So my understanding of extension functions is that you can add a function to an existing library as a static function. I've made this:

fun System.currentTimeSeconds(): Long {
    return System.currentTimeMillis()/1000
 }

and yet I can't do System.currentTimeSeconds(). Am I missing a concept or can you not do extension functions on System?

That will only work on an instance. Like

System().currentTimeSeconds()

The

System()...

doesn't work because <init> is private. Why does the example used here: https://kotlinlang.org/docs/reference/extensions.html not create an instance?

What do you mean? All of these examples operate on instances. There is one example that operates on the companion object.

@Jack Englund the extension functions won’t create static functions. The function created is a normal function which can be invoked on a

instance

of the class you’re extending

There's an issue for it https://youtrack.jetbrains.com/issue/KT-11968

I was primarily looking at the first example in that docs page.

val list = mutableListOf(1, 2, 3)

list.swap(0, 2) // 'this' inside 'swap()' will hold the value of 'list'

That’s exactly what we said, the

swap

function is called on an instance