Clojure recursive group-by

recursive-group-by.clj

  ;;; Based on https://stackoverflow.com/a/38842018/12373587
  
  (defn map-vals
    [m f]
    (persistent!
     (reduce-kv
      (fn [m k v]
        (assoc! m k (f v)))
      (transient {})
      m)))

  (defn nested-group-by
    "Like group-by but instead of a single function, this is given a list or vec
   of functions to apply recursively via group-by. An optional `final` argument
   (defaults to identity) may be given to run on the vector result of the final
   group-by."
    ([fs coll]
     (nested-group-by fs coll identity))
    ([[f & fs] coll final-fn]
     (if f
       (map-vals
        (group-by f coll)
        #(nested-group-by fs % final-fn))
       (final-fn coll))))

  
    (def foo [ ["A" 2011 "Dan"]
            ["A" 2011 "Jon"]
            ["A" 2010 "Tim"]
            ["B" 2009 "Tom"] ])

  (nested-group-by [first second] foo #(map (fn [x] (nth x 2)) %))