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<h1>
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Yesod tutorial for newbies
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</h1>
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<p><img alt="Title image" src="/Scratch/img/blog/Yesod-tutorial-for-newbies/warp-benchmark.png" /></p>
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<div class="intro">
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<p><span class="sc"><abbr title="Too long; didn't read">tl;dr</abbr>: </span> A simple yesod tutorial. You shouldn’t need to know Haskell very well. </p>
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<blockquote>
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<ul id="markdown-toc">
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<li><a href="#install">Install</a></li>
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<li><a href="#initialization">Initialization</a></li>
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<li><a href="#configure-git">Configure git</a></li>
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<li><a href="#a-last-point">A last point</a></li>
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<li><a href="#protected-echo">Protected echo</a></li>
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<li><a href="#cleaning-up">Cleaning up</a> <ul>
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<li><a href="#separate-handlers">Separate handlers</a></li>
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<li><a href="#use-datatext-instead-of-string">Use <code>Data.Text</code> instead of <code>String</code></a></li>
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<li><a href="#use-a-new-template-file">Use a new template file</a></li>
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</ul>
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</li>
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<li><a href="#protected-input">Protected input</a></li>
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</ul>
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</blockquote>
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</div>
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<p>You want the best technology to handle your new web application?
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Me too. After searching a lot, it appears if you focus only on technical aspect, Haskell is the way to go.</p>
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<p>It is <a href="http://www.yesodweb.com/blog/2011/03/preliminary-warp-cross-language-benchmarks">extremely fast</a>.
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It is secure by nature. Many typical programming bug are hard to make in Haskell.
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Haskell is also a “high level of abstraction” language. You can organize your code clearly.</p>
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<p>Actually there are three web frameworks in Haskell:</p>
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<ol>
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<li><a href="http://happstack.com">Happstack</a></li>
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<li><a href="http://snapframework.com">Snap</a></li>
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<li><a href="http://yesodweb.com">Yesod</a></li>
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</ol>
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<p>It is very hard to choose between these three.
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But my feeling goes to Yesod.
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It appears to be the one with most part done for you.
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As a beginner, let’s stay away of the detail as most as possible.</p>
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<p>The following tutorial contains some parts.</p>
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<ul>
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<li>Install → Install haskell and yesod. This can be long, but it is all automatic and this should be straightforward.</li>
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<li>Initialization → Initialize the project and configure it.</li>
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<li>Configure git → This is not mandatory, but it is a good practice.</li>
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<li>Verify the security → A first step to verify the yesod framework protect us from most common errors.</li>
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<li>Create a minimal blog → This is the “hello world” of web framework.</li>
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<li>Some tuning → Use html5 boilerplate for example.</li>
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</ul>
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<h2 id="install">Install</h2>
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<p>First you need to install <a href="http://www.haskell.org">Haskell</a>. The recommended way to do this is to use the <a href="http://www.haskell.org/platform">Haskell Platform</a>.</p>
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<p>Secondly you need to install yesod.</p>
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<pre class="twilight">
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<span class="Keyword">></span> cabal update
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<span class="Keyword">></span> cabal install yesod cabal-dev
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</pre>
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<p>That is all. It should take some time to
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do this as cabal will download all
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package and then compile them.</p>
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<h2 id="initialization">Initialization</h2>
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<p>Open a terminal and:</p>
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<pre class="twilight">
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<span class="Keyword">></span> yesod init
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</pre>
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<p>I entered my name, the name of the project was <code>yosog</code> and the name of the Foundation was <code>Yosog</code>, then I chosen <code>sqlite</code>.</p>
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<p>Perfect. Now you can start the development cycle:</p>
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<pre class="twilight">
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<span class="Keyword">></span> cd yosog
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<span class="Keyword">></span> cabal-dev install <span class="Keyword">&&</span> yesod --dev devel
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</pre>
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<p>This will compile the entire project.
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In the end you should now be able to see your local website by clicking this link:</p>
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<p><a href="http://localhost:3000"><code>http://localhost:3000</code></a></p>
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<p>Congratulation! You were able to see your yesod powered website.
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For the rest of the tutorial, use another terminal and let this one open in a corner to see what occurs.</p>
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<h2 id="configure-git">Configure git</h2>
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<p>It is not mandatory for a tutorial, but it is a good practice to have a CVS.</p>
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<p>To use <code>git</code> copy this <code>.gitignore</code> file into the <code>yosog</code> folder.</p>
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<div class="code"><div class="file"><a href="/Scratch/en/blog/Yesod-tutorial-for-newbies/code/.gitignore"> ➥ .gitignore </a></div><div class="withfile">
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<pre class="twilight">
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cabal-dev
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dist
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.static-cache
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static/tmp
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*.sqlite3
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</pre>
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</div></div>
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<p>Then initialize your git repository:</p>
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<pre class="twilight">
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<span class="Keyword">></span> git init .
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<span class="Keyword">></span> git add .
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<span class="Keyword">></span> git commit -a -m <span class="String"><span class="String">"</span>Initial yesod commit<span class="String">"</span></span>
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</pre>
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<p>Now we are ready to modify our web application.</p>
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<h2 id="a-last-point">A last point</h2>
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<p>What did we done:</p>
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<ol>
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<li>We have a directory containing a bunch of files</li>
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<li>We have a local web server on port 3000</li>
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</ol>
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<p>If we modify a file inside this directory, yesod should try
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to recompile as fast as possible the site. This way, you should
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see the modification you done.</p>
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<p>Instead of explaining the role of every file,
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let’s get straight to the point.</p>
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<p>Inside the <code>yosog</code> the important files/directories for this tutorial are:</p>
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<ol>
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<li><code>config/routes</code></li>
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<li><code>Handler/</code></li>
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<li><code>templates/</code></li>
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<li><code>static/</code></li>
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<li><code>config/models</code></li>
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</ol>
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<p>Obviously:</p>
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<ul>
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<li><code>config/routes</code> is where you’ll configure the map URL → Code.</li>
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<li><code>Handler/</code> contains the files that will contain the code called when a URL is accessed.</li>
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<li><code>templates/</code> contains HTML, JS and CSS templates. </li>
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<li><code>static/</code> contains static files.</li>
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<li><code>config/models</code> is where you’ll configure the persistent objects (database tables).</li>
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</ul>
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<p>With these informations we should be able to do a lot.
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Also note until here we don’t even typed any line of Haskell.</p>
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<h2 id="protected-echo">Protected echo</h2>
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<p>To verify the quality of the security of the yesod framework, let’s look at a minimal echo application.</p>
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<p>Our goal:</p>
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<p>Accessing <a href="http://localhost:3000/echo/some%20text"><code>http://localhost:3000/echo/some%20text</code></a>, should display “some text” in an %html web page.</p>
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<p>Let’s take a look at the file <code>config/routes</code>:</p>
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<pre class="twilight">
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/static StaticR Static getStatic
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/auth AuthR Auth getAuth
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/favicon.ico FaviconR GET
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/robots.txt RobotsR GET
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/ RootR GET
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</pre>
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<p>We want to add a route of the form <code>/echo/[anything]</code> somehow and do some action with this.
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We add the following:</p>
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<pre>
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/echo/#String EchoR GET
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</pre>
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<p>This line contains three elements: the <span class="sc">url</span> pattern, a handler name, an HTTP method.
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I am not particularly fan of the big R in the end of handler names.
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But this is the standard convention, then I use it.</p>
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<p>If you save <code>config/routes</code>, you should see your terminal in which you launched <code>yesod devel</code> do things.
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And certainly break in error.</p>
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<pre>
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Application.hs:31:1: Not in scope: `getEchoR'
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</pre>
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<p>Why? Simply because we didn’t written the code for the handler <code>EchoR</code>.
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Now, let’s do this. Edit the file <code>Handler/Root.hs</code> and append this:</p>
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<pre class="twilight">
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<span class="Entity">getEchoR</span> :: <span class="Constant">String</span> → <span class="Constant">Handler</span> <span class="Constant">RepHtml</span>
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getEchoR theText = <span class="Keyword">do</span>
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defaultLayout $ <span class="Keyword">do</span>
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[whamlet|<h1>#{theText}|]
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</pre>
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<p>After saving the file, you should see yesod recompile the application.
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When the compilation is finished you’ll see the message: <code>Starting devel application</code>.
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You can now visit: <a href="http://localhost:3000/echo/Yesod%20rocks!"><code>http://localhost:3000/echo/Yesod%20rocks!</code></a></p>
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<p>TADA! It works.</p>
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<p>Now, let’s try to attack our website by entering name with special characters:</p>
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<p><a href="http://localhost:3000/echo/<a>I'm <script>alert("Bad!");"><code>http://localhost:3000/echo/<a>I'm <script>alert("Bad!");</code></a></p>
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<p>The special characters are protected for us.
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If you have a malicious user, he could not hide some bad script inside his name for example.</p>
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<p>This is a direct consequence of <em>type safety</em>.
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The URL string is put inside a URL type.
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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 “<code>%20</code>” by space characters.
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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 “<code>&lt;</code>”.
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Thanks to yesod, most of tedious string transformation job is done for us.</p>
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<pre class="twilight">
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<span class="String"><span class="String">"</span>http://localhost:3000/echo/some%20text<a><span class="String">"</span></span> :: URL
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↓
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<span class="String"><span class="String">"</span>some text<a><span class="String">"</span></span> :: String
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↓
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<span class="String"><span class="String">"</span>some text &lt;a&gt;<span class="String">"</span></span> :: HTML
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</pre>
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<p>That was the first very minimal example, and we already
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verified Yesod protect us from many common errors.</p>
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<h2 id="cleaning-up">Cleaning up</h2>
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<p>This first example was nice, but for simplicity reason we didn’t used best practices.</p>
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<p>First we will separate the handler code into different files.
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After that we will use <code>Data.Text</code> instead of <code>String</code>. </p>
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<h3 id="separate-handlers">Separate handlers</h3>
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<p>In a first time create a new file <code>Handler/Echo.hs</code> containing:</p>
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<pre class="twilight">
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<span class="Keyword">module</span> <span class="Constant">Handler</span>.<span class="Constant">Echo</span> <span class="Keyword">where</span>
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<span class="Keyword">import</span> <span class="Constant">Import</span>
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<span class="Entity">getEchoR</span> :: <span class="Constant">String</span> → <span class="Constant">Handler</span> <span class="Constant">RepHtml</span>
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getEchoR theText = <span class="Keyword">do</span>
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defaultLayout $ <span class="Keyword">do</span>
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[whamlet|<h1>#{theText}|]
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</pre>
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<p>Do not forget to remove the getEchoR function inside the <code>Handler/Root.hs</code> file.</p>
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<p>We must declare the file inside the cabal configuration file <code>yosog.cabal</code>. Just after <code>Handler.Root</code> add:</p>
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<pre>
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Handler.Echo
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</pre>
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<p>We must also declare the new Handler module inside <code>Application.hs</code>.
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Just after the “<code>import Handler.Root</code>”, add:</p>
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<pre class="twilight">
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<span class="Keyword">import</span> <span class="Constant">Handler</span>.<span class="Constant">Echo</span>
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</pre>
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<h3 id="use-datatext-instead-of-string">Use <code>Data.Text</code> instead of <code>String</code></h3>
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<p>Now our handler is separated in another file.</p>
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<p>But we used <code>String</code> but it is a good practice to use <code>Data.Text</code> instead.</p>
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<p>To declare we will use <code>Data.Text</code> we modify the file <code>Foundation.hs</code>.
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Add an import directive just after the last one:</p>
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<pre class="twilight">
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import Data.Text
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</pre>
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<p>And also we must modify <code>config/routes</code> and our handler accordingly. Replace <code>#String</code> by <code>#Text</code> in <code>config/routes</code>:</p>
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<pre>
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/echo/#Text EchoR GET
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</pre>
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<p>And do the same in <code>Handler/Echo.hs</code>:</p>
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<div class="code"><div class="file"><a href="/Scratch/en/blog/Yesod-tutorial-for-newbies/code/Echo.hs"> ➥ Echo.hs </a></div><div class="withfile">
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<pre class="twilight">
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<span class="Keyword">module</span> <span class="Constant">Handler</span>.<span class="Constant">Echo</span> <span class="Keyword">where</span>
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<span class="Keyword">import</span> <span class="Constant">Import</span>
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||
<span class="Entity">getEchoR</span> :: <span class="Constant">Text</span> → <span class="Constant">Handler</span> <span class="Constant">RepHtml</span>
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||
getEchoR theText = <span class="Keyword">do</span>
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defaultLayout $ <span class="Keyword">do</span>
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[whamlet|<h1>#{theText}|]
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</pre>
|
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</div></div>
|
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|
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<h3 id="use-a-new-template-file">Use a new template file</h3>
|
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<p>The last thing to change in order to do things like in
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a real project is to use another template file.</p>
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<p>Just create a new file <code>template/echo.hamlet</code> containing:</p>
|
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<div class="code"><div class="file"><a href="/Scratch/en/blog/Yesod-tutorial-for-newbies/code/echo.hamlet"> ➥ echo.hamlet </a></div><div class="withfile">
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<pre class="twilight">
|
||
<h1> #{theText}
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</pre>
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</div></div>
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<p>and modify the handler <code>Handler/Echo.hs</code>:</p>
|
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|
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<pre class="twilight">
|
||
<span class="Entity">getEchoR</span> :: <span class="Constant">Text</span> → <span class="Constant">Handler</span> <span class="Constant">RepHtml</span>
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getEchoR theText = <span class="Keyword">do</span>
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defaultLayout $ <span class="Keyword">do</span>
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$(widgetFile <span class="String"><span class="String">"</span>echo<span class="String">"</span></span>)
|
||
</pre>
|
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|
||
<h2 id="protected-input">Protected input</h2>
|
||
|
||
<hr />
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<div style="background-color: #800; color: #FFF; font-weight: bold; border: 2px solid #300; box-shadow: 0 0 60px #300 inset;padding-left: 2em;">
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<h2 style="margin: 0 auto; text-align: center">TODO</h2>
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|
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<ul>
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<li> Display something, show it is protected.
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</li><li> Make the same as before, but with an input.
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</li><li> Create a minimal blog system.
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</li><li> Change template to html5 boilerplate.
|
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</li><li> Use Authentification.
|
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</li>
|
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</ul>
|
||
|
||
</div>
|
||
|
||
</div>
|
||
|
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