Hi all - I'm working through the JetBrains Academy course on Kotlin and I have a dumb question. I'm at the early point of looking at the output of

val result = println("text")
println(result)

text
kotlin.Unit

One of the practice questions asks about the Magic Of

println

and it implies that the value of

result

is

Unit

. While that's true, what's "text" doing in the output? Maybe this is something that I don't need to be worried about (and I'm not) but I'm curious why. println is called twice, once for "text", and then a second time for

println

itself, and the type value of the

println

function is

kotlin.Unit

? Is that true for all functions? Just

println

? Thanks for any clarity you're willing to share!

Are you familiar with other languages? Java for example?

Correct,

println()

is called twice here. The first time it prints

text

and the second time it prints the value of the

result

variable, which happens to be

Unit

(aka

kotlin.Unit

in its fully-qualified name). The fact that it prints something is a side-effect of calling the function.

Is that true for all functions? Just

println

?

In Kotlin, functions can return values of many types. The return type of the function is declared when declaring the function. Any function that returns "nothing interesting" usually omits the return type and automatically returns the value

Unit

, which is of type

Unit

(the type with only one value).

println

is one such function. Some functions do return more interesting types, for instance:

fun myCustomSum(a: Int, b: Int): Int {
    return a + b
}

This is a custom function that returns the sum of its 2 arguments

a

and

b

. If you were to print the result of this function, you would not get `kotlin.Unit`:

val result = myCustomSum(3, 5)
println(result) // prints 8

Hi @Vampire not with degree of proficiency. I'm pretty inexperienced.

I didn't ask for the degree, just whether you are. For example, if you know what a

void

method is in Java, then you know what a

Unit

method in Kotlin is

@Joffrey thanks for that response. That helps clarify things, I think!

It might not have been very clear from my first message, but in most programming languages, functions can do 2 types of things: • return a value • perform side-effects (do stuff themselves, like a network call, writing to a file, printing to the screen etc.)

println

is a function that has the side-effect of printing to the console a text representation of the value you pass to it. Not all functions have side effects, though (the ones without side-effect are called "pure functions"). However, all functions in Kotlin have a return type, so they return values (when completing successfully). If a function has nothing interesting to return, it will just return

Unit

.