Generic Functions

fun <T> identity(value: T): T = value

fun main() {
    println(identity(42))
    println(identity("Kotlin"))
}

fun <T> firstTwo(items: List<T>): List<T> = items.take(2)

fun main() {
    println(firstTwo(listOf(1, 2, 3, 4)))
    println(firstTwo(listOf("a", "b", "c")))
}

fun <A, B> combine(a: A, b: B): String = "$a and $b"

fun main() {
    println(combine(1, "one"))
    println(combine(true, 3.14))
}

fun <T> printTwice(value: T) {
    println(value)
    println(value)
}

fun main() {
    printTwice(100)
    printTwice("hello")
}

1. Placing the type parameter after the function name instead of before it; the correct order is fun <T> functionName(...).
2. Assuming a generic function can perform type-specific operations (like arithmetic) on T without any constraint restricting what T can be.
3. Redeclaring a type parameter already present on the enclosing class when it is not actually needed for that particular function.

• A generic function declares its type parameter before the function name: fun <T> functionName(...).
• The type parameter can appear in the parameter list, return type, or body of the function.
• Without constraints, T is treated as Any?, so only operations valid for any type (like storing or returning it) are allowed.
• Generic functions let one implementation work correctly and safely across many different types.