make sequence side-effectfull in Clojure

Question

What I want to do is like following.

(def mystream (stream (range 100)))

(take 3 mystream)
;=> (0 1 2)

(take 3 mystream)
;=> (3 4 5)

(first (drop 1 mystream))
;=> 7

The stream function make sequence side-effectfull like io stream.
I think this is almost impossible. Here is my attempt.

(defprotocol Stream (first! [this]))

(defn stream [lst]
  (let [alst (atom lst)]
    (reify Stream
       (first! [this]
         (let [[fs] @alst]
           (swap! alst rest)
           fs)))))

(let [mystream (stream (iterate inc 1))]
  (map #(if (string? %) (first! mystream) %)
       [:a "e" "b" :c "i" :f]))
;=> (:a 1 2 :c 3 :f)

Unfotunately this approach need to implement all function I will use.

Answer 1

This won't work with the standard take and drop, but you could quite easily write your own to work on a mutable atom, e.g. you could do something like this:

(def mystream (atom (range 100)))

(defn my-take [n stream]
  (let [data @stream
        result (take n data)]
    (reset! stream (drop n data))
    result))  

(my-take 3 mystream)
=> (0 1 2)

(my-take 3 mystream)
=> (3 4 5)

Answer 2

Judging by your followup comment to Maurits, you don't need mutation, but rather simply need to emit a new sequence with the elements in the right place.

For example:

(defn replace-when [pred coll replacements]
  (lazy-seq
    (when (seq coll)
      (if (seq replacements)
        (if (pred (first coll))
          (cons (first replacements) 
                (replace-when pred (rest coll) (rest replacements)))
          (cons (first coll) 
                (replace-when pred (rest coll) replacements)))
        coll))))

user=> (def seq1 [:a :b :c])
#'user/seq1
user=> (def seq2 [:x "i" "u" :y :z "e"])
#'user/seq2
user=> (replace-when string? seq2 seq1) 
(:x :a :b :y :z :c)