Learning Kotlin: Invoke

Today, we tackle a weird operator, invoke, which lets an instance of a class have a default function that I am not sure I've ever seen any language do. So, let us explore this with a simple example. We have a config class that returns the configuration for something, like this:

class Config {
fun get():String {
// do stuff
return "stuff"
}
}

fun main(args: Array<String>) {
val config = Config()
println(config.get())
}

Now, in our world, maybe get is the primary use, so we can actually make it that the instance config (line 9) can be called to get it:

class Config {
operator fun invoke(): String {
return this.get();
}

private fun get():String {
// do stuff
return "stuff"
}
}

fun main(args: Array<String>) {
val config = Config()
println(config())
}

Note that we add a new operator (line 2), and that calls the private get. It didn't need to be private, but I thought this would be cleaner, and now on line 14, we can just call the instance itself.

Now, you may be thinking, "nice, but so what — saving a few keystrokes isn't too awesome." Well, invoke can return anything, including itself, which opens up something crazy!

class Config {
var count = 0;
operator fun invoke(): Config {
count++
return this
}
}

fun main(args: Array<String>) {
val config = Config()
config()()()()()()()()()()
println("config was called ${config.count} times")
}

This will print out config was called 10 times. That is a lot more interesting, so let us ramp up another level and pass parameters to invoke:

class Config {
var word = ""
operator fun invoke(s: String): Config {
word += s
return this
}
}

fun main(args: Array<String>) {
val config = Config()
config("R")("o")("b")("e")("r")("t")
println(config.word)
}

While I do not know yet where I would use this myself, I do use invoke all the time. Since it is what makes lambdas possible in Kotlin because when we create a lambda, we get an object that is invoked with, well, invoke.