concurrent page fetcher

(:require [clojure.core.async :as a
             :refer [>! <! >!! <!! go chan buffer close! thread alts! alts!! timeout]])
             
(def URLS ["https://www.haskell.org"
           "https://www.amazon.com"
           "https://www.youtube.com"])

(defn time-now []
  (. System (nanoTime)))

(defn elapsed [start end]
  (/ (double (- end start)) 1000000000.0))

(defn fetch [url ch]
  (go
    (let [start (time-now)
          resp (slurp url)
          n (count resp)]
      (>! ch (format "%.2fs %7d %s" (elapsed start (time-now)) n url)))))

(defn -main
  "I don't do a whole lot ... yet."
  [& args]
  (let [start (time-now)
        ch (chan (count URLS))]
    (doseq [url URLS]
      (fetch url ch))
    (doseq [url URLS]
      (println (<!! ch)))
    (println (elapsed start (time-now)))))