The Clojure function cycle take a collections as argument and creates a lazy sequence by repeating the items in the collection. So if we pass a collection with the characters \a, \b and \c we get a lazy sequence of (\a \b \c \a \b \c …​).

(ns mrhaki.core.cycle
  (:require [clojure.test :refer [is]]))

;; The items in the collection are repeated
;; and return type is a lazy sequence.
(is (= [0 1 0 1 0 1]
       (take 6 (cycle [0 1]))))
(is (seq? (cycle [0 1])))

(is (= [\C \l \o \j \u \r \e \C \l \o \j \u \r \e]
       (take 14 (cycle "Clojure"))))

;; Useful for functions that want equally sized
;; collection arguments.
(is (= {:a 0 :b 1 :c 0 :d 1}
       (zipmap [:a :b :c :d] (cycle [0 1]))))

Written with Clojure 1.10.1.