lens.ts

// Copyright (c) 2020 Jozty. All rights reserved. MIT license.

import type { PH } from './utils/types.ts';
import curryN from './utils/curry_n.ts';

// @types
type LensF_2<T, F> = (setter: LensSetter<T, F>) => Lens<T, F>;

type LensF_1<T, F> = (getter: LensGetter<T, F>) => Lens<T, F>;

type LensF =
  & (<T, F>(getter: LensGetter<T, F>) => LensF_2<T, F>)
  & (<T, F>(getter: PH, setter: LensSetter<T, F>) => LensF_1<T, F>)
  & (<T, F>(getter: LensGetter<T, F>, setter: LensSetter<T, F>) => Lens<T, F>);

export type LensGetter<T, F> = (target: T) => F;

export type LensSetter<T, F> = (focus: F, target: T) => T;

export type GetTransformer<T, F, R> = (
  focus: F,
) => LensTransformer<T, F, R>;

export type LensTransformer<T, F, R> = {
  value: F;
  transform: (
    setter: LensSetter<T, F>,
    target: T,
  ) => LensTransformer<never, R, never>;
};

export type Lens<T, F> = <R>(
  getTransformer: GetTransformer<T, F, R>,
) => (target: T) => LensTransformer<never, R, never>;

function _lens<T, F>(
  getter: LensGetter<T, F>,
  setter: LensSetter<T, F>,
): Lens<T, F> {
  return function <R>(getTransformer: GetTransformer<T, F, R>) {
    return function (target: T) {
      const focusedObj = getter(target);
      const transformer = getTransformer(focusedObj);
      return transformer.transform(setter, target);
    };
  };
}

/**
 * Returns a lens for the given getter and setter functions. The `getter` "gets"
 * the value of the focus; the setter "sets" the value of the focus. The `setter`
 * should not mutate the data structure.
 */
export const lens = curryN(2, _lens) as LensF;