Kotlin Help

Returns and jumps

Kotlin has three structural jump expressions:

  • return by default returns from the nearest enclosing function or anonymous function.

  • break terminates the nearest enclosing loop.

  • continue proceeds to the next step of the nearest enclosing loop.

All of these expressions can be used as part of larger expressions:

val s = person.name ?: return

The type of these expressions is the Nothing type.

Break and continue labels

Any expression in Kotlin may be marked with a label. Labels have the form of an identifier followed by the @ sign, such as abc@ or fooBar@. To label an expression, just add a label in front of it.

loop@ for (i in 1..100) { // ... }

Now, we can qualify a break or a continue with a label:

loop@ for (i in 1..100) { for (j in 1..100) { if (...) break@loop } }

A break qualified with a label jumps to the execution point right after the loop marked with that label. A continue proceeds to the next iteration of that loop.

Return to labels

In Kotlin, functions can be nested using function literals, local functions, and object expressions. Qualified return s allow us to return from an outer function. The most important use case is returning from a lambda expression. Recall that when we write the following, the return -expression returns from the nearest enclosing function - foo:

//sampleStart fun foo() { listOf(1, 2, 3, 4, 5).forEach { if (it == 3) return // non-local return directly to the caller of foo() print(it) } println("this point is unreachable") } //sampleEnd fun main() { foo() }

Note that such non-local returns are supported only for lambda expressions passed to inline functions. To return from a lambda expression, label it and qualify the return:

//sampleStart fun foo() { listOf(1, 2, 3, 4, 5).forEach lit@{ if (it == 3) return@lit // local return to the caller of the lambda - the forEach loop print(it) } print(" done with explicit label") } //sampleEnd fun main() { foo() }

Now, it returns only from the lambda expression. Often it is more convenient to use implicit labels, because such a label has the same name as the function to which the lambda is passed.

//sampleStart fun foo() { listOf(1, 2, 3, 4, 5).forEach { if (it == 3) return@forEach // local return to the caller of the lambda - the forEach loop print(it) } print(" done with implicit label") } //sampleEnd fun main() { foo() }

Alternatively, you can replace the lambda expression with an anonymous function. A return statement in an anonymous function will return from the anonymous function itself.

//sampleStart fun foo() { listOf(1, 2, 3, 4, 5).forEach(fun(value: Int) { if (value == 3) return // local return to the caller of the anonymous function - the forEach loop print(value) }) print(" done with anonymous function") } //sampleEnd fun main() { foo() }

Note that the use of local returns in the previous three examples is similar to the use of continue in regular loops.

There is no direct equivalent for break, but it can be simulated by adding another nesting lambda and non-locally returning from it:

//sampleStart fun foo() { run loop@{ listOf(1, 2, 3, 4, 5).forEach { if (it == 3) return@loop // non-local return from the lambda passed to run print(it) } } print(" done with nested loop") } //sampleEnd fun main() { foo() }

When returning a value, the parser gives preference to the qualified return:

return@a 1

This means "return 1 at label @a" rather than "return a labeled expression (@a 1)".

Last modified: 06 October 2021