How transducers work (from https://www.youtube.com/watch?v=6mTbuzafcII)

transducers.clj

(defn filtering [pred]
  (fn [step]
    (fn [r x] (if (pred x) (step r x) r))))

(defn my-filter
  ([pred coll]
   "Returns the result immediately"
   (reduce ((filtering pred) conj)
           [] coll))
  ([pred]
   "Return the step function to use it later"
   ((filtering pred) conj)))