The document provides an overview of web development using the Ring framework in Clojure, detailing how to create a simple HTTP handler that responds with 'hello world!' and how to run a Ring application using Jetty. It also discusses the use of middleware for enhancing handlers, routing requests with Compojure, and constructing a JSON-based REST API for managing people information. Additionally, it briefly covers using Hiccup for generating HTML in conjunction with the created API.

1. Web development
by Carlo Sciolla

2. Ring handles HTTP
ring
GET /home
HTTP/1.1 200 OK
your app

3. Map in, map out
GET /home HTTP/1.1
User-Agent: curl/7.37.1
Host: 127.0.0.1:4000
Accept: *
{:request-method :get
:url /home HTTP/1.1
:headers {“User-Agent” ...}
...}
application
{:status 200
:body “Hello, world!”
:headers {...}
...}
HTTP/1.1 200 OK
Content-Type: text/plain
Hello, world!

4. Anatomy of a ring handler
(defn handler [request]
{:status 200
:headers {"Content-Type" "text/plain"}
:body "Hello World"})
Following the simplicity of the ring abstraction, ring handlers are plain functions. Compare this to a Java
Servlet.

5. How to run a ring application
(ns my.web
(:use ring.jetty.adapter))
(defn handler [request]
{:status 200
:headers {"Content-Type" "text/plain"}
:body "Hello World"})
(run-jetty handler 8080)
You can find ring adapters (-> wrappers) for the most common app servers. Ring will dispatch HTTP
requests to your handler.

6. How to run a ring application
; in project.clj:
:plugins [[lein-ring "0.9.3"]]
:ring {:handler hello-world.core/handler}
$ lein ring server
Started nREPL server on port 4500
2015-04-20 20:19:37.495:INFO:oejs.Server:jetty-7.6.13.v20130916
2015-04-20 20:19:37.548:INFO:oejs.AbstractConnector:Started
SelectChannelConnector@0.0.0.0:8090
Started server on port 8090
An alternative way to start your application is to run it from sources.

7. How to run a ring application
; in project.clj:
:plugins [[lein-ring "0.9.3"]]
:ring {:handler hello-world.core/handler}
$ lein ring uberwar
Created /Users/skuro/demo/target/demo-1.0-standalone.war
If you need a standard packaging, that’s available also.

8. Filtering with middleware
(defn wrap-content-type [handler type]
(fn [request]
(let [response (handler request)]
(assoc-in response [:headers “Content-Type”] type))))
(def wrapped
(wrap-content-type handler “text/plain”))
Middleware functions wrap regular handlers to enhance their functionalities. They accept in input a
handler plus other arguments, and must return a new handler. Compare them to Servlet Filters.

9. Compojure to route requests
(ns hello-world.core
(:require [compojure.core :refer :all]
[compojure.route :as route]))
(defn surname [name]
({"Alfred" "Hitchcock"} name))
(defroutes app
(GET "/:name" [name] (surname name)
(route/not-found "<h1>Page not found</h1>"))
Compojure helps you route request to the correct function depending on the request details. Route
matches are attempted in lexical order.

10. Compojure to route requests
(defroutes my-api
(context "/api" []
(GET "/people" [] (list-people)
(GET "/people/:name" [name] (describe-person name))))
; the above registers handles for these URLs:
; /api/people
; /api/people/:name
Routes can be grouped together via context, which establishes a common prefix and common
destructured parameters.

11. Middleware to convert to JSON
(use '[ring.middleware.json :only [wrap-json-response]]
'[ring.util.response :only [response]])
(defn handler [request]
(response {:foo "bar"}))
(def app
(wrap-json-response handler))
An handler can translate automatically your response bodies into JSON, and will also translate the
incoming request bodies into Clojure datastructures.

12. Questions?

13. create a JSON-based REST API that provides the following endpoints:
GET /people
Lists the current people. Ex.:
[{"id": 0, "name": "Carlo"}]
GET /people/:name
Returns the details of the selected person
POST /people
Adds a new person. The body is of the form:
{"name": "Carlo"}
Exercise time:
$ lein new compojure folks

14. A Clojure DSL for HTML
; in your project.clj
:dependencies [[hiccup "1.0.5"]]
user=> (use ‘hiccup.core)
nil
user=> (html [:span {:class "foo"} "bar"])
"<span class="foo">bar</span>"
Hiccup is a library that translates Clojure datastructures into HTML.

15. introduce a home page to manage people via HTML:
GET /
- The HTML contains the list of the current people
- The HTML contains a form to create a new person (using the POST
endpoint you created)
Exercise time:
Add HTML pages to your app

16. Demo
ClojureScript

17. Thanks
Carlo Sciolla CTO
carlo.sciolla@sytac.nl
+31 6 21205911
mail
phone