focusAtom.ts

// --- beauty begins ---

const createLens = (get, set) => ({ get, set });

const view = (obj, lens) => lens.get(obj)

const compose = (lenses) => {
  return createLens(
    (obj) => lenses.reduce(view, obj),
    (value, obj) =>
      lenses.reduceRight((acc, lens, index) => 
         lens.set(acc, lenses.slice(0, index).reduce(view, obj)), value)
  );
};

// --- bureaucracy begins ---

const shallowCopy = (obj) =>
  Array.isArray(obj)
    ? obj.slice()
    : Object.create(Object.getPrototypeOf(obj), Object.getOwnPropertyDescriptors(obj))

const prop = (key) => createLens(
  (obj) => obj[key], 
  (value, obj) => {
    const nextObj = shallowCopy(obj)
    nextObj[key] = value
    return nextObj
  }
);

// --- craziness begins ---

const SELECTOR_RESULT = Symbol()

const selectorProxyHandler = {
  get: (path, key) => {
    if (key === SELECTOR_RESULT) return path
    return new Proxy([...path, key], selectorProxyHandler)
  },
}

const selectorProxy = new Proxy([], selectorProxyHandler)

// --- destiny begins ---

const withSelector = (fn, selector) => {
  let prev = Symbol()
  return (value) => {
    const next = selector(value)
    if(next !== prev) {
      fn(next)
      prev = next
    }
  }
}

const atomMap = new WeakMap()
const getAtomFromCache = (atom, path) => {
  // ...
}

const focusAtom = <T, U>(atom: Atom<T>, selector: PropertyKey[] | PropertyKey | ((s: T) => U)) => {
  const path = typeof selector === 'function' 
    ? selector(selectorProxy)[SELECTOR_RESULT] 
    : typeof selector === 'string' 
      ? [selector]
      : selector

  // const maybeCachedAtom = getFocusAtomFromCache(atom, path)
  // if(maybeCachedAtom) return maybeCachedAtom

  const lens = compose(path.map(prop))

  return {
    get: () => selector(atom.get()),
    set: (value) => atom.set(lens.set(value, atom.get())),
    subscribe: (fn) => atom.subscribe(withSelector(fn, selector))
  }
}

// --- usage ---

const atom = atom({ deeply: { nested: ['hello', 'world'] } })

const nestedAtom = focusAtom(atom, (s) => s.deeply.nested[1])

console.log('nestedAtom.get()', nestedAtom.get())

nestedAtom.set('dunya')

console.log('nestedAtom.set("dunya")', atom.get())