What are underscore names in Kotlin reserved for?

Question

In Python you can use _ as a variable name. If I write e.g. val _ = 3 in Kotlin IntelliJ gives me an error with:

Names _, __, ___, ..., are reserved in Kotlin

What are they reserved for? What is their function?

Answer 1

The single underscore is already used in several ways where you want to skip a parameter or a component and don't want to give it a name:

• For ignoring parameters in lambda expressions:

  val l = listOf(1, 2, 3)
  l.forEachIndexed { index, _ -> println(index) }
  

• For unused components in destructuring declarations:

  val p = Pair(1, 2)
  val (first, _) = p
  

• For ignoring an exception in a try-catch statement:

  try {
      /* ... */
  } catch (_: IOException) {
      /* ... */
  }
  

These syntax forms were introduced in Kotlin 1.1, and that's why the underscore names had been reserved before Kotlin 1.1. The multiple-underscore names like __, ___ had also been reserved so that they are not misused where previously one would have used a single-underscore name.

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As @Willi Mentzel noted in a comment, another use of underscores, though not in a position of an identifier, is separating digit groups in numeric literals:

val oneMillion = 1_000_000
val creditCardNumber = 1234_5678_9012_3456L