export interface Lens<A, B> {
  get(root: A): B
  set(root: A, newValue: B): A
}

export function prop<A, K extends keyof A>(key: K): Lens<A, A[K]> {
  return {
    get: (root: A) => (root as any)[key],
    set: (root: A, newValue: any) => ({ ...root, [key]: newValue }),
  }
}

export function item<I>(index: number): Lens<I[], I | undefined> {
  return {
    get: (root: I[]) => (root as any)[index],
    set: (nums: I[], newValue: I | undefined) =>
      newValue === undefined
        ? [...nums.slice(0, index), ...nums.slice(index + 1)]
        : [...nums.slice(0, index), newValue, ...nums.slice(index + 1)],
  }
}

export function identityLens<A>() {
  return {
    get: (root: A) => root,
    set: (root: A, newValue: any) => newValue,
  }
}