Kotlin: Whats does “return@” mean?

In Kotlin, the return@label syntax is used for specifying which function among several nested ones this statement returns from.

It works with function literals (lambdas) and local functions. Non-labeled return statements return from the nearest (i.e. innermost) enclosing fun (ignoring lambdas). Consider this function:

fun foo(ints: List<Int>) {
    ints.forEach {
        if (it == 0) return
        print(it)
    }
}

^{Here, return will finish the execution of foo, not just the lambda.}

But if you want to return from any other function (a lambda or an outer fun) you have to specify it as a label at return statement:

fun foo(ints: List<Int>) {
    ints.forEach {
        if (it == 0) return@forEach // implicit label for lambda passed to forEach
        print(it)
    }
}

fun foo(ints: List<Int>): List<String> {
    val result = ints.map f@{
        if (it == 0) return@f "zero" // return at named label
        if (it == -1) return emptyList() // return at foo
        "number $it" // expression returned from lambda
    }
    return result
}

foo(listOf(1, -1, 1)) // []
foo(listOf(1, 0, 1)) // ["number 1", "zero", "number 1"]

Non-local return (i.e. return from outer functions) from a lambda is only supported for local and inline functions, because if a lambda is not inlined (or a function is placed inside an object), it is not guaranteed to be called only inside the enclosing function (e.g. it can be stored in a variable and called later), and the non-local return would make no sense in this case.

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There is also a similar syntax for qualified this, which is used to reference receivers of outer scopes: this@outer.