clojure recur stackoverflow

gistfile1.clj

(defn atest [state]
  (when-not (and (= "" state) (not (= (first state) \a))) 
          (list (first state) (. state (substring 1)))))
(defn op [state]
  (when-not (and (= "" state) (not (= (first state) \a)))
    (list #(list :| %1 %2) (. state (substring 1)))))
(defn chainl1-helper [x p op]
  (fn [state]
    (loop [x x
           state state]
      (if-let [xs (op state)]
        (when-let [xs2 (p (second xs))]
          (recur ((first xs) x (first xs2)) (second xs2)))
        (list x state)))))
      
(defn chainl1 [p op]
  (fn [state]
    (when-let [[v s] (p state)]
      ((chainl1-helper v p op) s))))
(def test-parse (chainl1 atest op))
(defn stress-test [n] (test-parse (apply str (take n (interleave (repeat "a") (repeat "+"))))))
(stress-test 99999)