defs name to have the root value of the expr iff the named var has no root value,
else expr is unevaluated
Source
(defmacrodefonce"defs name to have the root value of the expr iff the named var has no root value,
else expr is unevaluated"{:added"1.0"}[nameexpr]
`(let [v# (def ~name)]
(when-not (.hasRoot v#)
(def ~name ~expr))))