json-letcase.el

(defmacro access (trail value)
  (let ((trail_ (reverse trail))
        (result value))
    (dolist (focus trail_ result)
      (pcase focus
        (`(vector . ,index) (setq result `(aref ,result ',index)))
        (`(object . ,key) (setq result `(alist-get ',key ,result)))))))

(defmacro vector-alist-let (spec value body)
  (let ((varlist '())
        (stack `((,spec . nil))))
    (while stack
      (let* ((_ (pop stack))
            (focus (car _))
            (trail (cdr _)))
        (cond
         ((listp focus)
          (dolist (spec focus)
            (cond
             ((listp spec)
              (push `(,(cdr spec) . ((object . ,(car spec)) . ,trail)) stack))
             ((symbolp spec)
              (push `(,spec . ((object . ,spec) . ,trail)) stack)))))
         ((vectorp focus)
          (let ((index 0))
            (mapc (lambda (elt)
                    (push `(,elt . ((vector . ,index) . ,trail)) stack)
                    (setq index (1+ index)))
                  focus)))
         ((symbolp focus)
          (push `(,focus (access ,trail ,value)) varlist)))))
    `(let ,varlist ,body)))