Context Receivers

• 1.6.20
• Implicit Parameters
• Ext Fun with receiver, but MORE

Context Receivers

interface LoggingContext {
    val log: Logger // This context provides a reference to a logger
}

context(LoggingContext)
fun startBusinessOperation() {
    // You can access the log property
    log.info("Operation has started")
}

fun test(loggingContext: LoggingContext) {
    with(loggingContext) {
        // You need to have LoggingContext in a scope
        // to call startBusinessOperation()
        startBusinessOperation()
    }
}
                

Definitely non-nullable types

• 1.6.20
• Declare a generic type as non-nullable
• Esp. useful in Java interop (platform types)
• Uses type intersection

Definitely non-nullable types

fun <T> elvisLike(x: T, y: T): T = x ?: y
fun test(){
    elvisLike("", null) // valid
}
                

Definitely non-nullable types

fun <T : Any> elvisLike(x: T, y: T): T = x ?: y
fun test(){
    val s : String? = null
    elvisLike(s, "") // compile error at s
}
                

Definitely non-nullable types

fun <T> elvisLike(x: T, y: T & Any): T & Any = x ?: y
fun test(){
    val s : String? = null
    elvisLike(s, null) // compile error on null
}
                

Data Object

• 1.7.20
• Like data class, but for objects
• Particularly good in sealed class hierarchies

Data Object

sealed class ReadResult {
    data class Number(val value: Int) : ReadResult()
    data class Text(val value: String) : ReadResult()
    data object EndOfFile : ReadResult()
}

fun main() {
    println(ReadResult.Number(1)) // Number(value=1)
    println(ReadResult.Text("Foo")) // Text(value=Foo)
    println(ReadResult.EndOfFile) // EndOfFile
}
                

Generic Inline Classes

• 1.7.20
• Inline classes can be generic now

Generic Inline Classes

@JvmInline
value class UserId<T>(val value: T)

fun printId(s: UserId<String>) {
    println(s.value)
}

fun printId(s: UserId<UUID>) {
    println(s.value.toString())
}

fun test(){
    printId(UserId(""))
    printId(UserId(UUID.randomUUID()))
}
                

Compiler improvements

• Better Incremental Compilation in Gradle
• Support for parallel compilation of a single module in the IR JVM backend
• K2 Compiler has more features, still Alpha

Adoption

• Kotlin continues to be dominant in the Android space (95% of apps)
• Google announced that Kotlin a first-class backend JVM lang internally
• Google did a study about mixing code
• Mixing ecosystems (Java + C++) adds significant overhead
• Mixing languages within ecosystem (JVM Java + JVM Kotlin) does not
• Spring and Gradle continue to promote Kotlin as a preferred language