Option<A> is a container for an optional value of type A. If the value of type A is present, the Option<A> is an instance of Some<A>, containing the present value of type A. If the value is absent, the Option<A> is an instance of None.

Option.kt

sealed interface Option<out A> {
    data class Some<out B>(val value: B) : Option<B>

    object None : Option<Nothing>
}

fun <A> some(value: A): Option<A> {
    return Option.Some(value)
}

fun none(): Option<Nothing> {
    return Option.None
}

fun <A> Option<A>.ifNone(fallbackValue: A): A {
    return if (this is Option.Some) value else fallbackValue
}

fun <A, B> Option<A>.map(fa: (A) -> B): Option<B> {
    return if (this is Option.Some) some(fa(value)) else none()
}

fun <A> A?.toOption(): Option<A> {
    return if (this == null) none() else some(this)
}

fun <A> Option<A>.toNullable(): A? {
    return if (this is Option.Some) value else null
}

inline fun <A> Option<A>.ifSome(block: (A) -> Unit) {
    if (this is Option.Some) {
        block(value)
    }
}