canceler.ts

export interface Canceler {
  (): void;
}

/** No operation canceler which "does nothing".
 *
 * ```js
 * concatCancelers(a, emptyCanceler) ==== concatCancelers(emptyCanceler, a) ==== a
 * ```
 */
export const emptyCanceler = () => {};

/**
 * Concat two cancelers into one which will executed original cancelers from
 * left to right one by one sequentially.
 *
 * `concatCancelers` is associative for any input `a,b,c: Canceler`:
 * ```js
 * concatCancelers(a, concatCancelers(b, c)) ==== concatCancelers(concatCancelers(a, b), c)
 */
export const concatCancelers = (a: Canceler, b: Canceler): Canceler => () => {
  a();
  b();
};

/**
 * Combines arbitrary number of cancelers into one which will executing them
 * left to right one by one sequentially.
 */
export const foldCancelers = (cancelers: Canceler[]): Canceler =>
  cancelers.reduce(concatCancelers, emptyCanceler);