A lens is just a pair of functions that allow us to get and set a value in an object. The interface of a lens could be declared as follows:
interface Lens<T1, T2> { get(o: T1): T2; set(o: T2, v: T1): T1;}
As we can see in the previous code snippet, the lens generic interface declares two methods. The get method can be used to get the value of a property of type T2 in an object of type T1. The set method can be used for the value of a property with type T2 in an object of type T1. The following code snippet implements the Lens interface:
const streetLens: Lens<Address, Street> = { get: (o: Address) => o.street, set: (v: Street, o: Address) => new Address(o.city, v)};
The preceding implementation of the Lens interface is named streetLens ...
