fun main() {
    testMap()
    testFilter()
    testFold()
    shouldHandleNullableTypes()
}

fun testMap() {
    assertEquals(Option.just(3).map { it * 2 }.map { "$it" }.getOrNull(), "6")
    assertEquals(Option.just(listOf(1)).map { it.first() }.getOrNull(), 1)
    assertEquals(Option.none<Int>().map { error("Should not be executed") }.getOrNull(), null)
}

fun testFilter() {
    assertEquals(Option.just(6).filter { it > 5 }.getOrNull(), 6)
    assertEquals(Option.just(6).filter { it < 5 }.getOrNull(), null)
}

fun testFold() {
    assertEquals(Option.just(2).map { it * 2 }.fold({ it / 2 }, { error("Should not be executed") }), 2)
    assertEquals(Option.none<Int>().fold({ error("Should not be executed") }, { 2 }), 2)
}

fun shouldHandleNullableTypes() {
    assertEquals(Option.just<Int?>(null).map { it ?: 5 }.getOrNull(), 5)
    assertEquals(
        Option.just<List<String>>(emptyList())
            .map { it.firstOrNull() }
            .fold({ it }, { error("Should not be executed") }),
        null
    )
}

private fun <T> assertEquals(x: T, y: T) {
    if (x != y) {
        error("$x was not equal to $y")
    }
}