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.# Chapter 3: Starting The Project

Creating a Clojure project

We're going to start by asking Leiningen to start a web application for us.

  $: lein new compojure chatter

The command creates a new compojure project called "Chatter". This command results in the creation of a directory chatter.

Start the server with this command

  $: lein ring server-headless

lein will download a bunch of stuff from the Internet the first time it runs. After this finishes, open your browser to http://localhost:3000. It will display a message "Hello World".

Stop the server connection by hitting "Ctrl + C" in the command line.

Let's take a closer look at what's in the chatter directory. It looks like this:

.
├── project.clj
├── README.md
├── resources
│   └── public
├── src
│   └── chatter
│       └── handler.clj
└── test
    └── chatter
        └── handler_test.clj

6 directories, 4 files

project.clj is a Clojure file that describes what our project does and what libraries it needs to run.

README.md is a markdown file (.md), which describes this project.

resources is a folder where you store static HTML, CSS, and images.

src is where the source code lives.

test is where the tests are stored.

A closer look at the src directory

In the editor, open the file src/chatter/handler.clj.

The file that ends with ".clj" indicates this is a Clojure file. Clojure programs are made up of expressions. Expressions are either a single name, number, string, or a list of expressions beginning with a paren (or parenthesis). These expressions make your app appear and function in a web browser.

In the src/chatter/handler.clj file, the first expression (ns chatter.handler ... tells Clojure what we want to call the namespace (ns) being defined in this file. In this case we want to call it "chatter.handler". You can think of a namespace as similar to a module in ruby or python. A container for some code.

The sub-expression, the expression below, begins with (:require .... This is importing the ring and compojure libraries. Those are low-level Clojure libraries for building web apps.

The second expression is (defroutes app-routes .... defroutes is specific to Clojure web apps. It creates a set of routes and gives them the name app-routes. The expressions after symbol app-routes are the route definitions. There are two here.

The first is:

(GET "/" [] "Hello World")

There are four parts to this expression:

  1. GET: Name of function, GET. GETs are requests for information or data.
  2. "/": this is the url of the page.
  3. []: this is an empty parameter vector. When you do a search or fill out a form, the parameters narrow the result.
  4. "Hello World": the result that gets sent back to the requesting browser.

After the GET request is the second route definition app-route, which is:

(route/not-found "Not Found")

This means when the server gets any kind of request other than GET /, it should return "Not Found."

Stop the server by going back to the terminal and doing "Ctrl+C".

Next Chapter 5: Reorganize some things