clojure.core/defonce

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Type: macro Added: Clojure 1.0 Examples: 3 Runnable: 0

([name expr])

defs name to have the root value of the expr iff the named var has no root value, else expr is unevaluated

Examples

by dakrone on 2010-12-30 . May have evaluation errors. ▶

(defonce foo 5) #'user/foo foo 5 ;; defonce does nothing the second time (defonce foo 10) nil foo 5

by DanBurton on 2016-07-14 . May have evaluation errors. ▶

;; Supports ^:private (defonce ^:private foo 3) #'user/foo foo 3 (in-ns 'user2) user/foo java.lang.IllegalStateException: var: user/foo is not public

by luiszambon on 2020-09-10 . May have evaluation errors. ▶

(def code {:clojure "Yes"}) #'user/code code {:clojure "Yes"} ;; defonce will not be evaluated (defonce code (+ 10 10)) nil code {:clojure "Yes"}

Note by lnostdal

Note that this isn't thread safe. I.e. EXPR for the same NAME can be evaluated more than once.

See also

• clojure.core/def (4)

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