不知道为什么 collection fold 和 reduce 不等价

Question

I am going through the Kotlin Koans course. And I am in the fold section .

GIVEN

data class Shop(val name: String, val customers: List<Customer>)

data class Customer(val name: String, val city: City, val orders: List<Order>) {
    override fun toString() = "$name from ${city.name}"
}

data class Order(val products: List<Product>, val isDelivered: Boolean)

data class Product(val name: String, val price: Double) {
    override fun toString() = "'$name' for $price"
}

data class City(val name: String) {
    override fun toString() = name
}

and

fun Shop.getOrderedProducts(): Set<Product> =
    customers.flatMap(Customer::orders).flatMap(Order::products).toSet()

The question asks me to do the following: Return the set of products that were ordered by all customers. Here is my answer, which for whatever reason is not accepted ( would you point the error? )

// Return the set of products that were ordered by all customers
fun Shop.getProductsOrderedByAll(): Set<Product> =
    customers.flatMap(Customer::orders)
        .map(Order::products)
        .reduce{ acc, el -> (acc.intersect(el)).toList()}
        .toSet()

Here is the correct answer:

// Return the set of products that were ordered by all customers
fun Shop.getProductsOrderedByAll(): Set<Product> {
    val allProducts = customers.flatMap { it.getOrderedProducts() }.toSet()
    return customers.fold(allProducts, { orderedByAll, customer ->
        orderedByAll.intersect(customer.getOrderedProducts())
    })
}

While I don't dispute their answer -- I can follow it without problem -- I don't see why mine isn't correct.

Answer 1

I finally figured it out. I was assuming that each customer is equivalent to one order, but that's not the case. Each customer may place multiple orders. And the question is to intersect products per customer -- not product per order