extrapolate: like iterate, but different

extrapolate.clj

(defn extrapolate
  "Returns a lazy seq produced by extrapolating `coll` with given rules.
  On each iteration,
    `gen-next` is called with the entire seq generated till this iteration
    `shrink` is called with the extrapolated seq (`identity` by default)"
  ([gen-next coll]
   (extrapolate gen-next identity coll))
  ([gen-next shrink coll]
   (concat coll
           (let [curr (volatile! (seq coll))]
             (repeatedly (fn []
                           (let [next-val (gen-next @curr)]
                             (vswap! curr #(shrink (concat % [next-val])))
                             next-val)))))))


;; USAGE
;; =====
;; without shrink (retains head)
;; ==============
(defn fibs-bad []
  (extrapolate #(apply + (take-last 2 %))
               [0 1]))

(fibs-bad)
;; => (0 1 1 2 3 5 8 ..)

;; with shrink
;; ===========
(defn fibs []
  (extrapolate #(apply + %)
               rest
               [0 1]))

(fibs)
;; => (0 1 1 2 3 5 8 ..)