Implementation of the Either monad in a closure.

utils.either.js

const Either = Object.freeze((() => {
  const Right = input => ({
    get isRight () {
      return _ => true;
    },

    get isLeft () {
      return _ => false;
    },

    get chain () {
      return handle => handle(input);
    },

    get fold () {
      return (_, handle) => handle(input);
    },

    get map () {
      return handle => Right(handle(input));
    }
  });

  const Left = error => ({
    get isRight () {
      return _ => false;
    },

    get isLeft () {
      return _ => true;
    },

    get chain () {
      return _ => Left(error);
    },

    get fold () {
      return (handle, _) => handle(error);
    },

    get map () {
      return _ => Left(error);
    }
  });

  return Object.freeze({
    get right () {
      return input => Right(input);
    },

    get left () {
      return error => Left(error);
    }
  });