throwsFunction.swift

func canPurchaseItem(item: Item, forUser user: User) throws {
    let filtered = allItems.filter { (item: Item) -> Bool in
        return item === item;
    }
    
    if filtered.count == 0 {
        throw PurchaseError.OutOfStockItem
    }
    
    if user.money - item.price < 0 {
        throw PurchaseError.InsufficientFunds
    }
    
    purchaseItem(item, forUser: user)
}