Swift Functional Programming - Second Edition

Lenses

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, ...