Converting a vector to a map in Clojure

Question

I'm having a problem with a little function that's supposed to count nodes and leaves in a tree. This is what it looks like:

(defn count-tree
  "Count nodes and leaves in a tree as produced by grow-tree-hlp."
  [tree]
  ; acc is [nodes leaves]
  (letfn [(inc-acc [x y] (map + x y))]
    (let [result (cond
                   (leaf? tree) [0 1]          ; increase leaves by 1
                   (node? tree) (reduce        ; recurse over children while increasing nodes by 1
                                  (fn [acc child] (inc-acc acc (count-tree child)))
                                  [1 0]
                                  (drop 2 tree)))]
      {:nodes  (first result)
       :leaves (second result)})))

The error is:

(err) Execution error (ClassCastException) at screpl.core/count-tree (core.clj:282).
(err) class clojure.lang.MapEntry cannot be cast to class java.lang.Number (clojure.lang.MapEntry is in unnamed module of loader 'app'; java.lang.Number is in module java.base of loader 'bootstrap')

I did a bit of testing, and it looks like the offending bits are (first result) and (second result) in the map that I want to return. The core of the function, the cond part, works just fine when it's taken out of let, the only problem is it returns a vector when I want a map.

Answer 1

The issue is that the inc-acc function expects two vectors of numbers (it's much better when things are documented BTW), but count-tree returns a map.

Few ways to fix it:

• Use only vectors for both the acc and the result value of count-tree
• Use only maps for those and adjust inc-acc
• Change inc-acc so it accepts a vector and a map
• Just write it all in a different way

Here's how I'd do it:

(def node? vector?)

(def leaf? (complement node?))

(defn node-children [node]
  (drop 2 node))

(defn count-tree [tree]
  (letfn [(f [acc tree]
            (if (leaf? tree)
              (update acc :leaves inc)
              (reduce f
                      (update acc :nodes inc)
                      (node-children tree))))]
    (f {:nodes 0 :leaves 0} tree)))

(count-tree [:div {}
             [:span {} "hello"]
             [:span {} "there"]
             [:div {}
              [:span {} "!"]]])
=> {:nodes 5, :leaves 3}