Implements a guard function for Kotlin that acts similar to the Swift keyword

GuardDsl.kt

import java.lang.RuntimeException

inline fun guard(predicate: Boolean, orElse: () -> Nothing) {
    contract {
        returns() implies predicate
        callsInPlace(orElse, InvocationKind.AT_MOST_ONCE) // probably redundant
    }
    
    if (!predicate) {
        orElse()
    }
}

fun foo(a: Int?) {
    guard(a != null) {
        return@foo // Option A
        return // Option A'
        throw RuntimeException("blerg") // Option B
    }

    println(a + 1) // smart cast!
}

See also a discussion on the Kotlin forums.