Unused arguments when destructing an object in Kotlin

Question

When destructuring an object, is it possible to only declare the variables I need?

In this example I'm only using b and my IDE is giving me a warning that a is unused.

fun run() {
    fun makePair() = Pair("Apple", "Orange")

    val (a, b) = makePair()

    println("b = $b")
}

In other languages, you often have something like `val (_, b) = makePair()`. Nothing similar in Kotlin? — Thilo, Mar 15 '16 at 11:54
Somewhat similar question, and somewhat unsatisfactory answer: http://stackoverflow.com/questions/29046636/mark-unused-parameters-in-kotlin — Thilo, Mar 15 '16 at 11:57
@Thilo Scala has it, but doesn't seem like there is anything like it in Kotlin. At least not yet. — Trygve Laugstøl, Mar 15 '16 at 13:55

yole · Answer 1 · 2017-04-03T09:14:54.810

10

Since Kotlin 1.1, you can use an underscore to mark an unused component of a destructing declaration:

fun run() {
    fun makePair() = Pair("Apple", "Orange")

    val (_, b) = makePair()

    println("b = $b")
}

edited Apr 03 '17 at 09:14

answered Mar 15 '16 at 12:33

yole

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score 4 · Answer 2 · answered Mar 15 '16 at 11:47

4

You could use:

val b = makePair().component2()

answered Mar 15 '16 at 11:47

McDowell

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score 0 · Answer 3 · answered Mar 15 '16 at 16:02

If you're only interested in the first couple of arguments you can omit the remaining ones. In your code that's not possible, but if you change the order of arguments, you can write it this way:

fun run() {
    fun makePair() = Pair("Orange", "Apple")

    val (b) = makePair()

    println("b = $b")
}

Unused arguments when destructing an object in Kotlin

3 Answers3