A Yesod Tutorial

As of March 2020, School of Haskell has been switched to read-only mode.


Why Haskell?

Impressive Haskell Benchmark

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

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 for Haskell (even if Greg Weber did it, 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
  2. Snap
  3. Yesod

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.

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

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.

Before the real start


The recommended way to install Haskell is to download the Haskell Platform.

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

~ cabal update
~ cabal install yesod cabal-dev

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


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

~ yesod init

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:

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

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:


Congratulation! Yesod works!

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

\rm -rf dist/* ; cabal-dev install && yesod --dev devel

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.


Then initialize your git repository:

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

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


`config/routes` is where you'll configure the map %url โ†’ 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!


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


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:

/static StaticR Static getStatic
/auth   AuthR   Auth   getAuth

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

/ HomeR GET

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

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

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

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

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!

TADA! It works!


"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!");

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 "<" by "<". Thanks to yesod, this tedious job is done for us.

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

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

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:

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; }

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:

module Handler.Echo where

import Import

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

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

We must declare this new file intoyosog.cabal. Just after Handler.Home, add:


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

``` haskell
import Handler.Echo

This is it.

ps: 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.


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):

import Data.Text

We have to 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

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:

<h1> #{theText}

and modify the handler Handler/Echo.hs:

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

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.


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:

/mirror MirrorR GET POST

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

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")

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.

Create the two corresponding templates:

<h1> Enter your text
<form method=post action=@{MirrorR}>
    <input type=text name=content>
    <input type=submit>
<h1>You've just posted
<p>#{postedText}#{T.reverse postedText}
<p><a href=@{MirrorR}>Get back

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.

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:

/blog               BlogR       GET POST
/blog/#ArticleId    ArticleR    GET

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:

    title   Text
    content Html 

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.

module Handler.Blog
    ( getBlogR
    , postBlogR
    , getArticleR

import Import
import Data.Monoid

-- to use Html into forms
import Yesod.Form.Nic (YesodNic, nicHtmlField)
instance YesodNic App
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._Hint: Do not forget to put `YesodNic` and `nicHtmlField` inside the exported objects of the module._
entryForm :: Form Article
entryForm = renderDivs $ Article
    <$> areq   textField "Title" Nothing
    <*> areq   nicHtmlField "Content" Nothing

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.

-- 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")

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:


Articles $if null articles -- Show a standard message if there is no article

There are no articles in the blog $else -- Show the list of articles