clojure.core/cons

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Type: function Added: Clojure 1.0 Examples: 5 Runnable: 4

([x seq])

Returns a new seq where x is the first element and seq is the rest.

Examples

by dpritchett on 2010-07-14 ▶

;; prepend 1 to a list (cons 1 '(2 3 4 5 6)) ;;=> (1 2 3 4 5 6) ;; notice that the first item is not expanded (cons [1 2] [4 5 6]) ;;=> ([1 2] 4 5 6)

by phalphalak on 2015-03-05 ▶

;; may return results of different types but always a seq (map (juxt identity type seq? list?) [(cons 1 nil) (cons 1 '())]) ;; => ([(1) clojure.lang.PersistentList true true] ;; [(1) clojure.lang.Cons true false])

by edcaceres on 2016-11-22 ▶

;; Cons new-element into nested structures "cons-in" (def db {:users [{:name "Eduardo"}]}) (def new-element {:name "Eva"}) (assoc db :users (cons new-element (:users db))) ;; => {:users ({:name "Eva"} {:name "Eduardo"})}

by belun on 2017-02-03 . May have evaluation errors. ▶

(defn zeros [] (lazy-seq (cons 0 (zeros)))) ;; "cons" does not realize second parameter, ;; opening the world for recursive functions that create lazy sequences (first (zeroes)) 0 (first (rest (zeroes))) 0 (first (rest (rest (zeroes)))) 0 ;; example stolen from youtuber : ;; https://youtu.be/iaph8m63HQw?list=PLAC43CFB134E85266

by didiercrunch on 2018-05-26 ▶

;; this example might not work ;; conj behave differently with "set" like structures. (def a-set #{1 2 3}) (conj a-set 3) ;; will add 3 to the set ;; => #{1 3 2} (conj a-set 4) ;; => #{1 4 3 2} (first (conj a-set 4)) ;; => 1

Note by belun

useful for creating lazy sequences, because it does not need to realize the param "seq" (it just appends the whole thing to param "x"

http://stackoverflow.com/questions/12389303/clojure-cons-vs-conj-with-lazy-seq

See also

• clojure.core/conj (9)
• clojure.core/seq (5)

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