lazy-assoc-in

lazy-assoc-in.clj

(ns test.core
  (:import [clojure.lang IDeref]))

(require '[lazymap.core :as lm])

(defn lazy-assoc-in
  "Value should be either derefable (delay or future) which will be dereffed when value is needed
  or a function which will be called."
  [m [k & ks] f]
  (if (seq ks)
    (lm/delayed-assoc m k (delay (lazy-assoc-in (or (get m k) (lm/create-lazy-map {})) ks f)))
    (lm/delayed-assoc m k (if (instance? IDeref f)
                            f
                            (delay (f))))))

(def m (-> (lm/create-lazy-map {})
           (lazy-assoc-in [:a :b] (fn [] (do (println "lazy-get") 1)))
           (assoc-in [:a :c] 2)))

(-> m :a :c)
; => 2

(-> m :a :b)
; lazy-get
; => 1