pop

user=> (peek [1 2 3])
3
user=> (pop [1 2 3])
[1 2]
user=> (peek '(1 2 3))
1
user=> (pop '(1 2 3))
(2 3)

user=> (peek ())
nil
user=> (pop ())
IllegalStateException Can't pop empty list

user=> (peek [])
nil
user=> (pop [])
IllegalStateException Can't pop empty vector

user=> (peek (clojure.lang.PersistentQueue/EMPTY))
nil
user=> (into [] (pop (clojure.lang.PersistentQueue/EMPTY)))
[] ;; Can pop empty Queue

;; Use a vector as a LIFO stack to check for balanced brackets

(require '[clojure.set :refer [map-invert]])

(defn balance [form]
  (let [brackets {\[ \] \( \) \{ \}}
        scan (fn [q x]
               (cond
                 (brackets x) (conj q x)
                 ((map-invert brackets) x)
                 (if (= (brackets (peek q)) x)
                   (pop q)
                   (throw
                     (ex-info
                       (str "Unmatched delimiter " x) {})))
                 :else q))]
    (reduce scan [] form)))

(balance "(let [a (inc 1]) (+ a 2))")
;; ExceptionInfo Unmatched delimiter ]

(balance "(let [a (inc 1)] (+ a 2))")
;; []

;;basic example on vector and list

;;pop on vector returns a new vector removing the last element
(pop [1 2 3])
;; [1 2]

;;pop on list returns a new list removing the first element
(pop '(1 2 3)) ;; (2 3)