Lens in typescript

lens.ts

export const get = Symbol("get");
export const set = Symbol("set");

interface Lens<T, V> {
  [get]: (o: T) => V;
  [set]: (v: V) => (t: T) => T;
}

const identity = <T>(): Lens<T, T> => ({
  [get]: t => t,
  [set]: v => t => v
});

const property = <T, K extends keyof T>(k: K): Lens<T, T[K]> => ({
  [get]: (t: T): T[K] => t[k],
  [set]: (v: T[K]) => (t: T): T => ({ ...t, [k]: v })
});

type PropertiesLens<P, T> = Lens<P, T> &
  { [K in keyof T]: PropertiesLens<P, T[K]> };

export const properties = <T>(): PropertiesLens<T, T> =>
  propertiesRecursive(identity());

const propertiesRecursive = <P, T>(p: Lens<P, T>): PropertiesLens<P, T> =>
  new Proxy(p, {
    get(o, k) {
      if (k === get) return p[get];
      if (k === set) return p[set];
      else if (typeof k === "string")
        return propertiesRecursive(
          compose(
            p,
            property(k as keyof T)
          )
        );
      else return (o as any)[k];
    }
  }) as PropertiesLens<P, T>;

const compose = <A, B, C>(x: Lens<A, B>, y: Lens<B, C>): Lens<A, C> => ({
  [get]: (a: A) => y[get](x[get](a)),
  [set]: (c: C) => (a: A) => {
    const b: B = x[get](a);
    const b2: B = y[set](c)(b);
    return x[set](b2)(a);
  }
});

// well typed examples

type inner = { x: number };
const innerX = property<inner, "x">("x");

type outer = { y: inner };
const outerY = property<outer, "y">("y");

const num = innerX[get]({ x: 4 });

const outerYinnerX = compose(
  outerY,
  innerX
);

const innerLens = properties<inner>();

const inlineCcomposition = compose(
  properties<outer>().y,
  properties<inner>().x
);

const outerSample = { y: { x: 4 } };

console.log([
  inlineCcomposition[get](outerSample),
  inlineCcomposition[set](8)(outerSample)
]);

console.log([
  properties<outer>().y[get](outerSample),
  properties<outer>().y.x[set](8)(outerSample)
]);

const outerYInnerX = properties<outer>().y.x;
const outerLens = properties<outer>();

console.log([
  outerLens.y[get](outerSample),
  outerLens.y.x[set](8)(outerSample)
]);