Generic type <T> parameter BEFORE the function name

Zorgan :

What is the usage of the <T> type parameter before the function name in Kotlin?

Example:

fun <T> MutableList<T>.swap(index1: Int, index2: Int) {
    val tmp = this[index1] 
    this[index1] = this[index2]
    this[index2] = tmp
}

Referring to the first <T> above.

I've tried to look through the Kotlin docs regarding generics as well as the Java Generics however they mostly just touch on the 2nd <T> not the first.

Lino :

It is used to indicate that generics are used and not some type T is referenced.

Have a look at this completly valid example

fun <String> MutableList<String>.swap(index1: Int, index2: Int)

Now this can be called on any MutableList<*> and not only MutableList<String>. If you would not write <String> after the fun keyword, how would kotlin know that in fact you were referencing a generic and not kotlin.String?

The same goes for the example you've shown. The <> after the fun just introduces a new generic parameter, else kotlin would complain that it wouldn't know the type T