scratch.clj

(defn sieve [f a-seq]
  (lazy-seq
   (let [[x & xs] a-seq]
     (when (or x (seq xs))
       (let [[new-x new-f] (f x)]
         (if new-x
           (cons x (sieve new-f xs))
           (sieve new-f xs)))))))

;;re-implement filter
(defn sfilter [f a-seq]
  (sieve (fn thisfn [x] [(f x) thisfn]) a-seq))

;;drop the nth element from a seq
(defn drop-nth [n a-seq]
  (sieve
   (partial (fn thisfn [m x]
              [(not= m n) (partial thisfn (inc m))])
            0)
   a-seq))

;;stateful, yet functional sieve
(defn drop-all-odds-after-five [a-seq]
  (sieve
   (partial (fn thisfn [seen-five? x]
              [(if seen-five? (even? x) true)
               (partial thisfn (or seen-five? (= x 5)))])
            false)
   a-seq))