Clojure nested maps path

Question

This is really a followup to question Update hierarchical / tree structure in Clojure

I need to be able to change an Atom containing a map with lists of submaps. I would like to use fx assoc-in to do this, but i'm not sure how best to optain the path to the element i wich to change

My data structure:

(def x (atom {:name "A" 
          :id 1 
          :children [{:name "B" 
                      :id 2 
                      :children []} 
                     {:name "C" 
                      :id 3 
                      :children [{:name "D" 
                                  :id 4 
                                  :children []}]}]}))

How do i make a function to find the path go a given id, fx give me path to map which contain #(= (:id %) 3):

(find-path 3 @x) ; => [0 :children 1]

So i can do this to get a new map:

(assoc-in @x [(conj (find-path...) :name)] "Jim")

Or update the Atom like this:

(swap! x assoc-in [(conj (find-path...) :name)] "Bob")

I thought it should be easy to make a recursive function returning the path, but i cannot get it to work. — Drewes
In my opinion it's a lot easier to reason about and manipulate persistent nested structures using zippers than to try to figure out a recursive function to handle them. — juan.facorro

cgrand cgrand · Accepted Answer · 2014-03-11T11:26:29

There's a fundamental issue here is that your update process is now a two steps process: find-path and update-in. However you are working with atoms so the path returned by find-path may be incorrect by the time update-in gets to see the value of the atom.

So you should combine them in a single fn so as to not have one deref and one swap!.

(defn update-by-id [x id f & args]
  (apply update-in x (find-path x id) f args))

Now you can use a single swap!:

(swap! x update-by-id 3 assoc :name "Bob")

However if you constantly update and access through ids you should also evaluate switching to another (flatter) representation for this piece of data.

Clojure nested maps path

2 Answers