clojure.core/when-first

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Type: macro Added: Clojure 1.0 Examples: 3 Runnable: 2

([bindings & body])

bindings => x xs

Roughly the same as (when (seq xs) (let [x (first xs)] body)) but xs is evaluated only once

Examples

by gstamp on 2010-08-02 ▶

(when-first [a [1 2 3]] a) 1 (when-first [a []] :x) nil (when-first [a nil] :x) nil

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

;; Note that the 'when' switches on the truthiness of the sequence, not the ;; truthiness of the elements within the sequence. (when-first [a [nil 2 3]] (print (str "Picked: " (prn-str a)))) Picked: nil nil

by reborg on 2018-04-02 ▶

;; A concise way to write the basic lazy loop. (defn dechunk [xs] (lazy-seq (when-first [x xs] (cons x (dechunk (rest xs)))))) ;; would print 32 dots otherwise: (first (map #(do (print ".") %) (dechunk (range 100)))) ;; .0

See also

• clojure.core/when-let (6)
• clojure.core/when (3)

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