summary.clj

(defn summarize 
  "Summarizes a data set. Groups data by result of groupingf applied to each
  member of coll. Applies summarizingf to each group."
  [groupingf summarizingf coll]
  (let [grouped (group-by groupingf coll)]
    (map #(list (first %) (summarizingf (second %))) grouped)))

(defn summarize-count
  [groupingf coll]
  (summarize groupingf count coll))

; juxt is the non-str version of catf
(defn catf
  [sep & fs]
  (fn [x]
    (clojure.string/join sep ((apply juxt fs) x))))