scratch.clj

(defmacro generate-iter
  "the init form is a vector of name/initval pairs"
  [name initform has-item-expr item-expr move!-expr valueof-expr]
  (let [args (vec (map first (partition 2 initform)))
        inits (map second (partition 2 initform))]
    `(letfn
         [(~name ~args
                 (reify :as ~'this
                        ~'Iter
                        (~'has-item [] ~has-item-expr)
                        (~'item [] ~item-expr)
                        (~'move! [] ~move!-expr)
                        ~'Transient
                        (~'value-of
                         []
                         (reify ~'Editable
                                (transient-of [~'sentry]
                                              ~valueof-expr)))))]
       (iter-seq (~name ~@inits)))))


(defn filterx
  [pred coll]
  (generate-iter
   iter [p pred, seq-cell (cell (sequence coll))]
   ;; has-item
   (do
     (loop [cc seq-cell]
       (when (and (<< has-item cc) (not (pred (<< item cc))))
         (recur (>> move! cc))))
     (<< has-item seq-cell)) 
   ;; item
   (<< item seq-cell)
   ;; move!
   (do (>> move! seq-cell) this)
   ;; valueof... has 'sentry defined
   (iter pred (make-cell sentry (sequence @seq-cell)))))


(defn mapx
  [f coll]
  (generate-iter
   [f f, seq-cell (cell (sequence coll))]
   (<< has-item seq-cell)
   (f (<< item seq-cell))
   (do (>> move! seq-cell) this)
   (iter f (make-cell sentry (sequence @seq-cell)))))


(defn takex
  [n coll]
  (generate-iter iter
   [ai (atom 0), n n, seq-cell (cell (sequence (coll)))]
   (and (<< has-item seq-cell) (< @ai n))
   (<< item seq-cell)
   (do (swap! ai inc)
       (>> move! seq-cell)
       this)
   (iter (atom @ai) n (make-cell sentry (sequence @seq-cell)))))