extension Functions
fun MutableList<Int>.swap(index1: Int, index2: Int) {
val tmp = this[index1] // 'this' corresponds to the list
this[index1] = this[index2]
this[index2] = tmp
}
val l = mutableListOf(1, 2, 3)
l.swap(0, 2) // 'this' inside 'swap()' will hold the value of 'l'

Handling null values
var a: String = "abc"
a = null // compilation error
var a: String? = "abc"
a = null // ok
if (a != null) {
print("String of length ${a.length}") //a can’t be null in this scope
}
//val bob: Employee?
bob?.department?.head?.name?:"John Doe"

FP: Lambda
strings.filter { it.length == 5 }
.sortBy { it }
.map { it.toUpperCase() }

FP: Algebraic Data Types
sealed class Maybe<out T> {
object None : Maybe<Nothing>()
class Just<T>(val t: T) : Maybe<T>()
}
//Pattern matching
class User(val age: Int)
fun lookupFromDB(s: String): Maybe<User> = //........
fun printUser(username: String) {
val rec = lookupFromDB(username)
when (rec) {
is Maybe.None -> println("not found")
is Maybe.Just<User> -> println("${rec.t.age} years old")
}
}

FP: Immutability
//class
data class Person(val name: String, val age: Int)
val mike = Person("Mike", 31)
val olderMike = mike.copy(age = 32)
//collection
val people = listOf(mike, olderMike)
people.add(Person("Bob", 50)) // ERROR
val peopleDB = arrayListOf(mike, olderMike)
peopleDB.add(Person("Bob", 50))

FP: Currying and Partial
//curry
val sum2ints = { x: Int, y: Int -> x + y }
val curried: (Int) -> (Int) -> Int = sum2ints.curried()
assertEquals(curried(2)(4), 6)
val add5 = curried(5)
assertEquals(add5(7), 12)
//partial
val format = { prefix: String, x: String, postfix: String -> "${prefix}${x}${postfix}" }
val prefixAndBang = format(p3 = "!")
// Passing just the first parameter will return a new function
val hello = prefixAndBang(p1 = "Hello, ")
println(hello("world"))

Just Kotlin

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