iolist.clj

(require '[clojure.core.protocols :as p]
         '[clojure.java.io :as io])

(deftype ByteSource [source]
  p/CollReduce
  (p/coll-reduce
    [coll f]
    (p/coll-reduce coll f nil))
  (p/coll-reduce
    [coll f init]
    (with-open [x (io/input-stream source)]
      (loop [n (.read x)
             v init]
        (if (= -1 n)
          v
          (let [r (f v n)]
            (if (reduced? r)
              @r
              (recur (.read x) r))))))))

;; user=> (count (reduce conj [] (ByteSource. "/etc/passwd")))
;; (count (reduce conj [] (ByteSource. "/etc/passwd")))
;; 5086
;; user=>