scratch.clj

(defmacro gen-iter-fn
  [name [op coll] has-item-body item-body move!-body]
  `(defn ~name [~op ~coll]
     (letfn
         [(iter# [~op ~coll]
                 (reify :as ~'this
                        ~'Iter
                        (~'has-item [] ~@has-item-body)
                        (~'item [] ~@item-body)
                        (~'move! [] ~@move!-body)
                        ~'Transient
                        (~'value-of []
                                     (reify Editable
                                            (transient-of [sentry#]
                                                          (iter# ~op (make-cell sentry# (sequence (deref ~coll)))))))))]
       (iter-seq (iter# ~op (cell (sequence ~coll)))))))


(gen-iter-fn filterx [pred coll]
             [(loop [cc coll]
                (when (and (<< has-item cc) (not (pred (<< item cc))))
                  (recur (>> move! cc))))
              (<< has-item coll)]
             [(<< item coll)]
             [(>> move! coll) this])

(gen-iter-fn mapx [f coll]
             [(<< has-item coll)]
             [(f (<< item coll))]
             [(>> move! coll) this])