What does the arrow ("->") operator do in Kotlin?

The -> is a separator. It is special symbol used to separate code with different purposes. It can be used to:

• Separate the parameters and body of a lambda expression

  val sum = { x: Int, y: Int -> x + y }
  

• Separate the parameters and return type declaration in a function type

  (R, T) -> R
  

• Separate the condition and body of a when expression branch

  when (x) {
      0, 1 -> print("x == 0 or x == 1")
      else -> print("otherwise")
  }  
  

Here it is in the documentation.

From the Kotlin docs:

->

• separates the parameters and body of a lambda expression

• separates the parameters and return type declaration in a function type

• separates the condition and body of a when expression branch

The -> is part of Kotlin's syntax (similar to Java's lambda expressions syntax) and can be used in 3 contexts:

• when expressions where it separates "matching/condition" part from "result/execution" block

   val greet = when(args[0]) {
     "Apple", "Orange" -> "fruit"
     is Number -> "How many?"
     else    -> "hi!"
   }
  

• lambda expressions where it separates parameters from function body

    val lambda = { a:String -> "hi!" }
    items.filter { element -> element == "search"  }
  

• function types where it separates parameters types from result type e.g. comparator

    fun <T> sort(comparator:(T,T) -> Int){
    }
  

Details about Kotlin grammar are in the documentation in particular:

• functionType
• functionLiteral
• whenEntry