clojure.walk/postwalk

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Type: function Added: Clojure 1.1 Examples: 6 Runnable: 1

([f form])

Performs a depth-first, post-order traversal of form. Calls f on each sub-form, uses f's return value in place of the original. Recognizes all Clojure data structures. Consumes seqs as with doall.

Examples

by sunil.nandihalli on 2010-10-14 . May have evaluation errors. ▶

(require '[clojure.walk :refer [postwalk]]) (let [counter (atom -1) line-counter (atom 0) print-touch (fn [x] (print (swap! line-counter inc) ":" (pr-str x) "→ ")) change (fn [x] (let [new-x (swap! counter inc)] (prn new-x) [new-x x]))] (postwalk (fn [x] (print-touch x) (change x)) {:a 1 :b 2})) ;; printed output: 1 : :a → 0 2 : 1 → 1 3 : [[0 :a] [1 1]] → 2 4 : :b → 3 5 : 2 → 4 6 : [[3 :b] [4 2]] → 5 7 : {2 [[0 :a] [1 1]], 5 [[3 :b] [4 2]]} → 6 ;; returned value: [6 {2 [[0 :a] [1 1]], 5 [[3 :b] [4 2]]}]

by boxie on 2013-02-01 . May have evaluation errors. ▶

;;example of removing namespaces from all keys in a nested data structure (def thing {:page/tags [{:tag/category "lslsls"}]}) (postwalk #(if (keyword? %) (keyword (name %)) %) thing) {:tags [{:category "lslsls"}]}

by wmatson on 2017-04-05 . May have evaluation errors. ▶

(use 'clojure.walk) ;;example of evaluating an expression tree, starting at the leaves (def expression-tree {:function + :children [1 {:function * :children [2 6]}]}) (defn evaluate [node] (if-let [f (:function node)] (apply f (:children node)) node)) (postwalk evaluate expression-tree) => 13

by cljlc on 2018-02-19 . May have evaluation errors. ▶

;; an example to show the differences between postwalk and prewalk ;; (see the counterpart at prewalk) (let [counter (atom 0) print-touch (fn [x] (print (swap! counter inc) ":" (pr-str x) "→ ")) change-type (fn [x] (let [new-x (if (vector? x) (apply list x) (str x))] (prn new-x) new-x))] (clojure.walk/postwalk (fn [x] (print-touch x) (change-type x)) [:a [:ba :bb] :c])) ;; printed output: 1 : :a → ":a" 2 : :ba → ":ba" 3 : :bb → ":bb" 4 : [":ba" ":bb"] → (":ba" ":bb") 5 : :c → ":c" 6 : [":a" (":ba" ":bb") ":c"] → (":a" (":ba" ":bb") ":c") ;; returned value: => (":a" (":ba" ":bb") ":c")

by burinc on 2023-09-08 ▶

;; Recursively sort nested map (require '[clojure.walk :as w]) (def m {:b 2, :c 3, :a 1, :d {:x 10, :c 3, :a 11, :b 22}}) (w/postwalk (fn [x] (if (map? x) (into (sorted-map) x) x)) m) ;;=> {:a 1, :b 2, :c 3, :d {:a 11, :b 22, :c 3, :x 10}}

by tomdl89 on 2025-12-18 . May have evaluation errors. ▶

;; A simpler illustration of the difference between postwalk and prewalk… (defn walking-fn [x] (cond (number? x) (* 10 x) (vector? x) (map inc x))) (walk/postwalk walking-fn [1 2 3]) ;; => (11 21 31) (walk/prewalk walking-fn [1 2 3]) ;; => (20 30 40)

Note by pesterhazy

As of 1.9.0, postwalk passes the k/v pairs of a map to f not, as one might expect, as type clojure.lang.MapEntry but as clojure.lang.PersistentVector (JIRA). As a result, f cannot distinguish k/v pairs from other two-element vectors.

Note by finalfantasia

Alex Miller's article "Tree visitors in Clojure" might be helpful in understanding general tree traversal and the usage of clojure.walk/postwalk.

Note by leo-ppp

As of 1.9.0, postwalk passes the k/v pairs of a map to f not, as one might expect, as type clojure.lang.MapEntry but as clojure.lang.PersistentVector (JIRA). As a result, f cannot distinguish k/v pairs from other two-element vectors.

This was fixed in 1.10.0, and one can use (map-entry? v) to distinguish a k/v pair from a vector when walking.

Note by jeff303

Updated link to "Tree visitors in Clojure", mentioned above (PDF):

https://insideclojure.org/images/j-treevisit-pdf.pdf

See also

• clojure.walk/prewalk (0)
• clojure.walk/postwalk-demo
• clojure.walk/postwalk-replace

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