Add element to vector in nested tree structure

Question

Jumping right in, better way of doing this:

(assoc-in 
    {:children [{:children [{:children [{:children [{:children []}]}]}]}]} 
    [:children 0 :children 0 :children 0 :children 0 :children 0] 
    :hello)

I want to insert :hello into deepest :children vector. Above I am doing it with assoc-in.

Is there a better way than assoc-in?
Or, if when assoc-in is the only way, how would you handle assoc-in's 2nd argument [k & ks]?

Also good to know if there it something that also works for inserting :world into and arbitrary :children's vector... like the 3rd child or the 2nd child's 1st child.

Answer 1

The vector argument to assoc-in need not be a literal, so you can construct it as desired.

(def nested-map  
  {:children [{:children [{:children [{:children [{:children []}]}]}]}]})

(assoc-in nested-map (vec (take 10 (cycle [:children 0]))) :hello)
;=> {:children [{:children [{:children [{:children [{:children [:hello]}]}]}]}]}

Or for 3rd child of the 2nd child of the 1st child, construct a path like

(vec (interleave (repeat :children) [0 1 2]))
;=> [:children 0 :children 1 :children 2]

More generally you can use zippers to arbitrarily move around the nested map, e.g. descend last child. The movement functions are can be composed, etc.

(require '[clojure.zip :as zip])

(def z (zip/zipper map? :children #(assoc % :children (vec %2)) nested-map))

(-> (ffirst (filter (comp zip/end? second)  ; find last
                    (partition 2 1 (iterate zip/next z)))) 
    (zip/edit (constantly :hello)) ; change to :hello
    zip/root) ; bubble up changes

;=> {:children [{:children [{:children [{:children [{:children [:hello]}]}]}]}]}