In the previous section, we covered copy constructors. Here, we will examine a functional structure called a lens. Simply put, lenses are functional getters and setters that are implemented for a whole object and its parts:

• Getters: We can look through the lens at an immutable object to get its parts
• Setters: We can use the lens to change a part of an immutable object

Let's implement a lens:

struct Lens<Whole, Part> { 
    let get: (Whole) -> Part 
    let set: (Part, Whole) -> Whole 
} 

Let's use it to change our FunctionalProduct object to get and set the producer property:

let prodProducerLens: Lens<FunctionalProduct, Producer> = 
    Lens(get: { $0.producer }, 
         set: { 
             FunctionalProduct(name: $1.name, 
                               price: $1.price,    
                               quantity: $1.quantity, 
                               producer: $0) }) 

Let's change the producer for mexBananas:

let mexBananas2 = prodProducerLens.set(Producer(name: "QAZ",
                                                address: "Yucatan, 
                                                Mexico"), mexBananas) 

Through our lens, we can change it as shown in the preceding code.

Let's examine another example. Suppose that we have a Producer object as follows:

let chineeseProducer = Producer(name: "KGJ", 
                                address: "Beijing, China") 

We want to change the address:

let producerAddressLens: Lens<Producer, String> = 
    Lens(get: { 
             $0.address },
         set: {
             Producer(name: $1.name,
                      address: $0) }) 

let chineeseProducer2 = producerAddressLens.set("Shanghai, China", chineeseProducer) 

Suppose that we had mexBananas2 and needed to have a Chinese banana producer, then we could use:

let chineeseBananaProducer = prodProducerLens.set(producerAddressLens.set("Shanghai, China", chineeseProducer), mexBananas2) 

This syntax does not look very simple, and it seems that we did not gain much after all. In the next section, we will simplify it.