delay.lisp

(defclass promise ()
  ((action :initarg :action :initform (error "Required :action"))
   (is-called-p :initform nil)
   (result :initform nil)))

(defmacro delay (action)
  `(make-instance 'promise :action (lambda () ,action)))

(defun force (promise)
  (if (slot-value promise 'is-called-p)
      (slot-value promise 'result)
      (let ((r (funcall (slot-value promise 'action))))
	(setf (slot-value promise 'is-called-p) t
	      (slot-value promise 'result) r)
	r)))