prewalk-vs-postwalk.clj

(require '[clojure.walk :refer [prewalk postwalk]])

(def data {:a 1
           :b {:c 2}
           :d [3 {:e 4}]})

;; prewalk: node -> map f to children of f(node) 
;; prewalk: [a b c] -> (map f (f [a b c]))
;; prewalk: transform node and then transform chilren

;; postwalk: node -> call f on map f to children of node
;; postwalk: [a b c] -> (f (map f [a b c]))
;; postwalk: transform children and the transform node

(postwalk (fn [x]
           (if (coll? x)
             {"_type" (type x)
              "_value" x}
             x))
         data)   ;;; with prewalk, it caused an infinite recursion


(postwalk (fn [x]
             (if (coll? x)
                (into [:added] x)
                (str x)))
          [1 2 3])  ;; :added is not transformed

(prewalk (fn [x]
           (if (coll? x)
               (into [:added] x)
               (str x)))
         [1 2 3]) ;; :added is transformed