Don’t panic! [Clojure version]


Hi guys!
When it rains my internet connection is a pain in the neck, by the way I want to show you the Clojure version of Don’t Panic.
If you don’t remember it, Don’t Panic is a simple application developed with university students in mind, it periodically checks a site, looking for an update. It was a good occasion to play with Leningen!
Here is the full code (Leningen version):

;; Don't panic
(ns dontpanic.core (:gen-class))

(defn fetch-url
  "Fetches an URL. Returns a string made by the page's html."
  [address]
  (with-open [stream (.openStream (java.net.URL. address))]
    (let  [buf (java.io.BufferedReader.
		(java.io.InputStreamReader. stream))]
      (apply str (line-seq buf)))))

(defn hash-code
  "Simple wrapper. Returns a string hashcode."
  [string]
  (.hashCode string))

(defn -main
  "Periodically check if a site has been modified."
  [& args]
  (let
      [address (nth args 0)
       site-hashcode (hash-code (fetch-url address))]
    (while true
       (do
	 (if (not (= site-hashcode (hash-code (fetch-url address))))
	   (println "Maybe an update! Finger crossed.")
	   (println "Nothing changed, calm down."))
	 (Thread/sleep 5000)))))

The idea behind the scenes is very simple: fetch-url returns the entire page’s html as a string, the hash-code function returns the string hash code. The eternal loop (while true) fetch the url, computes the hash code: if it’s different from the previous one, an maybe an update is occurred!
Shortly I will post a standalone jar!!
Usage (code version):

(-main [<site url as string>])

Usage (jar version):

java -jar dontpanic.jar <site-url>

Enjoy!

2 thoughts on “Don’t panic! [Clojure version]

  1. Nice utility – and I share the enthusiasm for Clojure – a great language for sure.

    I just wanted to suggest a possible design improvement. Perhaps you’re aware of this already, but you don’t have to fetch and hash a page to know if it has changed. Web clients have long avoided unnecessary fetching by using timestamps and the 304 (not modified) HTTP response code.

    Here’s a pointer to the appropriate part of the spec. http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.3

    I’m afraid this would become more of a “calling java from Clojure” exercise than anything else, but it would probably still be a useful example

    What I’m not sure of though, is how hard this would be in clojure. You have to get into

    • In first place, thanks for your comment, the first one by long time🙂
      In second place, I will take a look to your suggestion: mine implementation is the easiest one, despite you can archive the same purpose looking at the “Last Modified” attribute on some html pages🙂
      Watch out, not all the pages have this attribute🙂
      Bye!
      Alfredo

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