reducers.clj

(defn count-words
  ([] {"balk" 1})
  ([freqs word]
   (assoc freqs word (inc (get freqs word 0)))))

(defn merge-counts
  ([] {"baz" 2})
  ([& m]
   (apply merge-with + m)))

(defn word-frequency [text]
  (r/fold 2 merge-counts count-words (clojure.string/split text #"\s+")))

(word-frequency "bar foo bar bar") ;{"foo" 1, "bar" 3, "baz" 4}