app.clj

(def input
  [{:t "A" :id 1}
   {:t "B" :id 2 :parent 1}
   {:t "C" :id 3 :parent 2}
   {:t "D" :id 4 :parent 1}
   {:t "E" :id 5 :parent 1}
   {:t "F" :id 6 :parent 5}
   {:t "G" :id 7 :parent 6}
   {:t "H" :id 8 :parent 1}])

;; desired output
(def desired-output
  {:t "A" :id 1
   :children [{:t "B" :id 2 :parent 1
               :children [{:t "C" :id 3 :parent 2}]}
              {:t "D" :id 4 :parent 1}
              {:t "E" :id 5 :parent 1
               :children [{:t "F" :id 6 :parent 5
                           :children [{:t "G" :id 7 :parent 6}]}]}
              {:t "H" :id 8 :parent 1}]})

;; potential solution:
(defn gather-children [m msgs]
  (assoc m :children (map #(gather-children % msgs)
                          (filter #(= (:id m) (:parent %)) msgs))))

(pprint 
 (gather-children (first input) input))