clojure - merge to set default values, but potentially expensive functions -

an idiomatic way set default values in clojure merge:

;; `merge` can used support setting of default values (merge {:foo "foo-default" :bar "bar-default"}         {:foo "custom-value"}) ;;=> {:foo "custom-value" :bar "bar-default"} 

in reality however, default values not simple constants function calls. obviously, i'd avoid calling function if it's not going used.

so far i'm doing like:

(defn ensure-uuid [msg]   (if (:uuid msg)     msg     (assoc msg :uuid (random-uuid)))) 

and apply ensure-* functions (-> msg ensure-uuid ensure-xyz).

what more idiomatic way this? i'm thinking like:

(merge-macro {:foo {:bar (expensive-func)} :xyz (other-fn)} my-map)  (associf my-map   [:foo :bar] (expensive-func)   :xyz (other-fn)) 

you can use delay combined force.

you can merge defaults like

(merge {:foo "foo-default" :bar "bar-default" :uuid (delay (random-uuid))}        {:foo "custom-value" :uuid "abc"}) 

and access values using

(force (:foo ...)) 

or

(force (:uuid ...)) 

random-uuid called when need value (and first time).

you can wrap call force in get-value function, or that.