Kotlin — Order model (data classes, sealed types, when)

// An order model in Kotlin: data classes, an enum, a sealed result type,
// extension functions, and when expressions.

enum class Status { PENDING, SHIPPED, DELIVERED }

data class LineItem(val name: String, val qty: Int, val unitCents: Int) {
    val totalCents: Int get() = qty * unitCents
}

data class Order(val id: String, val status: Status, val items: List<LineItem>)

sealed interface Result<out T> {
    data class Ok<T>(val value: T) : Result<T>
    data class Err(val message: String) : Result<Nothing>
}

fun Order.totalCents(): Int = items.sumOf { it.totalCents }

fun Order.summary(): String = when (status) {
    Status.PENDING -> "Order $id is awaiting shipment"
    Status.SHIPPED -> "Order $id is on the way"
    Status.DELIVERED -> "Order $id was delivered"
}

fun checkout(order: Order): Result<Int> =
    if (order.items.isEmpty()) Result.Err("empty order")
    else Result.Ok(order.totalCents())

fun main() {
    val order = Order(
        id = "A-1001",
        status = Status.SHIPPED,
        items = listOf(LineItem("Notebook", 3, 450), LineItem("Pen", 10, 120)),
    )

    println(order.summary())
    when (val result = checkout(order)) {
        is Result.Ok -> println("total cents: ${result.value}")
        is Result.Err -> println("error: ${result.message}")
    }
}