postwalk-with-side-effect.clj

(defn postwalk-with-side-effect [x]
  (let [state (atom [])
        side-effect (fn [x] (if (:data x) (swap! state conj (:data x))))]
    (clojure.walk/postwalk (fn [x] (side-effect x) x) x)
    @state))

(defn test []
  (= [1 2 3]
     (postwalk-with-side-effect
      {:data 3 :children [{:children {:children []}}
                          {:children [{:children [] :data 1}]}
                          {:children [] :data 2}]})
     ))

this avoids mutating, doesn't have to build a collection that nobody uses, and for those reasons is likely more performant

(defn tree-seq-and-reduce
  [x]
  (reduce (fn [state el]
            (if-let [datum (:data el)]
              (conj state datum)
              state))
          []
          (tree-seq coll? seq x)))

(defn test-it
  []
  (= (set [1 2 3])
     (set (tree-seq-and-reduce
           {:data 3 :children [{:children {:children []}}
                               {:children [{:children [] :data 1}]}
                               {:children [] :data 2}]}))))

(test-it)