(defun each (sequence function &key lifetime)
  (etypecase sequence
    (null
     (pend :success NIL))
    (cons
     (labels ((next (sequence)
                (when sequence
                  (let ((result (funcall function (pop sequence))))
                    (if (typep result 'promise)
                        (then result
                              (lambda (v)
                                (declare (ignore v))
                                (funcall #'next sequence)))
                        (then (pend :success sequence)
                              #'next))))))
       (then (pend :success sequence) #'next)))
    (vector
     (let ((length (length sequence)))
       (labels ((next (i)
                  (when (< i length)
                    (let ((result (funcall function (aref sequence i))))
                      (if (typep result 'promise)
                          (then result
                                (lambda (v)
                                  (declare (ignore v))
                                  (funcall #'next (1+ i))))
                          (then (pend :success (1+ i))
                                #'next))))))
         (then (pend :success 0) #'next))))))