[marcellus23]

I don't really understand what makes that better than something like (from e.g. Swift):

    func double(x: Int) -> Int { x * 2 }

It just seems less readable because it's introducing not only an additional syntax for declaring functions, but also a secondary meaning for the "=" operator.

edit: in my original version of this comment I forgot to add the return type syntax, "-> Int", which Swift needs. I suppose eliding the return type is a bit more terse. You could also assign a closure in Swift to avoid that, like:

    let double = { (x: Int) in x * 2 }

which is about as short as the Kotlin version, and has the advantage of not being "special" in any sense from any other Swift syntax (i.e., it's just assigning a standard closure to a variable).