group-by.clj

(defn map-keys [m f]
  (into {} (for [[k v] m] [k (f v)])))

;; recursive group-by
(defn group-by-> [starting-target starting-fns]
  (loop [target starting-target fns starting-fns]
   (if (nil? fns) target
      (map-keys (group-by (first fns) target) #(recur % (rest fns))))))

;; usage
(group-by-> [(time/date-time 2010 9 30)
	     (time/date-time 2010 10 1)
	     (time/date-time 2010 10 2)
	     (time/date-time 2010 10 2 1)]
	    [time/month time/day])

;; {9 {30 [#<DateTime 2010-09-30T00:00:00.000Z>]}
;;  10 {1 [#<DateTime 2010-10-01T00:00:00.000Z>]
;;      2 [#<DateTime 2010-10-02T00:00:00.000Z>
;; 	#<DateTime 2010-10-02T01:00:00.000Z>]}}