Hands-On Functional Programming with TypeScript

Lenses

We learned about lenses in some of the previous chapters. In one of the last examples, we implemented lenses in a way that is very close to the implementation provided by Ramda.

We implemented a higher-order function named lens that can be used to create a Lens implementation. The lens function takes two functions that must implement the Prop and Assoc interfaces:

interface Lens<T1, T2> {    get(o: T1): T2;    set(o: T2, v: T1): T1;}type Prop<T, K extends keyof T> = (o: T) => T[K];type Assoc<T, K extends keyof T> = (v: T[K], o: T) => T;const lens = <T1, K extends keyof T1>(    getter: Prop<T1, K>,    setter: Assoc<T1, K>,): Lens<T1, T1[K]> => {    return {        get: (obj: T1) => getter(obj),        set: (val: T1[K], obj: T1) => setter(val, obj),    };}const view ...