tree-walking-with-a-zipper.clj

; in clojure an if-then statement looks like: (if test do-this-if-true do-this-if-false)
; eg (if has-water (prn "content") (water-plant))

(defn find-replace [zipper pred replacement]
  (loop [loc zipper] ;begin a loop - when we call `recur` at the bottom,this is where we come back to. 
    (if (z/end? loc) ;if we are at the end, it means we've visited every node, so...
      (root loc) ;we return the zipper
      (if
        (pred loc) ;`pred` is a function that we supply as a "test" for each location. eg. is this an 'x'?
        (recur (z/next (z/replace loc (replacement loc)))) ;this is what we do if `pred` returned true 
        (recur (z/next loc)))))) ; this is what we do if `pred` returned false

; example usage of this `find-replace` function

(let [my-zipper (z/of-string "(+ 1 2 (- 4 3))") ;make a zipper out of a string of clojure code
      multiply-by-ten (fn [element] (* 10 element))] 
    (find-replace my-zipper even? multiply-by-ten))