bad_reduce.clj

(defn increment-in-map [map key]
  (update-in map [key] #(inc (or % 0))))

(defn count-occurrences [to-count]
  (loop [counted-so-far {}
        remaining-to-count to-count]
    (if (empty? remaining-to-count)
      counted-so-far
      (recur (increment-in-map counted-so-far (first remaining-to-count))
             (rest remaining-to-count)))))

 ;(describe
 ;  "counting occurrences"
 ;  (it "empty"
 ;      (should= {} (count-occurrences '())))
 ;  (it "one one"
 ;      (should= {1 1} (count-occurrences '(1))))
 ;  (it "one two"
 ;      (should= {2 1} (count-occurrences '(2))))
 ;  (it "one one, one two"
 ;      (should= {1 1 2 1} (count-occurrences '(1 2)))
 ;      )
 ;  (it "one one, one two, one three"
 ;      (should= {1 1 2 1 3 1} (count-occurrences '(1 2 3))))
 ;  (it "three ones"
 ;      (should= {1 3} (count-occurrences '(1 1 1)))))