Programming Clojure, 2nd Edition

Lazy and Infinite Sequences

Most Clojure sequences are lazy; in other words, elements are not calculated until they are needed. Using lazy sequences has many benefits:

• You can postpone expensive computations that may not in fact be needed.
• You can work with huge data sets that do not fit into memory.
• You can delay I/O until it is absolutely needed.

Consider the code and following expression:

(ns examples.primes)
;; Taken from clojure.contrib.lazy-seqs
; primes cannot be written efficiently as a function, because
; it needs to look back on the whole sequence. contrast with
; fibs and powers-of-2 which only need a fixed buffer of 1 or 2
; previous values.
(def primes
  (concat
   [2 3 5 7]
 (lazy-seq ...