CODEWITHSUNDEEP
kotlin
Ananth Raghuraman
Ananth Raghuraman

Reputation: 89

What is this syntax, parentheses around variable/property names at declaration site?

val (_,time) = time { Thread.sleep(1000) }

I see the Left Hand Side (LHS) has a val, so its declaring a variable. LHS also has some kind of a function syntax which does not look like a lambda declaration. What is (_,time)? Don’t you have to give a type to the time on the LHS? I understand the RHS perfectly well: it is a function which accepts a lambda as parameter and is named ‘time’. Original code

Upvotes: 4

Views: 919

Answers (1)

Adam Arold
Adam Arold

Reputation: 30528

The left hand side is called destructuring.

If you try to assign an instance of a data class (or any class which has componentN functions) to a variable you can desturcture it. This means that you can assign its internals to variables. The _ syntax indicates that you don't care about the first item.

Example:

class Foo(val first: String, val second: String) {

    operator fun component1() = first

    operator fun component2() = second
}

Usage:

val (first, second) = Foo("first", "second")

If you use data classes you don't need to create the componentN functions, they are generated for you.

Equivalent data class:

data class Foo(val first: String, val second: String)

Upvotes: 7

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