Quick and dirty approach to lenses

lenses.ts

type Lens<O, V> = {
  get: (o: O) => V
  set: (value: V) => (o: O) => O,
  update: (fn: (current: V) => V) => (o: O) => O
}
  
type Homework = {done: number};

type Person = {
  age: number;
  homework: Homework
}

function compose<O, A, B>(lens1: Lens<O, A>, lens2: Lens<A, B>): Lens<O, B> {
  return {
    get(o) {
      return lens2.get(lens1.get(o))
    },
    set(value) {
      return o => {
        const a = lens2.set(value)(lens1.get(o));
        return lens1.set(a)(o);
      }
    },
    update(fn) {
      return o => {
        const b = fn(this.get(o));
        return this.set(b)(o);
      }
    }
  }
}

function createLens<O>() {
  return <K extends keyof O>(key: K): Lens<O, O[K]> => ({
    get(o) {
      return o[key]
    },
    set(newValue) {
      return (o) => Object.assign({}, o, { [key]: newValue })
    },
    update(fn) {
      return o => this.set(
        fn(this.get(o))
      )(o)
    },
  })
}

const person1: Person = { age: 27, homework: { done: 0 } };

const doneLens = compose(createLens<Person>()('homework'), createLens<Homework>()('done'))

doneLens.set(2)(person1); // {age: 27, homework: {done: 2}}
doneLens.update(count => count+1)(person1); // {age: 27, homework: {done: 1}}