A simple Option Monad implementation in Kotlin

OptionMonad.kt

package dev.flexicon.monads

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

    object None : Option<Nothing>
}

fun <T> some(value: T): Option<T> = Option.Some(value)

fun <T, R> Option<T>.flatMap(f: (T) -> Option<R>): Option<R> =
    fold({ f(it) }, { it })

fun <T> Option<T>.get(): T? = fold({ it }, { null })

fun <T> Option<T>.getOrThrow(message: () -> String = { "Option is None" }): T =
    fold({ it }, { throw IllegalStateException(message()) })

fun <T, R> Option<T>.fold(onSome: (T) -> R, onNone: (Option.None) -> R): R =
    when (this) {
        is Option.Some -> onSome(value)
        is Option.None -> onNone(this)
    }