dive into transducers - extract from https://github.com/chpill/transducers-deep-dive/blob/a9d158b161fd963cb5c7ba1ab18531b76da8ae08/src/transducers_deep_dive/core.clj

original.clj

(defn error? [x] (:error x))

(defn original [chunks]
  (transduce (comp
              (halt-when error?)
              (map #(frequencies (map :a %))))
             (completing (partial merge-with +))
             {}
             chunks))