Extension functions

• Kotlin provides the ability to extend a class with new functionality without having to inherit from the class.
• For example, you can write new functions for a class from a third-party library that you can’t modify.
• Such functions are available for calling in the usual way as if they were methods of the original class.
• This mechanism is called extension functions.
• There are also extension properties that let you define new properties for existing classes.

//define extension function
fun MutableList<Int>.swap(index1: Int, index2: Int) {
    val tmp = this[index1] // 'this' corresponds to the list
    this[index1] = this[index2]
    this[index2] = tmp
}

//call extension function
val list = mutableListOf(1, 2, 3)
list.swap(0, 2) // 'this' inside 'swap()' will hold the value of 'list'

//generic example
fun <T> MutableList<T>.swap(index1: Int, index2: Int) {
    val tmp = this[index1] // 'this' corresponds to the list
    this[index1] = this[index2]
    this[index2] = tmp
}