Kotlin Guard

Guard.kt

import kotlin.contracts.ExperimentalContracts
import kotlin.contracts.InvocationKind
import kotlin.contracts.contract

@ExperimentalContracts
inline fun guard(predicate: Boolean, block: () -> Nothing) {
    contract {
        returns() implies predicate
        callsInPlace(block, InvocationKind.AT_MOST_ONCE)
    }

    if (!predicate) block()
}

inline fun <T> guard(receiver: T?, block: () -> Nothing): T {
    if (receiver == null) block()
    return receiver
}

@ExperimentalContracts
fun main() {
    val nullable: String? = null

    guard(nullable != null) {
        return
    }

    nullable.length

    val foo = guard(nullable) {
        return
    }

    foo.length
}