defonce

macro

since v0.0-2156

clojure.core/defonce

Edit

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Source docstring:

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

(core/defmacro defonce
  [x init]
  (core/let [qualified (if (namespace x)
                         x
                         (symbol (core/str (core/-> &env :ns :name)) (name x)))]
    `(when-not (exists? ~qualified)
       (def ~x ~init))))

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