(def v) with no init supplied destroys #'v metadata


  • Type: Enhancement Enhancement
  • Status: Open Open
  • Priority: Minor Minor
  • Resolution: Unresolved
  • Affects Version/s: Release 1.6
  • Fix Version/s: None
  • Component/s: None
  • Labels:
  • Approval:


(def a) destroys #'a metadata, check this:

(def ^:mykey a 1)

(meta #'a)              ;; ok, :mykey is present

(let [v (def a)]
   [(meta v)            ;; NO :mykey present, metadata destroyed
    (identical? v #'a)  ;; true, we are talking of the same var

(meta #'a)              ;; NO :mykey present

If this is not a bug but a "feature", then we have at least two problems:

1- The def special form documentation doesn't state this behaviour at all, it needs to be clarified. With the current documentation it seems as doing a def with no init supplied will not make any side-effect at all, and this is not true for the var metadata.

2- defmulti uses this form to lookup the var and check if it already binds to a MultiFn, if that is the case then defmulti does nothing... but it really does something, defmulti will destroy the original var metadata in the (supposedly non-destructive) check. This is the involved defmulti fragment:

(let [v# (def ~mm-name)]
  (when-not (and (.hasRoot v#) (instance? clojure.lang.MultiFn (deref v#)))



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