the same way you would a method with the signature

fun isWeekend(date: Date): Boolean

That doesn't seem to work, because isn't the isWeekend method in a different scope now? Let's say my class is as follows:

class ExtensionFunctions{
    fun Date.isWeekend() = day == 6 || day == 7
}

I won't be able to test it by just calling

isWeekend(Date())

on the class right?

In this case

Date.isWeekend

is both an extension function of

Date

and a member function of

ExtensionFunctions

, so you need both receivers in scope. In a member-function of

ExtensionFunctions

you can just call

Date().isWeekend()

, outside you would need

with (instanceOfExtensionFunctions) {
    Date().isWeekend()
}

So extension functions can be anywhere? And I will just be able to access them? I think this is the bit of knowledge I am lacking about extension functions... If I were to declarem them in a Kotlin file I should just be able to use them throughout my project? Because this sounds awesome!

Yes, you can make them top-level and they will be available anywhere. Just like normal top-level functions

anywhere in the same file you mean? Not the whole projct right? Say I had a Kotlin file called ExtensionFunctions.kt And its contents were:

import java.util.*

fun main(args: Array<String>) {
    fun Date.isWeekend() = day == 6 || day == 7
}

That's inside another function, that would be local to the function.

Ah! I feel silly now!