Sieve of Erastothenes as a data-flow using core.async (http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes)

core.clj

(ns sieve.core
  (:require [clojure.core.async :as async :refer :all]]))

(defn counter-stream [n]
  (let [c (chan)]
    (go
     (loop [i n]
       (>! c i)
       (recur (inc i))))
    c))

(defn filter-multiples [p in]
  (let [out (chan)]
    (go
     (loop []
       (let [v (<! in)]
         (when-not (zero? (mod v p))
           (>! out v)))
       (recur)))
    out))

(defn sieve []
  (let [primes (chan)]
    (go
     (loop [c (counter-stream 2)]
       (let [p (<! c)]
         (>! primes p)
         (recur (filter-multiples p c)))))
    primes))

(comment
  (def primes (sieve))
  (<!! primes)
  2
  (<!! primes)
  3
  etc...)