clojure.core/lazy-cat

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Type: macro Added: Clojure 1.0 Examples: 7 Runnable: 4

([& colls])

Expands to code which yields a lazy sequence of the concatenation of the supplied colls. Each coll expr is not evaluated until it is needed.

(lazy-cat xs ys zs) === (concat (lazy-seq xs) (lazy-seq ys) (lazy-seq zs))

Examples

by pashields on 2011-01-17 ▶

(lazy-cat [1 2 3] [4 5 6]) (1 2 3 4 5 6)

by kij on 2012-03-26 ▶

;; N.B. this example holds onto the head of a lazy seq which should generally be avoided (def fib-seq (lazy-cat [0 1] (map + (rest fib-seq) fib-seq))) (take 10 fib-seq)

by guruma on 2012-12-14 . May have evaluation errors. ▶

;; When the producer function produces a collection, not an element, ;; lazy-cat is usable. (defn n-repeat [n] (lazy-cat (repeat n n) (n-repeat (inc n)))) #'user/n-repeat (take 6 (n-repeat 1)) (1 2 2 3 3 3) (take 12 (n-repeat 1)) (1 2 2 3 3 3 4 4 4 4 5 5)

by mattvvhat on 2015-09-01 ▶

(lazy-cat (seq ["lazy-cat" "is" "my" "favorite" "function"]))

by parkeristyping on 2017-03-30 . May have evaluation errors. ▶

(defn loop-endlessly "Block thread with endless loop when evaluated" [] (while true)) #'user/loop-endlessly (take 3 (lazy-cat ["will" "it" "return?"] (loop-endlessly))) ("will" "it" "return?") (take 4 (lazy-cat ["will" "it" "return?"] (loop-endlessly))) ;; This gets stuck on loop-endlessly and never returns

by reborg on 2018-03-07 . May have evaluation errors. ▶

;; Prefer lazy-cat to build a lazy seq out of ;; non-lazy collections with different creation costs (time (first (concat (sort > (range 10)) (sort > (range 1e7))))) ;; "Elapsed time: 17442.084309 msecs" (time (first (lazy-cat (sort > (range 10)) (sort > (range 1e7))))) ;; "Elapsed time: 0.458283 msecs"

by sanel on 2024-10-09 ▶

;; Be aware that lazy-cat can be slower on smaller collections (time (first (concat (sort > (range 100)) (sort > (range 100))))) ;; "Elapsed time: 0.058254 msecs" (time (first (lazy-cat (sort > (range 100)) (sort > (range 100))))) ;; "Elapsed time: 0.182578 msecs" (time (first (concat (sort > (range 10)) (sort > (range 100))))) ;; "Elapsed time: 0.051935 msecs" (time (first (lazy-cat (sort > (range 10)) (sort > (range 100))))) ;; "Elapsed time: 0.149794 msecs"

Note by Dimagog

It looks like lazy-cat is on deprecation path in favor of lazy-seq.

See also

• clojure.core/lazy-seq (6)
• clojure.core/concat (7)
• clojure.core/conj (9)
• clojure.core/mapcat (7)

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