empty

(empty '(1 2)) ;; => ()
(empty  [1 2]) ;; => []
(empty  {1 2}) ;; => {}
(empty #{1 2}) ;; => #{}

;; Works for PersistentQueue as well, which is slightly harder to see
(def q (conj clojure.lang.PersistentQueue/EMPTY 1))
;; => #'user/q
q
;; => #<PersistentQueue clojure.lang.PersistentQueue@20>
(seq q)
;; => (1)

(def empty-q (empty q))
;; => #'user/empty-q
empty-q
;; => #<PersistentQueue clojure.lang.PersistentQueue@1>
(seq empty-q)
;; => nil

;; returns nil when input is not supported or not a collection
(empty (int-array [1 2])) ;; => nil
(empty 1) ;; => nil

(map empty [[\a \b] {1 2} (range 4)])
;; => ([] {} ())

(swap! (atom (range 10)) empty)      
;; => ()

;; The output will not necessarily be of the same JVM class as the input
user=> (class (seq [1]))
clojure.lang.PersistentVector$ChunkedSeq

user=> (class (empty (seq [1])))
clojure.lang.PersistentList$EmptyList

;; Create the same data structure with the same metadata, but empty
(def foo (with-meta #{1 2 3} {:some-meta 1}))
foo ;; => #{1 2 3}
(meta foo) ;; => {:some-meta 1}

(def empty-foo (empty foo))
empty-foo ;; => #{}
(meta empty-foo) ;; => {:some-meta 1}