my-assoc-in.clj

(defn my-assoc-in
  [m [k & ks] v]
  (assoc m k 
         (reduce (fn [v-in k-in] (assoc {} k-in v-in))
                 v
                 (reverse ks))))

(comment
  (my-assoc-in {} [:a] 1)
  ;; => {:a 1}
  (my-assoc-in {:a 1} [:b] 2)
  ;; => {:a 1, :b 2}
  (my-assoc-in {:a 1} [:a] 2)
  ;; => {:a 2}
  (my-assoc-in {} [:a :b :c] 4)
  ;; => {:a {:b {:c 4}}}
  (my-assoc-in {:a {:b 3}} [:a :b] {:c 123})
  ;; => {:a {:b {:c 123}}}
)