optional.kt

sealed class Optional<out T: Any> {
    abstract val isPresent: Boolean
}
object None : Optional<Nothing>() {
    override val isPresent: Boolean = false
}
data class Some<T: Any>(val value: T) : Optional<T>() {
    override val isPresent: Boolean = true
}
fun <T: Any> T?.asOptional(): Optional<T> = this?.let(::Some) ?: None
val <T: Any> Optional<T>.value: T? get() = when (this) { is Some -> value else -> null }
fun <T: Any> Optional<T>.asIterable(): Iterable<T> = listOfNotNull(value)
inline fun <T: Any> Optional<T>.filter(predicate: (T) -> Boolean): Optional<T> =
    value?.takeIf(predicate)?.let { this } ?: None
inline fun <T: Any, U: Any> Optional<T>.flatMap(mapper: (T) -> Optional<U>): Optional<U> =
    value?.let(mapper) ?: None
inline fun <T: Any> Optional<T>.ifPresent(consumer: (T) -> Unit) { value?.let(consumer) }
inline fun <T: Any, U: Any> Optional<T>.map(mapper: (T) -> U): Optional<U> =
    flatMap { Some(mapper(it)) }
inline fun <T: Any> Optional<T>.orElse(other: () -> T): T = value ?: other()