What language features of Kotlin would you like to see in other programming languages?

Extension functions: Allow adding new functions to existing classes without inheritance or using decorator patterns. Improves code readability and organization.

Null-safety: A type system that explicitly distinguishes nullable and non-nullable types, preventing NullPointerException errors at compile time.

Data classes: Simplified creation of classes for storing data with automatic generation of equals(), hashCode(), toString(), copy().

data class User(val name: String, val age: Int)

While and when expressions: when is a powerful replacement for switch with support for various conditions (types, ranges, boolean conditions), while can be an expression returning the value of the last executed condition.

val result = when (x) {
    1 -> "one"
    in 2..10 -> "from two to ten"
    is String -> "string"
    else -> "something else"
}

Coroutines: Lightweight threads that simplify asynchronous and non-blocking programming. Integrated at the language level.

Named and optional arguments: Enhance code readability when calling functions and allow omitting optional parameters.

fun greet(name: String, greeting: String = "Hello") {
    println("$greeting, $name!")
}

greet(name = "World") // Using default optional argument
greet(name = "World", greeting = "Hi") // Overriding optional argument

Smart casts: The compiler automatically determines the type of a variable after a type check (e.g., is), allowing access to members of that type without explicit casting.

fun printLength(obj: Any) {
    if (obj is String) {
        // Now obj is automatically considered a String within this block
        println("String length: ${obj.length}")
    }
}

Type inference: The compiler can often determine the type of a variable on its own, reducing code verbosity.

val number = 42 // Kotlin infers this as Int
val message = "Hello" // Kotlin infers this as String

Lambda expressions with implicit it parameter: Convenient for simple lambdas with one parameter.

val list = listOf(1, 2, 3)
list.forEach { println(it) } // 'it' is the list element