with-collects.lisp

(defmacro with-collects ((&rest syms) &body body)
  (labels
      ((collector-expander (sym gtail)
         `(,sym (arg)
                (if (null ,sym)
                    (progn
                      (setf ,sym (cons arg nil))
                      (setf ,gtail ,sym)
                      arg)
                    (let ((last (cons arg nil)))
                      (setf (cdr ,gtail) last
                            ,gtail last)
                      arg)))))
  (let ((gtails (mapcar #'(lambda (s) (gensym "WITH-COLLECTS-TAIL")) syms)))
    `(let ,syms
       (labels
           (,@(mapcar #'collector-expander syms gtails))
         ,@body)))))

;; example
(with-collect (a b)
  (a 10)
  (b 20)
  (a 30)
  (list a b))
=> ((10 30) 20)