pad-pairs.clj

(defn pad-by
  ([pred pad]
   (fn [rf]
     (let [pv (volatile! ::none)]
       (fn
         ([] (rf))
         ([result] (rf result))
         ([result input]
          (let [prior @pv]
            (vreset! pv input)
            (if (pred prior input)
              (rf (rf result pad) input)
              (rf result input))))))))
  ([pred pad coll] (sequence (pad-by pred pad) coll)))

(defn parse-props-spec
  [spec]
  (transduce (pad-by #(= (type %1) (type %2) nil)) conj [] spec)