@Suppress("NON_PUBLIC_PRIMARY_CONSTRUCTOR_OF_INLINE_CLASS", "UNCHECKED_CAST")
inline class Option<A> private constructor(val value: Any?) {

    inline fun <R> map(op: (A) -> R): Option<R> =
        if (this.value === EmptyOptionValue) {
            this as Option<R>
        } else {
            Option.just(op(this.value as A))
        }

    inline fun filter(op: (A) -> Boolean): Option<A> =
        if (this.value === EmptyOptionValue || !op(this.value as A)) {
            none()
        } else {
            this
        }

    inline fun <R> fold(ifPresent: (A) -> R, ifEmpty: () -> R): R =
        if (this.value === EmptyOptionValue) {
            ifEmpty()
        } else {
            ifPresent(this.value as A)
        }

    fun getOrNull(): A? =
        if (this.value === EmptyOptionValue) {
            null
        } else {
            this.value as A
        }

    companion object {
        private val EMPTY = Option<Any>(EmptyOptionValue)

        fun <A> none() = EMPTY as Option<A>

        fun <A> just(value: A) = Option<A>(value)
    }
}

/**
 * Marker for empty options.
 * This is necessary to allow nullable types in the option.
 * TODO: Somehow don’t expose this.
 */
object EmptyOptionValue