Kurt Harriger documents his process of creating a Clojure web application called Twitturl to search Twitter. He initializes the project using Leiningen, sets up dependencies like Ring and hiccup, defines functions for searching Twitter and formatting the results, and implements a handler to return the search results. He runs the application using the Ring server and packages it as a WAR file to deploy on services like Amazon Elastic Beanstalk. The document highlights features of Clojure like S-expressions, destructuring, and chaining that make the code more concise compared to Java.

1. Finding Clojure
Kurt Harriger @kurtharriger

2. The Challenge
Twitter search url filter

3. Project Folder
% lein new twitturl
Created new project in: /Users/kurtharriger/code/twitturl
├── README
├── project.clj
├── src
│ └── twitturl
│ └── core.clj
└── test
└── twitturl
└── test
└── core.clj
% git init && git add -A && git commit -m “create project”

4. project.clj
(defproject twitturl "1.0.0-SNAPSHOT"
:description "Twitturl aggregator"
:dependencies [[org.clojure/clojure "1.2.0"]
[org.clojure/clojure-contrib "1.2.0"]
[ring "0.3.7"]
[clj-http "0.1.2"]]
:dev-dependencies [[lein-ring "0.4.0"]]
:ring {:handler twitturl.core/app})

5. Appleholics
% brew install leiningen
Non Appleholics
Find Leiningen for Clojure on
https://github.com/technomancy/leiningen
% lein self-install

6. S-Expressions
Clojure:
(defn fn [arg1 & more] arg1)
defn is a macro used here with 3 parameters:
function name, arguments, and body.
Returns the function binding.
Clojure:
(println (fn value1 value2))
Java:
System.out.println(fn(value1, value2));

7. S-Expressions
Clojure:
(defn fn [arg1 & more] arg1)
defn is a macro used here with 3 parameters:
function name, arguments, and body.
Returns the function binding.
Clojure:
(println (fn value1 value2))
Java:
System.out.println(fn(value1, value2));

8. ring framework
(ns twitturl.core
(:require [com.twinql.clojure.http :as http])
(:use [ring.middleware.params])
(:use [hiccup.core]))
Import dependencies
(defn handler [] nil) ;todo create handler
(def app (-> handler wrap-params))
-> operator used apply multiple
Entry point functions to handler

9. De-structuring
(defn format-tweets [json] nil) ; todo
(defn handler [{{q "q"} :params}]
{ :content-type "text/html"
:body (html [:body
(-> (search q) format-tweets)])})
same as: Only need q
(defn handler [request]
(let [params (request :params) Local variable assignments
q (params "q")] request and params
{:content-type "text/html" unnecessary.
:body (html [:body
(-> (search q) format-tweets)])}))

10. The Search
(def url "http://search.twitter.com/search.json")
define readonly variable named url
(defn search [query]
(http/get url :query {:q query} :as :json))
returns result of com.twinql.clojure.http.get
using parameter list
1) url
2) :query
3) {:q query }
4) :as
5) :json

11. Enter the repl
% lein deps
Copying 22 files to /Users/kurtharriger/code/twitturl/lib
Copying 17 files to /Users/kurtharriger/code/twitturl/lib/dev
% lein repl
REPL started; server listening on localhost:24319.
user=> (use ‘twitturl.core)
nil
user=> (search “clojure”)
{:code 200,
:reason "OK",
:content
{:results
[{:from_user "planetclojure",
:text "Removing defined tests in Clojure REPL
http://goo.gl/fb/qeUty #clojure #SO",
...
}, ...]
}
}

12. The Handler
(defn format-tweets [json] nil) ; todo
(defn handler [{{q "q"} :params}]
{ :content-type "text/html"
:body hiccup library function
(html [:body
(-> (search q) format-tweets)])})
same as (format-tweets (search q))
returns a map containing
:content-type and :body

13. functions
(defn aslink [url]
(html [:a {:href url} url]))
user=> (aslink “http://url1”)
"<a href="http://url1">http://url1</a>"
(def urls (partial re-seq #"http://S+"))
same as:
(defn urls [text] (re-seq #“http://S+” text))
user=> (urls “blah blah http://url1 http://url2”)
("http://url1" "http://url2")

14. map reduce
(defn linkify [text]
(reduce #(.replace %1 %2 (aslink %2))
text (urls text)))
(fn [newText url]
(.replace newText url (aslink url))
String linkify(String text) {
String newText = text;
for(String url : getUrls(text)) {
newText = newText.replace(url, asLink(url));
}
return newText;
}

15. chaining
(defn format-tweet [tweet] 1 2
[:li (:from_user tweet)
[:blockquote (-> tweet :text linkify)]])
Java:
linkify(tweet.get(“text”))

16. chaining
(defn format-tweet [tweet] 1 2 2
[:li (:from_user tweet)
[:blockquote (-> tweet :text linkify)]])
Java:
linkify(tweet.get(“text”))
4
Java requires more(parentheses)?!

17. chaining
(defn format-tweets [json]
[:ul (->> json :content :results
(remove #(-> % :text urls nil?))
(map format-tweet))])
ArrayList<String> formatTweets(JSONObject json) {
StringBuilder tweets = new StringBuilder();
tweets.append(“<ul>”);
JSONObject content = json.getJSONObject(“content”);
JSONArray results = json.getJSONArray(“results”);
for(JSONObject tweet : results) {
String[] urls = getUrls(tweet.getString(“text”))
if(urls != null && urls.length > 0) {
tweets.append(“<li>” + formatTweet(tweet) “</li>”);
}
}
tweets.append(“</ul>”);
return tweets;
}

18. chaining
(defn format-tweets [json] 5
[:ul (->> json :content :results
(remove #(-> % :text urls nil?))
(map format-tweet))])
ArrayList<String> formatTweets(JSONObject json) {
StringBuilder tweets = new StringBuilder();
tweets.append(“<ul>”);
JSONObject content = json.getJSONObject(“content”);
JSONArray results = json.getJSONArray(“results”);
for(JSONObject tweet : results) {
12
String[] urls = getUrls(tweet.getString(“text”))
if(urls != null && urls.length > 0) {
tweets.append(“<li>” + formatTweet(tweet) “</li>”);
}
}
Java usually requires more(parentheses)?!
tweets.append(“</ul>”);
return tweets;
}

19. ring server
% lein ring server
2011-04-16 21:18:54.965:INFO::Logging to STDERR via org.mortbay.log.StdErrLog
2011-04-16 21:18:54.968:INFO::jetty-6.1.26
2011-04-16 21:18:54.998:INFO::Started SocketConnector@0.0.0.0:3000
Started server on port 3000
% lein ring war
Created /Users/kurtharriger/code/twitturl/twitturl-1.0.0-SNAPSHOT.war
Amazon Elastic Beanstalk plugin
https://github.com/weavejester/lein-beanstalk

21. src/twitturl/core.clj
1 (ns twitturl.core
2 (:use [ring.middleware.params])
3 (:use [hiccup.core])
4 (:require [com.twinql.clojure.http :as http]))
5
6 (def url "http://search.twitter.com/search.json")
7
8 (defn search [query] (http/get url :query {:q query} :as :json))
9 (defn aslink [url] (html [:a {:href url} url]))
10
11 (def urls (partial re-seq #"http://S+"))
12
13 (defn linkify [text]
14 (reduce #(.replace %1 %2 (aslink %2)) text (urls text)))
15
16 (defn format-tweet [tweet]
17 [:li (:from_user tweet)
18 [:blockquote (-> tweet :text linkify)]])
19
20 (defn format-tweets [json]
21 [:ul (->> json :content :results
22 (remove #(-> % :text urls nil?))
23 (map format-tweet))])
24
25 (defn handler [{{q "q"} :params}]
26 { :content-type "text/html"
27 :body (html [:body (-> (search q) format-tweets)])})
28
29 (def app (-> handler wrap-params ))

22. Finding Clojure
https://github.com/kurtharriger/twitturl
Leiningen Ring framework
DSLs Lists are awesome
Less () than Java De-structuring
Method chaining High-order functions
Kurt Harriger @kurtharriger