Kotlin

val name: String? = null
val length = name?.length ?: 0
println(length)

val name: String? = null
val len = name?.length ?: 0
// val crash = name!!.length // throws if name is null

val user = User("Ada")
  .apply { active = true } // returns receiver

val nameLen = user.name.let { it.length } // returns lambda result

coroutineScope {
  val a = async { 1 }
  val b = async { 2 }
  val sum = a.await() + b.await()
  println(sum)
}

sealed interface Result

data class Ok(val value: Int) : Result

data class Err(val message: String) : Result

fun handle(r: Result) = when (r) {
  is Ok -> r.value
  is Err -> 0
}

flow {
  emit(1)
  emit(2)
}.collect { value ->
  println(value)
}

inline fun <reified T> Any?.asTypeOrNull(): T? = this as? T

val x: Any = 123
val n: Int? = x.asTypeOrNull<Int>()

suspend fun loadUser(id: String): User {
  // can suspend here (e.g., awaiting IO)
  return api.getUser(id)
}

open class Base
class Child : Base()

fun Base.say() = "base"
fun Child.say() = "child"

val x: Base = Child()
println(x.say()) // "base" (static dispatch)

val config by lazy { loadConfig() }

fun loadConfig(): String = "ok"

@JvmInline
value class UserId(val value: String)

fun loadUser(id: UserId) = id.value

interface Logger { fun log(msg: String) }

class ConsoleLogger : Logger {
  override fun log(msg: String) = println(msg)
}

class Service(private val logger: Logger) : Logger by logger {
  fun work() = log("working")
}

var name: String by Delegates.observable("<init>") { _, old, new ->
  println("$old -> $new")
}

name = "Ala"