(defun iterate (end-p cur-fun step-fun start function)
  (labels ((next (object)
             (unless (funcall end-p object)
               (let* ((cur (funcall cur-fun object))
                      (next (funcall step-fun object))
                      (result (funcall function cur)))
                 (if (typep result 'promise)
                     (then result
                           (lambda (v)
                             (declare (ignore v))
                             (funcall #'next next)))
                     (then (pend :success next)
                           #'next))))))
    (then (pend :success start) #'next)))

(defun each (sequence function)
  (etypecase sequence
    (null
     (pend :success NIL))
    (list
     (iterate #'null #'car #'rest sequence function))
    (vector
     (let ((length (length sequence)))
       (iterate (lambda (i) (<= length i))
                (lambda (i) (aref sequence i))
                #'1+ 0 function)))))