richhickey

;sample use idea:

(defn mapx
  "Returns a lazy sequence consisting of the result of applying f to the
  set of first items of each coll, followed by applying f to the set
  of second items in each coll, until any one of the colls is
  exhausted.  Any remaining items in other colls are ignored. Function
  f should accept number-of-colls arguments."
  [f coll]
  (gen-iter iter [f seq-cell]

richhickey

(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)