What is a "receiver"?

[Wasabi375]

@Mervil

Now when it comes to “this” vs “it”: these are just shortcut keywords. “This” is used to refer to the object or “receiver” that an extended function is called on, while “it” is used to refer to a parameter being passed in a lambda/anonymous function. Do I have that right?

Yes.

@Mervil

They clearly refer to different things, but both are still shortcuts available to make code more concise. I’m assuming that “this” and “it” can NOT be used in the same block of code, is that correct? For example, with Kotlin’s standard functions, like Let, “this” isn’t available because the scope is on the receiver as a parameter, not an object, whereas “it” can be used to refer to the parameter which is the receiver itself. But for functions like apply and run, “this” can be used because the receiver isn’t being passed into the lambda as a parameter, but they are acting on the receiver as an object, whereas “it” can’t be used because there is no parameter scoped in the apply or run function call.
Am I getting it?

Yes and no. For for functions like let and run this is right. You can construct a function which takes a lambda with both it and this.

fun foo(code: String.(Int) -> Unit){
    "test".code(5)
}

fun main(){
   foo {
       println(this)
       println(it)
   }
}

Here the code lambda has a reciever (of type String) and a single parameter (of type Int). So you can mix this and it but I can’t remember any real world example where this is done.

---

So the reciever is the part in an extension function before the function name. It pretends to be the class the function belongs to. It can either be accessed with this but same as for normal methods you can omitt the this keyword for properties like this.foo or function calls like this.bar().

It is another special case. Normaly when you create a lambda you need to name the parameters of that lambda

someList.filter { entry -> entry !=  foo }

However if a lambda only has a single parameter it will have a default name it.

someList.filter { it != foo }

If you want to you can still give it any name you like with the above syntax.