How to create a Traversal for a List of arbitrary length

[JohnMcGuinness]

Since there's no usable documentation for this library I'll ask a question here.

I have a Cart which may have an Item in it. An Item has a Discount, which may be NoDiscount or PercentDiscount.

public record Cart(String id, Item item, int quantity)

public record Item(String sku, double price, int leftInStock, Discount discount)

sealed interface Discount permits NoDiscount, PercentDiscount

public record NoDiscount() implements Discount

public record PercentDiscount(double value) implements Discount

I have some Optics:

private static final Lens<Cart, Item> itemL = Lens.lens(Cart::item, (Item i) -> (Cart c) -> new Cart(c.id(), i, c.quantity()));

private static final Lens<Item, Discount> discountL = Lens.lens(Item::discount, d -> i -> new Item(i.sku(), i.price(), i.leftInStock(), d)); 

private static final Prism<Discount, Double> onlyPctDiscount
	= Prism.prism(
		d -> {
			if(d instanceof PercentageOff) {
				return Option.some(((PercentageOff) d).value());
			}
			else {
				return Option.none();
			}
		},
		PercentageOff::new
	);

This code recalulates the Item's Discount given a Cart:

itemL.composeLens(discountL).composePrism(onlyPctDiscount).modify(calculation).f(cart);

If I change the Cart to have a List of Item's, to recalculate the discount for each item in the cart I'm pretty sure I need to change that Prism to a Traversal.

How do I create a traversal for a list with any number of Items. From what I can make out FJ only allows Traversals for 2 to 6 items in a List.

[fourofclubs]

Creating a custom Traversal can be tricky, but there is a Traversal for Lists. It's at List.Optic.traversal().
If your Cart changes to include a List of Items, that Prism is still fine, but itemL would need to change to a Lens<Cart, List<Item>> (and probably change the name to itemsL). Then, I believe you should only need to compose an extra traversal:
itemsL.composeTraversal(List.Optic.traversal()).composeLens(discountL).composePrism(onlyPctDiscount).modify(calculation).f(cart)
That should modify each item's discount.