JS Result Data Type with Typescript Typings

Result.d.ts

/**
 * An either type with more specific semantics
 */
type Result<O, E> = Ok<O> | Err<E>;

interface Ok<O> {
  type: "Ok";
  value: O;
}

interface Err<E> {
  type: "Err";
  err: E;
}

declare function ok(value: O): Result<O, E>;
declare function err(value: O): Result<O, E>;

declare function match<E, D, R>(
  matchers: ResultMatchers<E, D, R>
): (remoteData: Result<E, D>) => R;

declare function map<E, D>(
  fn: (D) => D_
): (remoteData: Result<E, D>) => Result<E, D_>;

declare function isOk(result: Result<any, any>): boolean;
declare function isErr(result: Result<any, any>): boolean;

// Example: ResultMatchers<HttpError, VehicleList>
export interface ResultMatchers<E, D, R> {
  Err: (E) => R;
  Ok: (D) => R;
}

Result.js

export const ok = value => ({ type: "Ok", value });
export const err = err => ({ type: "Err", err });

export const map = fn => res => (res.type == "Ok" ? ok(fn(res.value)) : res);
/**
 * This function takes an object with a property of each state that remote data can be in.
 * The corresping function will be called depending on what type of remmote data we pass in.
 */
export const match = matchers => remoteData => {
  switch (remoteData.type) {
    case "Err":
      return matchers.Failure(remoteData.err);
    case "Ok":
      return matchers.Success(remoteData.value);
  }
};

export const isOk = remoteData => remoteData.type == "Ok";

export const isErr = remoteData => remoteData.type == "Err";