clojure.core/reduced

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Type: function Added: Clojure 1.5 Examples: 3 Runnable: 3

([x])

Wraps x in a way such that a reduce will terminate with the value x

Examples

by silasdavis on 2014-10-02 ▶

;; Suppose you want to short-circuit a sum like: (reduce (fn [a v] (+ a v)) (range 10)) ;;=> 45 ;; So that it returns the sum of the integers if less than 100: (reduce (fn [a v] (if (< a 100) (+ a v) (reduced :big))) (range 10)) ;;=> 45 ;; But the keyword :big otherwise: (reduce (fn [a v] (if (< a 100) (+ a v) (reduced :big))) (range 20)) ;;=> :big ;; The value returned by (reduced :big) short-circuits the reduction so that ;; it returns the wrapped value without ranging over the entire sequence. ;; This is useful for infinite lazy sequences: (reduce (fn [a v] (if (< a 100) (+ a v) (reduced :big))) (range)) ;;=>:big ;; Which would otherwise not terminate.

by teymuri on 2015-11-18 ▶

;;re-implementing (some) using (reduce) and (reduced): (defn resome [pred koll] (reduce (fn [_ c] (when-let [x (pred c)] (reduced x))) nil koll)) ;;user> (resome #{4} [3 4 2 3 2]) ;;>>> 4 ;;user> (resome even? [3 41 25 3 2]) ;;>>> true ;;user> (resome even? [3 41 25 3 27]) ;;>>> nil

by TravisHeppner on 2018-12-19 ▶

;;note the return from f must be from reduced for reduce to terminate early. ;;no early termination as the return of f is nil (reduce (fn f [a b] (when (> b 2) (reduced "Done early!")) (println b)) [1 2 3 4 5]) ;;2 ;;3 ;;4 ;;5 ;;=> nil ;;early termination as the return of f is "Done early!" wrapped in a reduce object. (reduce (fn f [a b] (if (> b 2) (reduced "Done early!") (println b))) [1 2 3 4 5]) ;;2 ;;=> "Done early!"

See also

• clojure.core/reduce (22)
• clojure.core/reduced?
• clojure.core/unreduced (1)
• clojure.core/ensure-reduced (2)

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