Hi team! I understand what extension functions are...

# announcements

[Jordan Foo]

Hi team! I understand what extension functions are, such as adding a newFunction

fun Bar.newFunction() {}

What exactly does this do?

fun Bar.() {}

[Marc Knaup]

That code is not valid 🤔

[Jordan Foo]

It is though! We've had it in production... for a few months and the person that wrote it is no longer at the company haha

[Marc Knaup]

image.png

[Jordan Foo]

Screen Shot 2021-03-18 at 08.23.19.png

[Jordan Foo]

Screen Shot 2021-03-18 at 08.23.35.png

[christophsturm]

thats the return parameter of the method

[Marc Knaup]

As an expression, okay.

It means to create/return a function (closure/lambda) that expects an Application instance to be passed as receiver (this).

[christophsturm]

val x: Bar.() -> Unit = {}

[christophsturm]

thats valid

[Jordan Foo]

ah, so it's a javascript bind

[Jordan Foo]

Thanks!

[Marc Knaup]

val foo1 = fun Bar.() {}
fun Bar.foo2() {}

Is roughly the same. Just that the former is an extension lambda and the second a real extension function.

[Jordan Foo]

Gotcha. Thanks Marc!

[Marc Knaup]

Similar to JS, yes. Just in JS you cannot specify the type of this. Bar in this case.

[Zach Klippenstein (he/him) [MOD]]

i think the kotlin docs distinguish between “anonymous functions” (e.g. fun () {}) and “lambdas” which don’t use the fun keyword, but yea they’re effectively the same thing