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

-----
isHidden:       false
menupriority:   1
kind:           article
created_at:     2012-01-15T19:14:40+02:00
title: Haskell web programming
subtitle: A Yesod tutorial
author_name: Yann Esposito
author_uri: yannesposito.com
tags:
  - yesod
  - haskell
  - programming
  - web
macros:
 html5: '<span class="sc">html5</span>' 
 html: '<span class="sc">html</span>' 
 http: '<span class="sc">http</span>' 
 url: '<span class="sc">url</span>' 
 css: '<span class="sc">css</span>' 
-----
<%= blogimage("flying_neo.jpg","Neo Flying at warp speed") %>

begindiv(intro)

_update_: updated for yesod 0.10

<%= tldr %> A simple yesod tutorial.
Yesod is a Haskell web framework.
You shouldn't need to know Haskell. 


> <center><sc><b>Table of content</b></sc></center>
> 
> * Table of Content (generated)
> {:toc}

enddiv

Why Haskell?

<%= blogimage("haskell-benchmark.png","Impressive Haskell Benchmark") %>

Its efficiency (see [Snap Benchmark][snapbench] _&_ 
[Warp Benchmark][warpbench][^benchmarkdigression]).
Haskell is an order of magnitude faster than interpreted languages 
like [Ruby][haskellvsruby] and [Python][haskellvspython][^speeddigression].

Haskell is a high level language and make it harder to shoot you in the foot
than `C`, `C++` or `Java` for example.
One of the best property of Haskell being:

> "If your program compile it will be 
>  very close to what the programmer intended".

Haskell web frameworks handle parallel tasks perfectly. 
For example even better than node.js[^nodejstroll].

<%= leftblogimage("thousands_smiths.jpg","Thousands of Agent Smith") %>

From the pure technical point of view,
Haskell seems to be the perfect web development tool.
Weaknesses of Haskell certainly won't be technical:

- Hard to grasp Haskell
- Hard to find a Haskell programmer
- The Haskell community is smaller than the community for `/.*/` 
- There is no [heroku](http://heroku.com) for Haskell (even if [Greg Weber did it](http://www.yesodweb.com/blog/2011/07/haskell-on-heroku), it was more a workaround).

I won't say these are not important drawbacks.
But, with Haskell your web application will have both properties
to absorb an impressive number of parallel request securely
and to adapt to change.

Actually there are three main Haskell web frameworks:

1. [Happstack](http://happstack.com)
2. [Snap](http://snapframework.com)
3. [Yesod](http://yesodweb.com)

I don't think there is a real winner between these three framework.
The choice I made for yesod is highly subjective.
I just lurked a bit and tried some tutorials.
I had the feeling yesod make a better job at helping newcomers.
Furthermore, apparently the yesod team seems the most active.
Of course I might be wrong since it is a matter of feeling.

<%= blogimage("owl_draw.png","1. Draw some circles. 2. Draw the rest of the fucking owl") %>

Why did I write this article?
The yesod documentation and particularly the book are excellent.
But I missed an intermediate tutorial.
This tutorial won't explain all details.
I tried to give a step by step of how to start from a five minute tutorial 
to an almost production ready architecture.
Furthermore explaining something to others is a great way to learn.
If you are used to Haskell and Yesod, this tutorial won't learn you much.
If you are completely new to Haskell and Yesod it might hopefully helps you.
Also if you find yourself too confused by the syntax, it might helps to read this 
[article](http://blog.ezyang.com/2011/11/how-to-read-haskell/)

During this tutorial you'll install, initialize and configure your first yesod project.
Then there is a very minimal 5 minutes yesod tutorial to heat up and verify the awesomeness of yesod.
Then we will clean up the 5 minutes tutorial to use some "best practices".
Finally there will be a more standard real world example; a minimal blog system.

[warpbench]: http://www.yesodweb.com/blog/2011/03/preliminary-warp-cross-language-benchmarks
[snapbench]: http://snapframework.com/blog/2010/11/17/snap-0.3-benchmarks
[^benchmarkdigression]: One can argue these benchmark contains many problems. But the benchmarks are just here to give an order of idea. Mainly Haskell is very fast.
[^speeddigression]: Generally _high level_ Haskell is slower than C, but _low level_ Haskell is equivalent to C speed. It means that even if you can easily link C code with Haskell, this is not needed to reach the same speed. Furthermore writing a web service in C/C++ seems to be a very bad idea. You can take a look at a [discussion on HN about this](http://news.ycombinator.com/item?id=3449388). 
[^nodejstroll]: If you are curious, you can search about [the Fibonacci node.js troll](http://www.unlimitednovelty.com/2011/10/nodejs-has-jumped-shark.html). Without any tweaking, [Haskell handled this problem perfectly](http://mathias-biilmann.net/posts/2011/10/is-haskell-the-cure). I tested it myself using yesod instead of Snap.
[haskellvsruby]: http://shootout.alioth.debian.org/u64q/benchmark.php?test=all&lang=ghc&lang2=yarv
[haskellvspython]: http://shootout.alioth.debian.org/u64q/benchmark.php?test=all&lang=ghc&lang2=python3

## Before the real start

### Install

The recommended way to install [Haskell][haskell] 
is to download the [Haskell Platform][haskellplatform].

[haskell]: http://www.haskell.org
[haskellplatform]: http://www.haskell.org/platform

Once done, you need to install yesod.
Open a terminal session and do:

<code class="zsh">
~ cabal update
~ cabal install yesod cabal-dev
</code>

There are few steps but it should take some time to finish.

### Initialize

You are now ready to initialize your first yesod project.
Open a terminal and type:

<code class="zsh">
~ yesod init
</code>

Enter your name, choose `yosog` for the project name and enter `Yosog` for the name of the Foundation.
Finally choose `sqlite`.
Now, start the development cycle:

<code class="zsh">
~ cd yosog
~ cabal-dev install && yesod --dev devel
</code>

This will compile the entire project. Be patient it could take a while the first time.
Once finished a server is launched and you could visit it by clicking this link:

[`http://localhost:3000`](http://localhost:3000)

Congratulation! Yesod works!

<blockquote>

Note: if something is messed up use the following command line inside the project directory.

<code class="zsh">
\rm -rf dist/* ; cabal-dev install && yesod --dev devel
</code>

</blockquote>

Until the end of the tutorial, use another terminal and let this one open 
in a corner to see what occurs.

### Configure git

> Of course this step is not mandatory for the tutorial 
> but it is a good practice.

Copy this `.gitignore` file into the `yosog` folder.

<code class="zsh" file=".gitignore">
cabal-dev
dist
.static-cache
static/tmp
*.sqlite3
</code>

Then initialize your git repository:

<code class="zsh">
~ git init .
~ git add .
~ git commit -a -m "Initial yesod commit"
</code>

We are almost ready to start.

### Some last minute words

Up until here, we have a directory containing a bunch of files 
and a local web server listening the port 3000.
If we modify a file inside this directory, yesod should try
to recompile as fast as possible the site. 
Instead of explaining the role of every file,
let's focus only on the important files/directories for this tutorial:

1. `config/routes`
2. `Handler/`
3. `templates/`
4. `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.  |
| `config/models`   | is where you'll configure the persistent objects (database tables). |

During this tutorial we'll modify other files as well,
but we won't explore them in detail.

Also note, shell commands are executed in the root directory of your project instead specified otherwise.

We are now ready to start!

## Echo

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

> Goal:
> 
> Make a server that when accessed `/echo/[some text]` should return a web page containing "some text" inside an `h1` bloc.

In a first time, we must declare the %url of the form `/echo/...` are meaningful.
Let's take a look at the file `config/routes`:

<pre>
/static StaticR Static getStatic
/auth   AuthR   Auth   getAuth

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

/ RootR GET
</pre>

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

<pre>
/echo/#String EchoR GET
</pre>

This line contains three elements: the %url pattern, a handler name, an %http method.
I am not particularly fan of the big R notation but this is the standard convention.

If you save `config/routes`, you should see your terminal in which you launched `yesod devel` activate and certainly displaying an error message.

<pre>
Application.hs:31:1: Not in scope: `getEchoR'
</pre>

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

<code class="haskell">
getEchoR :: String -> Handler RepHtml
getEchoR theText = do
    defaultLayout $ do
        [whamlet|<h1>#{theText}|]
</code>

Don't worry if you find all of this a bit cryptic. 
In short it just declare a function named `getEchoR` with one argument (`theText`) of type String.
When this function is called, it return a `Handler RepHtml` whatever it is. 
But mainly this will encapsulate our expected result inside an %html text.

After saving the file, you should see yesod recompile the application.
When the compilation is finished you'll see the message: `Starting devel application`.

Now you can visit: [`http://localhost:3000/echo/Yesod%20rocks!`](http://localhost:3000/echo/Yesod%20rocks!)

TADA! It works! 

### Bulletproof?

<%= blogimage("neo_bullet_proof.jpg","Neo stops a myriad of bullets") %>

Even this extremely minimal web application has some impressive properties.
For exemple, imagine an attacker entering this %url:

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

The special characters are protected for us. 
A malicious user could not hide some bad script inside.

This behavior 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 "<code><</code>" by "`&lt;`".
Thanks to yesod, this tedious job is done for us.

<code class="zsh">
"http://localhost:3000/echo/some%20text<a>" :: URL
                    ↓
              "some text<a>"                :: String
                    ↓
          "some text &lt;a&gt;"             :: Html 
</code>

Yesod is not only fast, it helps us to remain secure.
It protects us from many common errors in other paradigms.
Yes, I am looking at you PHP!

### Cleaning up

Even this very minimal example should be enhanced.
We will clean up many details:

- Use a general %css (cleaner than the empty by default)
- Dispatch handler code into different files
- Use `Data.Text` instead of `String`
- Put our "views"[^explainviewwidget] inside the `template` directory

[^explainviewwidget]: By view I mean yesod widget's hamlet, lucius and julius files. 

#### Use a better %css

It is nice to note, the default template is based on %html5 boilerplate.
Let's change the default %css. 
Add a file named `default-layout.lucius` inside the `templates/` directory containing:

<code class="css" file="default-layout.lucius">
body {
    font-family: Helvetica, sans-serif; 
    font-size: 18px; }
#main {
    padding: 1em;
    border: #CCC solid 2px;
    border-radius: 5px;
    margin: 1em;
    width: 37em;
    margin: 1em auto;
    background: #F2F2F2;
    line-height: 1.5em;
    color: #333; }
.required { margin: 1em 0; }
.optional { margin: 1em 0; }
label { width: 8em; display: inline-block; }
input, textarea { background: #FAFAFA}
textarea { width: 27em; height: 9em;}
ul { list-style: square; }
a { color: #A56; }
a:hover { color: #C58; }
a:active { color: #C58; }
a:visited { color: #943; }
</code>

Personally I would prefer if such a minimal %css was put with the scaffolding tool.
I am sure somebody already made such a minimal %css which give the impression 
the browser handle correctly %html without any style applied to it.
But I digress.

#### Separate Handlers

Generally you don't want to have all your code inside a unique file.
This is why we will separate our handlers.
In a first time create a new file `Handler/Echo.hs` containing:

<code class="haskell">
module Handler.Echo where

import Import

getEchoR :: String -> Handler RepHtml
getEchoR theText = do
    defaultLayout $ do
        [whamlet|<h1>#{theText}|]
</code>

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

We must declare this new file into`yosog.cabal`. 
Just after `Handler.Root`, add:

<pre>
    Handler.Echo
</pre>

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

<code class="haskell">
import Handler.Echo
</code>

This is it. 

<small><em>ps:</em> I am sure not so far in the future we could simply write 
`yesod add-handler Echo` to declare it and create a new handler file.</small>

#### `Data.Text`

It is a good practice to use `Data.Text` instead of `String`.

To declare it, add this import directive to `Foundation.hs` (just after the last one):

<code class="diff">
import Data.Text
</code>

We have to modify `config/routes` and our handler accordingly. 
Replace `#String` by `#Text` in `config/routes`:

<pre>
/echo/#Text EchoR GET
</pre>

And do the same in `Handler/Echo.hs`:

<code class="haskell" file="Echo.hs">
module Handler.Echo where

import Import

getEchoR :: Text -> Handler RepHtml
getEchoR theText = do
    defaultLayout $ do
        [whamlet|<h1>#{theText}|]
</code>

#### Use templates

Some %html (more precisely hamlet) is written directly inside our handler.
We should put this part inside another file.
Create the new file `templates/echo.hamlet` containing:

<code class="haskell" file="echo.hamlet">
<h1> #{theText}
</code>

and modify the handler `Handler/Echo.hs`:

<code class="haskell">
getEchoR :: Text -> Handler RepHtml
getEchoR theText = do
    defaultLayout $ do
        $(widgetFile "echo")
</code>

At this point, our web application is structured between different files.
Handler are grouped, we use `Data.Text` and our views are in templates.
It is the time to make a slightly more complex example.

## Mirror

<%= leftblogimage("mirror.jpg","Neo touching a mirror") %>

Let's make another minimal application.
You should see a form containing a text field and a validation button.
When you enter some text (for example "Jormungad") and validate,
the next page present you the content and its reverse appended to it. 
In our example it should return "JormungaddagnumroJ". 

First, add a new route:

<pre>
/mirror MirrorR GET POST
</pre>

This time the path `/mirror` will accept GET and POST requests.
Add the corresponding new Handler file:

<code class="haskell" file="Mirror.hs">
module Handler.Mirror where

import Import
import qualified Data.Text as T

getMirrorR :: Handler RepHtml
getMirrorR = do
    defaultLayout $ do
        $(widgetFile "mirror")

postMirrorR :: Handler RepHtml
postMirrorR =  do
        postedText <- runInputPost $ ireq textField "content"
        defaultLayout $ do
            $(widgetFile "posted")
</code>

Don't forget to declare it inside `yosog.cabal` and `Application.hs`.

We will need to use the `reverse` function provided by `Data.Text` 
which explain the additional import.

The only new thing here is the line that get the POST parameter named "content".
If you want to know more detail about it and form in general you can take 
look at [the yesod book](http://www.yesodweb.com/book/forms).

Create the two corresponding templates:

<code class="html" file="mirror.hamlet">
<h1> Enter your text
<form method=post action=@{MirrorR}>
    <input type=text name=content>
    <input type=submit>
</code>

<code class="html" file="posted.hamlet">
<h1>You've just posted
<p>#{postedText}#{T.reverse postedText}
<hr>
<p><a href=@{MirrorR}>Get back
</code>

And that is all.
This time, we won't need to clean up.
We may have used another way to generate the form
but we'll see this in the next section.

Just try it by [clicking here](http://localhost:3000/mirror).

Also you can try to enter strange values.
Like before, your application is quite secure.

## A Blog

We saw how to retrieve %http parameters.
It is the time to save things into a database.

As before add some routes inside `config/routes`:

<pre>
/blog               BlogR       GET POST
/blog/#ArticleId    ArticleR    GET
</pre>

This example will be very minimal:

- `GET`  on `/blog` should display the list of articles.
- `POST` on `/blog` should create a new article
- `GET`  on `/blog/<article id>` should display the content of the article.

First we declare another model object.
Append the following content to `config/models`:

<pre>
Article
    title   Text
    content Html 
    deriving
</pre>

As `Html` is not an instance of `Read`, `Show` and `Eq`,
we had to add the `deriving` line.
If you forget it, there will be an error.

After the route and the model, we write the handler.
First, declare a new Handler module.
Add `import Handler.Blog` inside `Application.hs` and add it into `yosog.cabal`.
Let's write the content of `Handler/Blog.hs`.
We start by declaring the module and by importing some block necessary to 
handle Html in forms.

<code class="haskell">
module Handler.Blog
    ( getBlogR
    , postBlogR
    , getArticleR
    )
where

import Import

-- to use Html into forms
import Yesod.Form.Nic (YesodNic, nicHtmlField)
instance YesodNic Yosog
</code>

<small>Remark: it is a best practice to add the YesodNic instance inside `Foundation.hs`. 
I put this definition here to make things easier but you should see a warning about this orphan instance.
To put the include inside Foundation.hs is left as an exercice to the reader.</small>

<small>_Hint: Do not forget to put `YesodNic` and `nicHtmlField` inside the exported objects of the module._
</small>

<code class="haskell">
entryForm :: Form Article
entryForm = renderDivs $ Article
    <$> areq   textField "Title" Nothing
    <*> areq   nicHtmlField "Content" Nothing
</code>

This function defines a form for adding a new article.
Don't pay attention to all the syntax. 
If you are curious you can take a look at Applicative Functor.
You just have to remember `areq` is for required form input.
Its arguments being: `areq type label default_value`.

<code class="haskell">
-- The view showing the list of articles
getBlogR :: Handler RepHtml
getBlogR = do
    -- Get the list of articles inside the database.
    articles <- runDB $ selectList [] [Desc ArticleTitle]
    -- We'll need the two "objects": articleWidget and enctype
    -- to construct the form (see templates/articles.hamlet).
    ((_,articleWidget), enctype) <- generateFormPost entryForm
    defaultLayout $ do
        $(widgetFile "articles")
</code>

This handler should display a list of articles.
We get the list from the DB and we construct the form.
Just take a look at the corresponding template:

<code class="html" file="articles.hamlet">
<h1> Articles
$if null articles
    -- Show a standard message if there is no article
    <p> There are no articles in the blog
$else
    -- Show the list of articles
    <ul>
        $forall Entity articleId article <- articles
            <li> 
                <a href=@{ArticleR articleId} > #{articleTitle article}
<hr>
  <form method=post enctype=#{enctype}>
    ^{articleWidget}
    <div>
        <input type=submit value="Post New Article">
</code>

You should remark we added some logic inside the template.
There is a test and a "loop".

Another very interesting part is the creation of the form.
The `articleWidget` was created by yesod.
We have given him the right parameters 
(input required or optional, labels, default values).
And now we have a protected form made for us.
But we have to create the submit button.

Get back to `Handler/Blog.hs`.

<code class="haskell">
-- we continue Handler/Blog.hs
postBlogR :: Handler RepHtml
postBlogR = do
    ((res,articleWidget),enctype) <- runFormPost entryForm
    case res of 
         FormSuccess article -> do 
            articleId <- runDB $ insert article
            setMessage $ toHtml $ (articleTitle article) <> " created"
            redirect $ ArticleR articleId 
         _ -> defaultLayout $ do
                setTitle "Please correct your entry form"
                $(widgetFile "articleAddError")
</code>

This function should be used to create a new article.
We handle the form response.
If there is an error we display an error page.
For example if we left some required value blank.
If things goes right: 

- we add the new article inside the DB (`runDB $ insert article`)
- we add a message to be displayed (`setMessage $ ...`)
- we are redirected to the article web page.

Here is the content of the error Page:

<code class="haskell">
<form method=post enctype=#{enctype}>
    ^{articleWidget}
    <div>
        <input type=submit value="Post New Article">
</code>

Finally we need to display an article:

<code class="haskell">
getArticleR :: ArticleId -> Handler RepHtml
getArticleR articleId = do
    article <- runDB $ get404 articleId
    defaultLayout $ do
        setTitle $ toHtml $ articleTitle article
        $(widgetFile "article")
</code>

The `get404` function try to do a get on the DB. 
If it fails it return a 404 page.
The rest should be clear. 
Here is the content of `templates/article.hamlet`:

<code class="html" file="article.hamlet">
<h1> #{articleTitle article}
<article> #{articleContent article}
</code>

The blog system is finished.
Just for fun, you can try to create an article with the following content:

<code class="html">
<p>A last try to <em>cross script</em> 
   and <em>SQL injection</em></p>
<p>Here is the first try: 
   <script>alert("You loose");</script></p>
<p> And Here is the last </p>
"); DROP TABLE ARTICLE;;
</code>

## Conclusion

This is the end of this tutorial.
I made it very minimal.

If you already know Haskell and you want to go further, 
you should take a look at the
recent [i18n blog tutorial](http://yesodweb.com/blog/2012/01/blog-example).
It will be obvious I inspired my own tutorial on it.
You'll learn in a very straightforward way how easy it is to use authorizations,
Time and internationalization. 

If, on the other hand you don't know Haskell.
Then you shouldn't jump directly to web programming.
Haskell is a very complex and unusual language.
My advice to go as fast as possible in using Haskell for web programming is:

1. Start by [try Haskell in your browser](http://tryhaskell.org)
2. Then read the excellent [Learn you a Haskell for Great Good](http://learnyouahaskell.com)
3. If you have difficulties in understanding concepts like monads, you should really read [these articles](http://homepages.inf.ed.ac.uk/wadler/topics/monads.html). For me they were enlightening.
4. If you feel confident, you should be able to follows the [yesod book](http://yesodweb.com/book) and if you find difficult to follows the yesod book, you should read [real world Haskell](http://book.realworldhaskell.org) first (it is a must read).

Also, note that:

- [haskell.org](http://haskell.org) is full of excellent resources.
- [hoogle](http://www.haskell.org/hoogle/) will be very useful
- Use [hlint](http://community.haskell.org/~ndm/hlint/) as soon as possible to get good habits.

As you should see, if you don't already know Haskell,
the path is long but I guaranty you it will be very rewarding!

_ps:_ You can download the source of this yesod blog tutorial at 
[github.com/yogsototh/yosog](http://github.com/yogsototh/yosog).