completing

;; Fix apparently inconsistent behaviour of "-" (minus) with transduce:
(transduce (map inc) - 0 (range 10))
;; 55
(transduce (map inc) (completing -) 0 (range 10))
;; -55

;; the reducing fn arity-1 executes the last transformation in transduce.
;; completing defaults to "identity" but you can change it.
;; Use this fact for example with transients and go back to persistent once done.
(require '[clojure.string :refer [lower-case]])
(transduce
 (comp
  (remove nil?)
  (map lower-case))
 (completing #(assoc! %1 %2 (inc (get %1 %2 0))) persistent!)
 (transient {})
 ["hi" "ho" "Hello" "hoi" "Hi" "Ha" "ha" "hello"])
;; {"hi" 2, "ho" 1, "hello" 2, "hoi" 1, "ha" 2}

;; Example calculating the average from 
;; https://aphyr.com/posts/360-loopr-a-loop-reduction-macro-for-clojure#comment-3800

(transduce (map inc)
           (completing (fn [[sum cnt] x] [(+ sum x) (inc cnt)])
                       (fn [[sum cnt]] (/ sum cnt)))
           [0 0]
           (range 7))

;; 4