catamorphism or something.clj

(def tree
  {:nodes
   [
    {:function "start"
     :nodes [{:call "end" :nodes [{:call "middle"}]}]
     }
    {:function "middle"
     :nodes [{:call "start"} {:call "end"}]}
    {:function "end"
     :nodes [{:call "start"}]}
    ]})

(defn catvec [a b]
  (vec (concat a b)))

(defn collect [nodes state]
  (reduce (fn [{:keys [known declare] :as res} {:keys [call nodes function]}]
            (let [updated
                  (if call
                    (if (or (.contains declare call) (.contains known call))
                      res
                      (update-in res [:declare] conj call))
                    ; Function can't be redefined
                    (if function
                      (update-in res [:known] conj function)
                      res))

                  nested
                  (if nodes (:declare (collect nodes updated)))]
              {:known (:known updated)
               :declare (distinct (vec (concat (:declare updated) nested)))}))
            state nodes))

  (collect (:nodes tree) {:declare [] :known []})