Web programming in clojure

Web Programming in Clojure
Barry Demchak
CSE294
November 12, 2010

Agenda
• Different REPLs
– Eclipse with Counterclockwise
– Leiningen
• Compojure Web Framework
– Application structure
– Framework execution model
– High level walkthrough
– Addons
• Alternate Frameworks

REPL with Eclipse
• http://www.assembla.com/wiki/show/clojure/Gettin
g_Started_with_Eclipse_and_Counterclockwise
• Install Counterclockwise
– Plugin for Eclipse
– Create and run simple Clojure project
(firstClojureProject)
– Run REPL in console window
• Install the labrepl project (and git/maven)
– Good online tutorial

REPL with Leiningen (“lein”)
• First, install Leiningen – a build tool for Clojure
– http://www.assembla.com/wiki/show/clojure/Get
ting_Started_with_Leiningen
– Installation downloads itself
• Use Lein to create a project
– Create .clj project file
– Use editor to add dependencies
– Use Lein to satisfy dependencies
– Use Lein to start a REPL
https://github.com/weavejester/compojure/wiki/Getting-Started

Compojure Components
Compojure Web App
Ring
Clout Hiccup
Middleware
«uses»
«call»
«call»
Handler
«call»
«uses»
Abstract HTTP as
{request} and
{response}
Enhance {request}
and route to user-
defined handler
Transform
{request} into
{response}
Compojure component
Application component
DSL for writing
HTML

Compojure Execution Model
What is a “handler”?
• {request} is a map
– :server-port 80
– :server-name “127.0.0.1”
– :remote-addr “127.0.0.1”
– :uri “/”
– :scheme :http
– :headers {}
– :request-method :get
• {response} is a map
– :status 200
– :headers {:X-lang “Clojure” :X-
Framework “Compojure”}
– :body “Hello world”
{request}  handler  {response}
http://groups.google.com/group/compojure/msg/9b9c01a737242b58?
http://efreedom.com/Question/1-3488353/Big-Idea-Behind-Compojure-Routes#3490479
Preview: Simplifying handler construction is Compojure’s key value proposition

The “Ring”
http://www.slideshare.net/mmcgrana/ring-web-apps-in-idiomatic-clojure

• (ns example-app
(:use compojure.server.jetty))
• (defn hello-world [request]
{:status 200
:headers {}
:body "Hello World"})
• (run-server
{:port 8080} "/*"
(servlet hello-world))
A Simple Servlet Using Ring

Getting Started with Compojure
• https://github.com/weavejester/compojure/w
iki/Getting-Started
• Use lien to create a project and dependencies

A Simple Compojure Web Server
• (ns hello-www.core
(:use compojure.core)
(:require [compojure.route :as route]))
• (defroutes example
(GET "/" [] "<h1>Hello World Wide Web!</h1>")
(route/not-found "Page not found"))
http://weavejester.github.com/compojure/docs/getting-started.html

Clojure Interlude – Useful Macros
• (def hello-world
(-> hello-world
(with-header "X-Lang" "Clojure")
(with-header "X-Framework" "Compojure")))
• (with-header
(with-header
hello-world "X-Lang" "Clojure")
"X-Framework" "Compojure")
• (decorate hello-world
(with-header "X-Framework" "Compojure"))

Compojure Middleware
• (defn with-header[handler header value]
(fn [request]
(let [response (handler request)]
(assoc-in response
[:headers header] value))))
• (decorate hello-world
(with-header "X-Framework" "Compojure"))
handler & args  middleware  handler

Compojure Middleware (cont’d)
• Route Handlers return nil if criteria not met
– (defn index-route [request]
(if (and
(= (:request-method request) :get)
(= (:uri request) "/"))
{:status 200
:headers {}
:body "The index page"))
– (def index-route
(GET “/” “The index page”))

Route Components
Route
HTTP
Method
Response
Function
URI
Pattern
Destructuring
Form
HTTP command
to match
Pattern of URI
to match
Extract attributes
into local values
Generate
response

Index Routes
• (def index-route
(GET "/" "The index page"))
• … (GET "/product/:id"
(str "You chose product: "
(-> request :route-params :id)))
• … (GET #"/product/(d+)"
(str "You chose product: "
((:route-params request) 0)))

Index Routes (cont’d)
• (def echo-typed-url-route
(GET "*"
{:keys [scheme server-name server-port uri]}
(str (name scheme) "://" server-name ":“
server-port uri)))
• (defroutes main-routes
(POST "/save" {form-params :form-params}
(str form-params))
http://efreedom.com/Question/1-3488353/Big-Idea-Behind-Compojure-Routes#3490479

Destructuring and Local Bindings
• Additional functions update namespace local
vars
– with-params (:params)
– with-cookies (:cookies)
– with-multipart
– with-session (:session)
– with-stacktrace
• Many more documented at http://v-182-163-
94-96.ub-freebit.net:8080/docs/api

Response Values
• Base class - {:status 200, :headers {}}
• Modification depends on class of modifier
– Integer sets status code
– String adds to response body
– ISeq, File, and InputStream set response body
– URL opens stream and sets response body
– Map merges into return value map
– Fn produces response from request & response maps
– IPersistentVector updates response map

Response Value Examples
• …(GET “/” “Index page”)
• …(ANY “*” [404 “Page not found”])
• …(GET “/image”
(File. “./public/image.png”))
• …(GET “/new-product”
(if product-released? “Cool!” :next))
• …(GET “/map-example” {:body “Hi there!”})

HTML Compojure (Hiccup)
• [:h1 {:id “title”} “Hello”]
[:h1#id “title” “Hello”]
• [:div (list “Hello” “ Ingolf”)]
• [:div [:span “Hello”]
[:span “ Ingolf”]]
{tag attribute-map content}
• <h1 id=“title”>Hello</h1>
• <div>Hello Ingolf</div>
• <div><span>Hello</span>
<span>Ingolf</span>
</div>
Clojure HTML

Compojure Addons
• Sandbar
– Session and flash as a global map
– Authorization and authentication (form-based)
– Forms and form validation
https://github.com/brentonashworth/sandbar

Other Web Frameworks
• Enlive
– light weight, non-inference routing
• Webjure
– light weight, provides routing and parsing on top of Java
servlets, Clojure Page Templates
• Conjure
– Rails-like framework for Clojure
• Moustache
– a micro web framework/internal DSL to wire Ring handlers
and middlewares
• Funkyweb
– ring based framework with route inference
https://github.com/cgrand/moustache
https://github.com/tatut/Webjure
http://www.lshift.net/blog/2010/02/27/a-simple-web-application-in-clojure-using-ring-and-enlive
http://groups.google.com/group/clojure/browse_thread/thread/657adc458151e79a/5dfb6362542cf124

Enlive Example
(ns sprout.core)
(defn router [req]
(condp = (:uri req)
"/error" {:status 404
:headers {"Content-Type" "text/html"}
:body (str "Error")}
"/info" {:status 200
:body (str "Ring on " (:server-name req)) }
{:status 200
:body (str "Welcome")}))
(defn app [req] (router req))
http://www.lshift.net/blog/2010/02/27/a-simple-web-application-in-clojure-using-ring-and-enlive

Questions
• Why is Compojure named Compojure?
• What are the requirements of the underlying Java-
based web server to support?
– Scalability
– Extensibility
• How would Compojure participate in web standards?
– Web Services (non-Rest)
– WS-*
• In the long run, does Compojure need Java?
• Does Java need Compojure?

Other References
• Clojure Web Development with Ring
http://mmcgrana.github.com/2010/03/clojure-web-development-ring.html
• Compojure: Clojure Web Framework https://github.com/blog/308-compojure-
clojure-web-framework
• How do you make a web application in Clojure?
http://stackoverflow.com/questions/167262/how-do-you-make-a-web-
application-in-clojure
• Introduction to the Compojure web Framework
http://groups.google.com/group/clojure/browse_frm/thread/448865254f9bd293?
pli=1
• mmcgrana’s ring at master https://github.com/mmcgrana/ring
• weavejester‘s compojure at master https://github.com/weavejester/compojure
• weavejester’s hiccup at master https://github.com/weavejester/hiccup
• WEB DEVELOPMENT – Mature Clojure web frameworks
http://efreedom.com/Question/1-3551805/Mature-Clojure-Web-Frameworks
• arbscht‘s clout at string-keys https://github.com/arbscht/clout/tree/string-keys
• cgrand‘s enlive at master https://github.com/cgrand/enlive

Web programming in clojure

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