10분이면 여러분도 Future Monad를 만들 수 있습니다.

Future.js

module.exports = ( () => {
  const STATE = { PENDING: 0, DONE: 1, FAIL: 2};
  class Future {
    constructor(worker) {
      this.state = STATE.PENDING;
      this.value = undefined;
      this.handlers = [];
      try{
        worker(Future.resolver(this), Future.rejector(this));
      } catch (error) {
        console.error(error);
      }
    }

    static resolver(future) {
      return (value) => {
        if(future.state !== STATE.PENDING) throw new Error('this Future is already end');
        future.state = STATE.DONE;
        future.value = value;
        for(let handler of future.handlers) {
          try {
            setImmediate(() => handler(value));
          } catch(handleError) {
            console.error(handleError);
          }
        }
      };
    }

    static rejector(future) {
      return (error) => {
        if(future.state !== STATE.PENDING) throw new new Error('this Future is already end');
        future.state = STATE.FAIL;
        future.value = error;
        for(let handler of future.handlers) {
          try {
            setImmediate(() => handler(error));
          } catch(handleError) {
            console.error(handleError);
          }
        }
      };
    }

    ready(handler) {
      let resultFuture = new Future( resolver => {
        if(this.state === STATE.DONE) setImmediate(() => resolver(handler(this.value)));
        else this.handlers.push(value => resolver(handler(value)));
      });
      return resultFuture;
    }
  }

  return {
    STATE: Object.assign(STATE),
    Future
  }
})();