type Item (name : string, price : float) =
    member this.Price =
        price;
    member this.Name =
        name;

type Customer (shoppingList : list<Item>, inBasket : list<Item>) =
    member this.ShoppingList =
        shoppingList;
    member this.Shop =
        this.AddItem(shoppingList.Head);
    member this.AddItem(item : Item) =
        new Customer(List.filter (fun x -> x <> item) shoppingList, List.append inBasket [item]);
    member this.Done =
        shoppingList.IsEmpty

type Checkout (customers : list<Customer>) =
    member this.AddCustomer(customer : Customer) =
        Checkout(List.append customers [customer]);
    member this.Tick =
        // todo - remove items from customer basket, remove customer when empty
        Checkout(customers);

type Supermarket (checkouts : list<Checkout>, customers : list<Customer>) =
    member this.Tick =
        let tickedCustomers = List.map (fun (x:Customer) -> x.Shop) customers;
        let tickedCheckouts = List.map (fun (x:Checkout) -> x.Tick) checkouts;
        let doneCustomers = List.filter (fun (x:Customer) -> x.Done) tickedCustomers;
        let notDoneCustomers = List.filter (fun (x:Customer) -> not x.Done) tickedCustomers;
        let filledCheckouts = this.SendCustomersToCheckouts(doneCustomers, tickedCheckouts);
        Supermarket(filledCheckouts, notDoneCustomers);

    member this.SendCustomersToCheckouts (doneCustomers : list<Customer>, readyCheckouts : list<Checkout>) =
        // todo - add customers to checkouts
        readyCheckouts

let rec simulate (supermarket : Supermarket) =
    simulate supermarket.Tick;

let singleCustomerShoppingList = [Item("Fishycakes", 5.0); Item("Nommycakes", 7.0)];
let singleCustomer = Customer(singleCustomerShoppingList, []);
let singleCheckout = Checkout([]);
let supermarket = Supermarket([singleCheckout], [singleCustomer]);
simulate(supermarket);