gistfile1.clj

(defn unfold [continue? producer seed]
  (reify
    clojure.core.protocols/CollReduce
    (coll-reduce [_ f]
      (loop [init seed
             seed seed]
        (if (reduced? init)
          @init
          (if (continue? seed)
            (let [next (producer seed)]
              (recur (f init next) next))
            init))))
    (coll-reduce [_ f init]
      (loop [init (f init seed)
             seed seed]
        (if (reduced? init)
          @init
          (if (continue? seed)
            (let [next (producer seed)]
              (recur (f init next) next))
            init))))))

#'user/unfold
user=> (reduce + (unfold pos? dec 10))
55