Non-local returns in Kotlin

nlr.kt

// Sometimes you are inside a lambda and want to return a value from some surrounding function (or surrounding lambda)

fun foo(): Any? {
    return withNLR(default = 25) {
        listOf(1, 2, 3).forEach {
            if (it > 2) doReturn(it)
        }
    }
}

// Here's the implementation of withNLR():

private class NonLocalReturnException(val r: Any?): RuntimeException()

public class NonLocalReturnAvailable<R> {
    public fun doReturn(r: R): Nothing = throw NonLocalReturnException(r)
}

public fun withNLR<R>(default: R, body: NonLocalReturnAvailable<R>.() -> Unit): R {
    try {
        NonLocalReturnAvailable<R>().body()
        return default
    }
    catch (e: NonLocalReturnException) {
        [suppress("UNCHECKED_CAST")]
        return e.r as R
    }
}