Hi, I'm just starting to learn Kotlin and I'm a little confused by something. How come if I assign a function to a varibale, I am able to pass it in to map like this:

fun main() {
  val strings = listOf("Hello", "World", "!")
  val getLengthOfString = fun(str: String) : Int {
    return str.length
  }
  val lengths = strings.map(getLengthOfString)
  println(lengths)
}

But if the function is a regular function, I cant pass in the function name like so:

fun main() {
  val strings = listOf("Hello", "World", "!")
  fun getLengthOfString(str: String): Int {
    return str.length
  }
  val lengths = strings.map(getLengthOfString)
  println(lengths)
}

you’ll need to use

::getLengthOfString

Ok, thanks.

what does :: do here?

Have a look at this https://kotlinlang.org/docs/reference/reflection.html#function-references

@Al Warren Thanks, I'm still a little fuzzy though. Is

::

basically used to satisfy the types? Also, in most other languages,

::

means bind, is that not the case here? Thanks.

::

isn’t an operator is used to specify function references. It looks a little weird since we’re referencing a function in the local scope. If you wanted to reference a method of a specific class it’d look like

Foo::bar

. Since the function exists within the local scope, there isn’t a name to really qualify its location by, so it comes out to just

::bar

.

Ah, got it, thanks. 🙂

Btw, the docs actually call it an operator. See the link above.

Apologies, I was imprecise in my wording, I meant more that

::

isn’t like bind or otherwise like

plus

or other infix operators - in this case it would fall in the same category as the dot operator or even

->

in C