Transducers in a nut-shell

transducers.clj

;; From tbaldridge on Slack

(defn map [f coll]
  (reduce (fn [acc i]
                  (conj acc (f i)))
                []
                coll))

(map inc [1 2 3]) ;;=> [2 3 4]

;; refactor to pull out reduce
(defn mapping [f]
   (fn [acc i]
     (conj acc (f i)))

(reduce (mapping inc) [] [1 2 3]) ;;=> [2 3 4]

;; But this isn't good enough because our accumulator may not support `conj`
;; Refactor once more:

(defn mapping [f]
  (fn [xf]
    (fn [acc i]
      (xf acc (f i)))))

(reduce ((mapping inc) conj) [] [1 2 3]) ;;=> [2 3 4]
(reduce ((mapping inc) +) 0 [1 2 3]) ;;=> 9
  
;; Now you can reuse `mapping` in a dozen different situations and we never have to define a mapping transducer again.