Nil Finder

(ns user)

(def app
  "Intenal Helper"
  (fnil conj []))

(defprotocol PathSeq
  (path-seq* [form path] "Helper for path-seq"))

(extend-protocol PathSeq
  java.util.List
  (path-seq*
   [form path]
   (->> (map-indexed
         (fn [idx item]
           (path-seq* item (app path idx)))
         form)
        (mapcat identity)))

  java.util.Map
  (path-seq*
   [form path]
   (->> (map
         (fn [[k v]]
           (path-seq* v (app path k)))
         form)
        (mapcat identity)))

  java.util.Set
  (path-seq*
   [form path]
   (->> (map
         (fn [v]
           (path-seq* v (app path v)))
         form)
        (mapcat identity)))
  
  
  java.lang.Object
  (path-seq* [form path] [[form path]])
  
  nil
  (path-seq* [_ path] [[nil path]]))

(defn path-seq
  "Returns a sequence of paths into a form, and the elements found at
   those paths.  Each item in the sequence is a map with :path
   and :form keys. Paths are built based on collection type: lists
   by position, maps by key, and sets by value, e.g.

   (path-seq [:a [:b :c] {:d :e} #{:f}])

   ({:path [0], :form :a}
    {:path [1 0], :form :b}
    {:path [1 1], :form :c}
    {:path [2 :d], :form :e}
    {:path [3 :f], :form :f})
   "
  [form]
  (map
   #(let [[form path] %]
      {:path path :form form})
   (path-seq* form nil)))

(comment
  (path-seq [:a [:b :c] {:d :e} #{:f}])
  
  ;; finding nils hiding in data structures:
  (->> (path-seq [:a [:b nil] {:d :e} #{:f}])
       (filter (comp nil? :form)))
    
  ;; finding a nil hiding in a Datomic transaction
  (->> (path-seq {:db/id 100
                  :friends [{:firstName "John"}
                            {:firstName nil}]})
       (filter (comp nil? :form)))
  

  )