Posts Tagged ‘clojure’
Compojure Demystified with an example – Part 3
In part2 we saw how to setup a skeleton project with Compojure. In this part we will see how to add static files to a Compojure project.
For our application, we will use JQuery as front end and Clojure as a backend. I will concentrate more on Compojure.
1) Create folders for static files.
mkdir public mkdir public/css mkdir public/js
It is better to have static files under a seperate directory that way you could use your webserver to serve these files. For now we will use Compojure to serve these files.
2) Create index.html under public folder
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
<title>My Address book</title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
</head>
<body id="main">
<p>
My Address Book
</p>
<body>
</html>
3) Start your server
lein repl src/address_book/core.clj
4) Access index.html
Go to http://localhost:8080/index.html . You will get “Page not found” instead of “My Address Book”. This is happening because we have not told compojure about our static files folder.
Route helper functions in Compojure
In Compojure we have two route helper functions. We have seen one already in core.clj file.
1) not-found :
This function is used to capture all other routes that are not defined in Compojure. As we still have not defined index.html we got “Page not found”.
2) files :
This function is used to inform Compojure where static files are present.
Lets change our core.clj to handle static files
(ns address_book.core
(:use [compojure.core]
[ring.adapter.jetty])
(:require [compojure.route :as route]))
(defroutes example
(GET "/" [] "<h1>My Address Book!</h1>")
(route/files "/" {:root "public"})
(route/not-found "Page not found"))
(run-jetty example {:port 8080})
Restart your server (there is a way to avoid restarting using futures. I will show that in a later blog) and access http://localhost:8080/index.html . Now you should see “My Address Book”.
In this part we saw how we can handle static files. In next part we will implement Add , View and View All functionalities.
PS: Using Hiccup you can create HTML and css in clojure. I will try to cover this in future blogs.
Source code is now available at github . Created branches for each part.
Checkout PART4 …
Compojure Demystified with an example – Part 2
In part 1 I introduced the address book application that we are going to build.
In this part we will setup our skeleton project with compojure.
1) Install Leiningen
http://github.com/technomancy/leiningen/blob/master/README.md
2) Create a new project using Leiningen
lein new address_book
3) Add Compojure to our project:
a) Edit project.clj
(defproject address_book "1.0.0-SNAPSHOT"
:description "Address Book"
:dependencies [[org.clojure/clojure "1.1.0"]
[org.clojure/clojure-contrib "1.1.0"]
[compojure "0.4.1"]
[ring/ring-jetty-adapter "0.2.3"]])
b) Install dependencies
lein deps
This should install all dependencies of compojure
c) Test whether our setup is working
Edit src/address_book/core.clj
(ns address_book.core
(:use [compojure.core]
[ring.adapter.jetty])
(:require [compojure.route :as route]))
(defroutes example
(GET "/" [] "My Address Book!")
(route/not-found "Page not found"))
(run-jetty example {:port 8080})
Run the server
lein repl src/address_book/core.clj
Goto http://localhost:8080 and you should see “My Address Book!”
In next part we will start implementing our functionalities.
Compojure Demystified with an example – Part 1
In this part, I am going to talk about what application we are going to implement.
Lets build a simple address book web application using compojure. This will be a RESTFUL application. As we go through this example we will see some interesting aspects of compojure.
Functionalities
- Add / Edit / Delete Address book
- View All Addresses
- View a single Address
Domain
Address
- Name
- Street1
- Street2
- City
- Country
- ZipCode
REST interfaces we will implement
- View All Addresses – GET – http://localhost:8080/addresses
- View single address – GET – http://localhost:8080/addresses/:id
- Add Address - POST – http://localhost:8080/addresses
- Edit Address - PUT – http://localhost:8080/addresses/:id
- Delete Address – DELETE – http://localhost:8080/addresses/:id
Next part 2 we will setup our base project with compojure.
Map, Reduce and Filter in Clojure
Map
Map applies a function to each element in a collection and returns a new collection with the results of each function.
user> (map #(+ 10 %1) [ 1 3 5 7 ]) (11 13 15 17)
Reduce
Reduce applies a function on all elements of a collection and returns a value. This value could also be another collection.
user> (reduce * [2 3 4]) 24
Filter
Filter applies a predicate function to each element in a collection and returns a new filtered collection.
user> (filter even? [1 2 3 4 5 6]) (2 4 6)
Different tools to help you test your clojure code
Problem :
We need to parse a xml string and be able to query using xpath style tag list.
Ex :
<friends> <person> <name>Siva</name> </person> </friends>
I need a function that can do this,
(get-value xml :person :name)returns “Siva”
Solution :
To parse and query xml we need to do these following three things in clojure.
1) Convert xml string (file) to Struct Map
Clojure core comes with a build in xml library (clojure.xml http://richhickey.github.com/clojure/clojure.xml-api.html) that has a parse function which takes in InputStream and returns a struct map that represents xml.
(defn get-struct-map [xml]
(let [stream (ByteArrayInputStream. (.getBytes (.trim xml)))]
(xml/parse stream)))
user> (get-struct-map xml)
{:tag :friends, :attrs nil, :content [{:tag :person, :attrs nil, :content [{:tag :name, :attrs nil, :content ["Siva"]}]}]}
This struct map is cumbersome to query.
2) Convert Struct Map to Zipper Data Structure
“A zipper is a technique of representing an aggregate data structure so that it is convenient for writing programs that traverse the structure arbitrarily and update its contents, especially in purely functional programming languages.” http://en.wikipedia.org/wiki/Zipper_%28data_structure%29
To make it easy for us to traverse we will change struct map to zipper data structure. Clojure comes with zip library ( it is short form for zipper ) http://richhickey.github.com/clojure/clojure.zip-api.html
We will this zip library to convert xml struct map to zipper data structure.
user> (clojure.zip/xml-zip xml-struct)
[{:tag :friends, :attrs nil, :content [{:tag :person, :attrs nil, :content [{:tag :name, :attrs nil, :content ["Siva"]}]}]} nil]
3) Use Zip-filter library and query zipper data structure.
Now that we have our xml in zipper data structure we could use zip-filter library that is present in clojure.contrib. http://richhickey.github.com/clojure-contrib/zip-filter-api.html
user> (clojure.contrib.zip-filter.xml/xml-> zipper-struct :person :name)
("Siva")
Putting this altogether
(ns com.sivajag.utils.xml
(:import (java.io ByteArrayInputStream))
(:require [clojure.xml :as xml])
(:require [clojure.zip :as zip])
(:require [clojure.contrib.zip-filter.xml :as zf]))
(defn get-struct-map [xml]
(if-not (empty? xml)
(let [stream (ByteArrayInputStream. (.getBytes (.trim xml)))]
(xml/parse stream))))
(defn get-value [xml & tags]
(apply zf/xml1-> (zip/xml-zip (get-struct-map xml)) (conj (vec tags) zf/text)))
user> (get-value xml :person :name)
"Siva"
Happy Coding!!!
Clojure futures – A Walkthrough
What is Future?
It is a simple mechanism to execute a snippet of code on a background thread.
How to use Future?
We can use Future in two ways.
1) Using macro “future”
(def f (future (prn "going to sleep...") (Thread/sleep 10000) (prn "slept good")))
2) Using function “future-call”
(defn long-process[] (Thread/sleep 10000)) (future-call long-process)
Under the shell both these ways are equivalent. The macro is syntax sugar and it gets converted to future-call.
Even though “long-process” function is put to sleep for 10 secs, it will return immediately. It returns a “future” object. We can use this object to inspect whether the function has been executed in background thread.
(future-done? f) (future-cancelled? f) (future-cancel f)
Limitations of Future?
As you known clojure comes with a bounded thread pool and unbounded thread pool. Mostly CPU bound operations should be using bounded thread pool and IO operations should be using unbound thread pool. Clojure Future uses unbounded thread pool, so it is good for IO operations but not for CPU bound operations. So for CPU bound operations Future might not be a good choice. But there is a open source project that helps to overcome this limitations. Check out http://github.com/amitrathore/medusa
clj-web-crawler – Web Crawling tool in clojure
I was looking for a simple web crawler in clojure and came across clj-web-crawler (http://github.com/heyZeus/clj-web-crawler) . It took me less than 2 minutes to get started. (thanks to Leiningen)
1) In project.clj add these two lines for :dependencies
[com.jmeeks/clj-web-crawler "0.1.0-SNAPSHOT"]
[org.clojars.kjw/commons-httpclient "3.1"]
2) Run lein deps and you ready to play with clj-web-crawler.
Checkout http://github.com/heyZeus/clj-web-crawler for more information
Leiningen Tips
Tip 1 : Adding License info
To add your license info, add these following lines in project.clj
:license {:name "Eclipse Public License - v 1.0"
:url "http://www.eclipse.org/legal/epl-v10.html"
:distribution :repo
:comments "same as Clojure"}
Tip 2: Turning off auto delete of lib folder
:disable-implicit-clean true
Tip 3: Turning on reflection warning calls
:warn-on-reflection true
Tip 4: Changing JVM level options
:jvm-opts "-Xmx1g"
Parsing XML in Clojure
Problem :
We need to parse a xml string and be able to query using xpath style tag list.
Ex :
<friends> <person> <name>Siva</name> </person> </friends>
I need a function that can do this,
(get-value xml :person :name)
returns “Siva”
Solution :
To parse and query xml we need to do these following three things in clojure.
1) Convert xml string (file) to Struct Map
Clojure core comes with a build in xml library (clojure.xml http://clojure.github.com/clojure/clojure.xml-api.html) that has a parse function which takes in InputStream and returns a struct map that represents xml.
(defn get-struct-map [xml]
(let [stream (ByteArrayInputStream. (.getBytes (.trim xml)))]
(xml/parse stream)))
user> (get-struct-map xml)
{:tag :friends, :attrs nil, :content [{:tag :person, :attrs nil, :content [{:tag :name, :attrs nil, :content ["Siva"]}]}]}
This struct map is cumbersome to query.
2) Convert Struct Map to Zipper Data Structure
“A zipper is a technique of representing an aggregate data structure so that it is convenient for writing programs that traverse the structure arbitrarily and update its contents, especially in purely functional programming languages.” http://en.wikipedia.org/wiki/Zipper_%28data_structure%29
To make it easy for us to traverse we will change struct map to zipper data structure. Clojure comes with zip library ( it is short form for zipper ) http://clojure.github.com/clojure/clojure.zip-api.html
We will this zip library to convert xml struct map to zipper data structure.
user> (clojure.zip/xml-zip xml-struct)
[{:tag :friends, :attrs nil, :content [{:tag :person, :attrs nil, :content [{:tag :name, :attrs nil, :content ["Siva"]}]}]} nil]
3) Use Zip-filter library and query zipper data structure.
Now that we have our xml in zipper data structure we could use zip-filter library that is present in clojure.contrib. http://clojure.github.com/clojure-contrib/zip-filter-api.html
user> (clojure.contrib.zip-filter.xml/xml-> zipper-struct :person :name)
("Siva")
Putting this altogether
(ns com.sivajag.utils.xml
(:import (java.io ByteArrayInputStream))
(:require [clojure.xml :as xml])
(:require [clojure.zip :as zip])
(:require [clojure.contrib.zip-filter.xml :as zf]))
(defn get-struct-map [xml]
(if-not (empty? xml)
(let [stream (ByteArrayInputStream. (.getBytes (.trim xml)))]
(xml/parse stream))))
(defn get-value [xml & tags]
(apply zf/xml1-> (zip/xml-zip (get-struct-map xml)) (conj (vec tags) zf/text)))
user> (get-value xml :person :name)
"Siva"
Happy Coding!!!
Clojure test-is library examples and explanation – Basic
Clojure contrib comes with a testing framework test-is. Even though it is a small library it does its job. test-is consists many useful functions and macros to help us test our code. I am going to go through some of them and show some examples. Some examples I took it from http://richhickey.github.com/clojure-contrib/test-is-api.html
deftest
Is a macro to define a test function.
Usage
(deftest name & body)
Example
(deftest test-addition (is (= (+ 1 2) 3)))
is
It is a generic assertion macro. You will end up using this a lot in your test code.
Usage
(is form) (is form msg)
Examples
(is (= (+ 1 2) 3))
[or]
user> (is (= (+ 2 1) 3) "should be 3") true user> (is (= (+ 3 1) 3) "should be 3")
FAIL in clojure.lang.PersistentList$EmptyList@1 (NO_SOURCE_FILE:1) should be 3 expected: (= (+ 3 1) 3) actual: (not (= 4 3)) false
This “is” macro could be used to test exceptions too.
(is (thrown? c body)) (is (thrown-with-msg? c re body))
are
Sometimes just using “is” will become tedious.
(deftest test-addition (is (= (+ 2 1) 3)) (is (= (+ 0 0) 0)) (is (= (+ -1 -1) -2)))
Using “are” we could clean up this test function as
(deftest test-addition
(are [x y] (= x y)
3 (+ 2 1)
0 (+ 0 0)
-2 (+ -1 -1)))
The only problem using “are” is when a test fails it screws up line number where failure happened.
testing
This is my favorite macro. It allows me organize my tests.
Lets take this test
(deftest test-addition (is (= (+ 2 1) 3)) (is (= (+ 0 0) 0)) (is (= (+ -1 -1) -2)))
Imagine it is doing some complicated stuff. This test does not express intent. Using “testing” we can rewrite
(deftest test-addition
(testing "should add positive numbers"
(is (= (+ 2 1) 3)))
(testing "should add zeros"
(is (= (+ 0 0) 0)))
(testing "should be able to add negative numbers"
(is (= (+ -1 -1) -2))))
The best part if a test fails it prints the string next to “testing”
(test-addition) FAIL in (test-addition) (NO_SOURCE_FILE:1) should be able to add negative numbers expected: (= (+ -1 -1) -3) actual: (not (= -2 -3))
Happy testing …
