typescript optics

lens.ts

type Lens<State, Value> = {
  (_: State): Value
  set: (_: Value) => (_: State) => State

  map<T>(_: Lens<Value, T>): Lens<State, T>
}

type Prop<K extends string, V = any> = Lens<Record<K, V>, V> 

const prop = <K extends string, V>(key: K, defaultVal?: V): Prop<K, V> => {
  const get = (obj: any) => obj[key] || defaultVal
  const set = (val: any) => (obj: any) => ({ ...obj, [key]: val })
  
  return lens(get, set) as any
}

const lens = <S, T>(
  get: (obj: S) => T,
  set: (val: T) => (_: S) => S
): Lens<S, T> => {
  const lens1 = (obj: S) => get(obj)
  
  return Object.assign(lens1, { set, map:
    <V extends any>(lens2: Lens<T, V>) => composeLenses(lens1 as any, lens2)
  })
}

const lensMap = <P extends string[]>(...props: P): {
  [K in P[number]]: Prop<K>
} => Array<any>({}, ...props).reduce(
  (map, key) => ({ ...map, [key]: prop(key, {}) })
)

const composeLenses = <X, Y, Z>(lens1: Lens<X, Y>, lens2: Lens<Y, Z>): Lens<X, Z> => {
  const get = (obj: X) => lens2(lens1(obj))
  const set = (val: Z) => (obj: X) => lens1.set(lens2.set(val)(lens1(obj)))(obj)
  
  return lens(get, set) as any
}