I've got a `List<Int>` that I want to map ov...

# getting-started

[Chris Cordero]

I've got a List<Int> that I want to map over and do some calculations on. Since I'm learning Kotlin, I thought I'd try to do it two ways. This one works:

val foo: List<Int> = listOf(100, 200, 300)
foo.map { it / 3 - 2 } .forEach { println(it) }

However, I wanted to see what to do if I wanted to make a reusable transform function inside the map . So I tried this:

fun myMapFunc(it: Int): Int {
    return it / 3 - 2
}

val foo: List<Int> = listOf(100, 200, 300)
foo.map(myMapFunc).forEach { println(it) } // error

I already know I can fix it by doing this:

foo.map { myMapFunc(it) }

But I feel like the first way should work too... Why doesn't it?

[Shawn]

it’s because unlike python and similar other languages, function references must be deliberately qualified in order to be passed as an argument or assigned to a variable

[Shawn]

try foo.map(::myMapFunc)

[Chris Cordero]

That works

[Chris Cordero]

Some follow up questions:
1. What do you mean by "deliberately qualified"?
2. What is the :: doing?

[Chris Cordero]

I can see here: https://kotlinlang.org/docs/reference/keyword-reference.html#operators-and-special-symbols

That the :: means that it is "creating a member reference or class reference", but I'm not sure what that means here exactly.

[Shawn]

So the way symbols work in Kotlin is kinda sorta similar to how it’s done in Java, which doesn’t treat methods or classes as first-class tokens

[Shawn]

if you want to pass in data about a class’s type to a function, you can’t just use the class’s name, like foo(Bar)

[Shawn]

you can pass in foo(Bar::class) which will generate a KClass<Bar> for you and send that to the function

[Shawn]

i.e. a class reference

[Shawn]

this isn’t just to mirror how Java does things — using a class name without :: will actually let you reference a class’s companion object if one is defined on it

[Shawn]

as for the member reference bit, technically speaking, Kotlin allows you to represent a reference to any member of a given class (or top-level declarations in a Kotlin file, which technically compiles to a class on the JVM anyhow)

[Shawn]

references to variables aren’t supported yet, but you can reference method/functions as well as properties (variables that are class members and have accessors generated for them) just fine (you really should just have a read of this whole page https://kotlinlang.org/docs/reference/reflection.html)

[Shawn]

in a very loose, imperfect sense :: is kinda like using * to get a pointer to something in C/C++

[Shawn]

(this also has some implications for representing references to constructors but I won’t go down that path unless you ask lol)

[Chris Cordero]

lol that was a great explanation thank you very much for the detailed answer

[Chris Cordero]

No worries on the constructor thing, I’ve got enough to let marinade in my head