The missing guard clauses for Kotlin. A simple and elegant way to guard against invariants.

GuardClauses.kt

/**
 * Invokes [otherwise] if the [value] is false.
 */
@OptIn(ExperimentalContracts::class)
inline fun guard(value: Boolean, otherwise: () -> Unit) {
    contract {
        returns() implies value
    }

    if (!value) otherwise()
}

/**
 * Invokes [otherwise] if the [value] is null, or returns the not null [value].
 */
@OptIn(ExperimentalContracts::class)
inline fun <T : Any> guardNotNull(value: T?, otherwise: () -> T): T {
    contract {
        returns() implies (value != null)
    }

    return value ?: otherwise()
}

GuardExamples.kt

var num: Int? = null

// Example with Guard
fun printStateWithNum1(state: String) {
    val notNullNum = guardNotNull(num) { return } 
    guardNotNull(state) { return }
    guard(state.isNotBlank()) { return }
    guard(state.all { it.isLetterOrDigit() }) { return }
    println("$state:$notNullNum")
}

// Example without Guard
fun printWithNum2(state: String) {
    val notNullNum = num ?: return
    if (state != null) return
    if (state.isNotBlank()) return
    if (state.all { it.isLetterOrDigit() }) return
    println("$state:$notNullNum")
}