scratch/content/html/en/blog/Yesod-tutorial-for-newbies.md

isHidden	menupriority	kind	created_at	title	author_name	author_uri	tags
false	1	article	2011-12-28T15:14:40+02:00	Yesod tutorial for newbies	Yann Esposito	yannesposito.com	yesod haskell programming web

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begindiv(intro)

<%= tldr %> A simple yesod tutorial. You shouldn't need to know Haskell very well.

• Will be replaced by Table of Content {:toc}

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You want the best technology to handle your new web application? Me too. After searching a lot, it appears if you focus only on technical aspect, Haskell is the way to go.

It is extremely fast. It is secure by nature. Many typical programming bug are hard to make in Haskell. Haskell is also a "high level of abstraction" language. You can organize your code clearly.

Actually there are three web frameworks in Haskell:

1. Happstack
2. Snap
3. Yesod

It is very hard to choose between these three. But my feeling goes to Yesod. It appears to be the one with most part done for you. As a beginner, let's stay away of the detail as most as possible.

The following tutorial contains some parts.

• Install → Install haskell and yesod. This can be long, but it is all automatic and this should be straightforward.
• Initialization → Initialize the project and configure it.
• Configure git → This is not mandatory, but it is a good practice.
• Verify the security → A first step to verify the yesod framework protect us from most common errors.
• Create a minimal blog → This is the "hello world" of web framework.
• Some tuning → Use html5 boilerplate for example.

Install

First you need to install Haskell. The recommended way to do this is to use the Haskell Platform.

Secondly you need to install yesod.

> cabal update > cabal install yesod cabal-dev

That is all. It should take some time to do this as cabal will download all package and then compile them.

Initialization

Open a terminal and:

> yesod init

I entered my name, the name of the project was yosog and the name of the Foundation was Yosog, then I chosen sqlite.

Perfect. Now you can start the development cycle:

> cd yosog > cabal-dev install && yesod --dev devel

This will compile the entire project. In the end you should now be able to see your local website by clicking this link:

http://localhost:3000

Congratulation! You were able to see your yesod powered website. For the rest of the tutorial, use another terminal and let this one open in a corner to see what occurs.

Configure git

It is not mandatory for a tutorial, but it is a good practice to have a CVS.

To use git copy this .gitignore file into the yosog folder.

cabal-dev dist .static-cache static/tmp *.sqlite3

Then initialize your git repository:

> git init . > git add . > git commit -a -m "Initial yesod commit"

Now we are ready to modify our web application.

A last point

What did we done:

1. We have a directory containing a bunch of files
2. We have a local web server on port 3000

If we modify a file inside this directory, yesod should try to recompile as fast as possible the site. This way, you should see the modification you done.

Instead of explaining the role of every file, let's get straight to the point.

Inside the yosog the important files/directories for this tutorial are:

1. config/routes
2. Handler/
3. templates/
4. static/
5. config/models

Obviously:

• config/routes is where you'll configure the map URL → Code.
• Handler/ contains the files that will contain the code called when a URL is accessed.
• templates/ contains HTML, JS and CSS templates.
• static/ contains static files.
• config/models is where you'll configure the persistent objects (database tables).

With these informations we should be able to do a lot. Also note until here we don't even typed any line of Haskell.

Protected echo

To verify the quality of the security of the yesod framework, let's look at a minimal echo application.

Our goal:

Accessing http://localhost:3000/echo/some%20text, should display "some text" in an %html web page.

Let's take a look at the file config/routes:

/static StaticR Static getStatic /auth AuthR Auth getAuth

/favicon.ico FaviconR GET /robots.txt RobotsR GET

/ RootR GET

We want to add a route of the form /echo/[anything] somehow and do some action with this. We add the following:

/echo/#String EchoR GET

This line contains three elements: the url pattern, a handler name, an HTTP method. I am not particularly fan of the big R in the end of handler names. But this is the standard convention, then I use it.

If you save config/routes, you should see your terminal in which you launched yesod devel do things. And certainly break in error.

Application.hs:31:1: Not in scope: `getEchoR'

Why? Simply because we didn't written the code for the handler EchoR. Now, let's do this. Edit the file Handler/Root.hs and append this:

getEchoR :: String -> Handler RepHtml getEchoR theText = do defaultLayout $ do [whamlet|

#{theText}|]

After saving the file, you should see yesod recompile the application. When the compilation is finished you'll see the message: Starting devel application. You can now visit: http://localhost:3000/echo/Yesod%20rocks!

TADA! It works.

Now, let's try to attack our website by entering name with special characters:

[http://localhost:3000/echo/<a>I'm <script>alert("Bad!");](http://localhost:3000/echo/I'm " %>

The special characters are protected for us. If you have a malicious user, he could not hide some bad script inside his name for example.

This is a direct consequence of type safety. The URL string is put inside a URL type. Then the interesting part in the URL is put inside a String type. To pass from URL type to String type some transformation are made. For example, replace all "%20" by space characters. Then to show the String inside an HTML document, the string is put inside an HTML type. Some transformations occurs like replace "<" by "<". Thanks to yesod, most of tedious string transformation job is done for us.

"http://localhost:3000/echo/some%20text" :: URL ↓ "some text" :: String ↓ "some text <a>" :: HTML

That was the first very minimal example, and we already verified Yesod protect us from many common errors.

Cleaning up

This first example was nice, but for simplicity reason we didn't used best practices.

First we will separate the handler code into different files. After that we will use Data.Text instead of String.

Separate handlers

In a first time create a new file Handler/Echo.hs containing:

module Handler.Echo where

import Import

getEchoR :: String -> Handler RepHtml getEchoR theText = do defaultLayout $ do [whamlet|

#{theText}|]

Do not forget to remove the getEchoR function inside the Handler/Root.hs file.

We must declare the file inside the cabal configuration file yosog.cabal. Just after Handler.Root add:

    Handler.Echo

We must also declare the new Handler module inside Application.hs. Just after the "import Handler.Root", add:

import Handler.Echo

Use Data.Text instead of String

Now our handler is separated in another file.

But we used String but it is a good practice to use Data.Text instead.

To declare we will use Data.Text we modify the file Foundation.hs. Add an import directive just after the last one:

import Data.Text

And also we must modify config/routes and our handler accordingly. Replace #String by #Text in config/routes:

/echo/#Text EchoR GET

And do the same in Handler/Echo.hs:

module Handler.Echo where

import Import

getEchoR :: Text -> Handler RepHtml getEchoR theText = do defaultLayout $ do [whamlet|

#{theText}|]

Use a new template file

The last thing to change in order to do things like in a real project is to use another template file.

Just create a new file template/echo.hamlet containing:

#{theText}

and modify the handler Handler/Echo.hs:

getEchoR :: Text -> Handler RepHtml getEchoR theText = do defaultLayout $ do $(widgetFile "echo")

Protected input

---

<%= startTodo %>

• Display something, show it is protected.
• Make the same as before, but with an input.
• Create a minimal blog system.
• Change template to html5 boilerplate.
• Use Authentification.

<%= endTodo %>