Hi, I'm just starting to learn Kotlin and I'm a li...

# getting-started

[CoopCoding]

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)
}

[leodeng]

you’ll need to use ::getLengthOfString

[CoopCoding]

Ok, thanks.

[b00m]

what does :: do here?

[Al Warren]

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

[CoopCoding]

@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.

[Shawn]

:: 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.

[Shawn]

:: is used probably because reusing . is ambiguous

[CoopCoding]

Ah, got it, thanks. 🙂

[Al Warren]

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

[Shawn]

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