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<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
<title>Also Sprach Leif</title>
<link href="http://hrothen.github.com/atom.xml" rel="self" />
<link href="http://hrothen.github.com" />
<id>http://hrothen.github.com/atom.xml</id>
<author>
<name>Leif Grele</name>
<email></email>
</author>
<updated>2015-04-02T00:00:00Z</updated>
<entry>
<title>Switching from Jekyll Bootstrap to Hakyll</title>
<link href="http://hrothen.github.com/posts/switching-from-jekyll-bootstrap-to-hakyll.html" />
<id>http://hrothen.github.com/posts/switching-from-jekyll-bootstrap-to-hakyll.html</id>
<published>2015-04-02T00:00:00Z</published>
<updated>2015-04-02T00:00:00Z</updated>
<summary type="html"><![CDATA[<article>
<div class="info">
Posted on April 2, 2015
</div>
<p>I recently decided to switch from using <a href="http://jekyllbootstrap.com/">Jekyll Bootstrap</a> to build my blog, to using <a href="http://jaspervdj.be/hakyll/index.html">Hakyll.</a> Mostly just because I was rewriting all the html and css anyway and I wanted to give Hakyll a try, but also because I wanted to have a better idea of what was going on with each part of the site.</p>
<p>Getting started with Hakyll is pretty easy, the package comes with a program <code>hakyll-init</code> which creates a basic site for you to build on, and there are lots of sites using it that open source their code. Some of the stuff I wanted to do took some work to figure out or find though, so I thought I’d collect a list of it here.</p>
<h2 id="adding-next-and-previous-buttons-to-posts">Adding Next and Previous Buttons to Posts</h2>
<p>There are a number of ways to add next and previous buttons to the posts, I liked the method I found on <a href="https://github.com/rgoulter/my-hakyll-blog/blob/master/site.hs">Richard Goulter’s blog</a>, but hakyll has added some new functions since that was written and we can cut it down a bit now.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell">postList <span class="ot"><-</span> sortRecentFirst <span class="fu">=<<</span> getMatches <span class="st">"posts/*"</span>
match <span class="st">"posts/*"</span> <span class="fu">$</span> <span class="kw">do</span>
<span class="co">-- strip date from filename when producing route</span>
route <span class="fu">$</span> gsubRoute postDateRegex (const <span class="st">"posts/"</span>) <span class="ot">`composeRoutes`</span>
setExtension <span class="st">"html"</span>
compile <span class="fu">$</span> <span class="kw">do</span>
<span class="kw">let</span> postLocationContext <span class="fu">=</span>
field <span class="st">"nextPost"</span> (nextPostURL postList) <span class="ot">`mappend`</span>
field <span class="st">"prevPost"</span> (prevPostURL postList) <span class="ot">`mappend`</span>
postCtx
pandocCompiler
<span class="fu">>>=</span> loadAndApplyTemplate <span class="st">"templates/post.html"</span> postLocationContext
<span class="fu">>>=</span> saveSnapshot <span class="st">"content"</span>
<span class="fu">>>=</span> loadAndApplyTemplate <span class="st">"templates/default.html"</span> postLocationContext
<span class="fu">>>=</span> relativizeUrls</code></pre>
<p>With <code>sortRecentFirst</code> from Hakyll, I now only need to define <code>nextPostURL</code> and <code>prevPostURL</code>, which are almost identical.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">findPostUrl ::</span> ([<span class="dt">Identifier</span>] <span class="ot">-></span> <span class="dt">Identifier</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">Identifier</span>)
<span class="ot">-></span> [<span class="dt">Identifier</span>] <span class="ot">-></span> <span class="dt">Item</span> <span class="dt">String</span>
<span class="ot">-></span> <span class="dt">Compiler</span> <span class="dt">String</span>
findPostUrl p posts post <span class="fu">=</span> <span class="kw">do</span>
<span class="kw">let</span> id <span class="fu">=</span> itemIdentifier post
<span class="kw">case</span> p posts id <span class="kw">of</span>
<span class="dt">Just</span> m <span class="ot">-></span> maybe empty toUrl <span class="fu"><$></span> getRoute m
<span class="dt">Nothing</span> <span class="ot">-></span> empty
<span class="ot">prevPostURL ::</span> [<span class="dt">Identifier</span>] <span class="ot">-></span> <span class="dt">Item</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">Compiler</span> <span class="dt">String</span>
prevPostURL <span class="fu">=</span> findPostUrl lookupPrev
<span class="ot">nextPostURL ::</span> [<span class="dt">Identifier</span>] <span class="ot">-></span> <span class="dt">Item</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">Compiler</span> <span class="dt">String</span>
nextPostURL <span class="fu">=</span> findPostUrl lookupNext
<span class="ot">lookupPrev ::</span> <span class="dt">Eq</span> a <span class="ot">=></span> [a] <span class="ot">-></span> a <span class="ot">-></span> <span class="dt">Maybe</span> a
lookupPrev ids id <span class="fu">=</span> <span class="kw">case</span> elemIndex id ids <span class="kw">of</span>
<span class="dt">Just</span> i <span class="ot">-></span> <span class="kw">if</span> i <span class="fu">>=</span> (length ids <span class="fu">-</span> <span class="dv">1</span>) <span class="kw">then</span> <span class="dt">Nothing</span> <span class="kw">else</span> <span class="dt">Just</span> <span class="fu">$</span> ids<span class="fu">!!</span>(i<span class="fu">+</span><span class="dv">1</span>)
<span class="dt">Nothing</span> <span class="ot">-></span> <span class="dt">Nothing</span>
<span class="ot">lookupNext ::</span> <span class="dt">Eq</span> a <span class="ot">=></span> [a] <span class="ot">-></span> a <span class="ot">-></span> <span class="dt">Maybe</span> a
lookupNext ids id <span class="fu">=</span> <span class="kw">case</span> elemIndex id ids <span class="kw">of</span>
<span class="dt">Just</span> i <span class="ot">-></span> <span class="kw">if</span> i <span class="fu"><=</span> <span class="dv">0</span> <span class="kw">then</span> <span class="dt">Nothing</span> <span class="kw">else</span> <span class="dt">Just</span> <span class="fu">$</span> ids<span class="fu">!!</span>(i<span class="fu">-</span><span class="dv">1</span>)
<span class="dt">Nothing</span> <span class="ot">-></span> <span class="dt">Nothing</span></code></pre>
<p>I also changed <code>lookupNext</code> and <code>lookupPrev</code> to use indicies because I managed to get them subtly wrong like, five times while setting up the site (in fact at the time of this writing they are wrong, and will hopefully be finally fixed for real when I post this).</p>
<h2 id="setting-up-redirects-on-github-pages">Setting up Redirects on Github Pages</h2>
<p>Github doesn’t let me directly tell the server to issue redirects, but fortunately HTML already has a way to do this. I’ll create a template <code>redirect.html</code> that contains the following lines:</p>
<pre class="sourceCode html"><code class="sourceCode html"><span class="dt"><!DOCTYPE </span>html<span class="dt">></span>
<span class="kw"><html></span>
<span class="kw"><head></span>
<span class="kw"><meta</span><span class="ot"> http-equiv=</span><span class="st">"content-type"</span><span class="ot"> content=</span><span class="st">"text/html; charset=utf-8"</span> <span class="kw">/></span>
<span class="kw"><meta</span><span class="ot"> http-equiv=</span><span class="st">"refresh"</span><span class="ot"> content=</span><span class="st">"0; url=/$postName$"</span> <span class="kw">/></span>
<span class="kw"></head></span>
<span class="kw"></html></span></code></pre>
<p>So then I just need to get a list of all the pages I want to redirect from and their targets, and then produce a tiny page for each one with <code>postName</code> replaced. This turns out to be fairly simple once you spend some time looking at the Hakyll docs. For extensibility, I’ll use post metadata to decide what redirect pages to generate, this way I don’t need to artificially separate my old posts and new posts, and if I want to I can create multiple aliases to a page.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell">aliasList <span class="ot"><-</span> getAliases <span class="fu"><$></span> getAllMetadata <span class="st">"posts/*"</span>
<span class="kw">let</span> aliases <span class="fu">=</span> map fromFilePath <span class="fu">$</span> snd <span class="fu">$</span> unzip aliasList
create aliases <span class="fu">$</span> <span class="kw">do</span>
route idRoute
compile <span class="fu">$</span> <span class="kw">do</span>
<span class="kw">let</span> aliasCtx <span class="fu">=</span> field <span class="st">"postName"</span> (getRealName aliasList) <span class="ot">`mappend`</span>
defaultContext
makeItem <span class="st">""</span>
<span class="fu">>>=</span> loadAndApplyTemplate <span class="st">"templates/redirect.html"</span> aliasCtx
<span class="fu">>>=</span> relativizeUrls</code></pre>
<p>I grab all the metadata for each file in <code>posts/</code>, and convert them into <code>(target,alias)</code> pairs. The list <code>aliases</code> contains only the alias names, which I use to generate the redirect pages, looking up the target page name in <code>aliasList</code> with <code>getRealName</code>. The actual work is done in <code>getAliases</code>, which filters out pages that don’t define the <code>aliases</code> keyword and reformats titles to match my url scheme.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">stripPostDate ::</span> FilePath <span class="ot">-></span> FilePath
stripPostDate <span class="fu">=</span> replaceAll postDateRegex (const <span class="st">"posts/"</span>)
<span class="co">-- convert a list of (Identifier,Metadata) into a list of (target,alias)</span>
<span class="ot">getAliases ::</span> [(<span class="dt">Identifier</span>,<span class="dt">Metadata</span>)] <span class="ot">-></span> [(FilePath,FilePath)]
getAliases ids <span class="fu">=</span>
<span class="kw">let</span> pairs <span class="fu">=</span> filter (not <span class="fu">.</span> null <span class="fu">.</span> snd) <span class="fu">$</span> map expand ids
paths <span class="fu">=</span> map (second (map addIndex) <span class="fu">.</span> first idToPath) pairs
<span class="kw">in</span> concatMap unzipSecond paths
<span class="kw">where</span>
<span class="ot"> expand ::</span> (<span class="dt">Identifier</span>,<span class="dt">Metadata</span>) <span class="ot">-></span> (<span class="dt">Identifier</span>,[FilePath])
expand <span class="fu">=</span> second (maybe [] read <span class="fu">.</span> M.lookup <span class="st">"aliases"</span>)
<span class="ot"> idToPath ::</span> <span class="dt">Identifier</span> <span class="ot">-></span> FilePath
idToPath <span class="fu">=</span> stripPostDate <span class="fu">.</span> toFilePath
<span class="ot"> addIndex ::</span> FilePath <span class="ot">-></span> FilePath
addIndex f <span class="fu">=</span> dropWhile (<span class="fu">==</span><span class="ch">'/'</span>) <span class="fu">$</span> dropExtension f <span class="fu"></></span> <span class="st">"index.html"</span>
<span class="co">-- get the path of the page an alias is pointing to</span>
<span class="ot">getRealName ::</span> [(FilePath,FilePath)] <span class="ot">-></span> <span class="dt">Item</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">Compiler</span> <span class="dt">String</span>
getRealName as i <span class="fu">=</span> <span class="kw">do</span>
<span class="kw">let</span> id <span class="fu">=</span> toFilePath <span class="fu">$</span> itemIdentifier i
path <span class="fu">=</span> fst <span class="fu">$</span> fromJust <span class="fu">$</span> find ((<span class="fu">==</span>id) <span class="fu">.</span> snd) as
return <span class="fu">$</span> replaceExtension path <span class="st">"html"</span>
<span class="ot">unzipSecond ::</span> (a,[b]) <span class="ot">-></span> [(a,b)]
unzipSecond (x,ys) <span class="fu">=</span> map (\a<span class="ot">-></span>(x,a)) ys</code></pre>
<p>Some of this is specific to my site layout and title format, but most of it is general-purpose. If you haven’t seen them before, <code>first</code> and <code>second</code> from <code>Control.Arrow</code> have kind of scary types, but the way most people use them is just to apply a function inside a tuple: <code>first foo (a,b) == (foo a,b)</code> and <code>second bar (a,b) == (a, bar b)</code>.</p>
<h2 id="actually-deploying-the-thing">Actually Deploying the Thing</h2>
<p>So, if you’re not using Jekyll, github wants you to just upload the static site directly, but this is a problem because I’ve got all this stuff that isn’t part of the site sitting in the repo. The solution is to keep all the source in a separate branch, which I call <code>hakyll</code>, and then deploy by pushing only the built site in <code>_site/</code> to <code>master</code>. For this purpose I modified the <code>deploy.sh</code> script used by <a href="https://github.com/blaenk/blaenk.github.io/blob/source/src/deploy.sh">Jorge Israel Peña</a> for his blog. Hakyll has a deploy command you can set in the site configuration, so I add</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">siteConfig ::</span> <span class="dt">Configuration</span>
siteConfig <span class="fu">=</span> defaultConfiguration{ deployCommand <span class="fu">=</span> <span class="st">"bash deploy.sh deploy"</span> }</code></pre>
<p>And replace the call to <code>hakyll</code> with <code>hakyllWith siteConfig</code> in <code>main</code>. Then I can just deploy the site by calling <code>./site deploy</code>.</p>
<p>Overall switching to Hakyll was pretty simple, and I didn’t really have any issues figuring out how to do anything I wanted, so I’d call this a win.</p>
<nav id="postNav">
<a href="/posts/lets-write-a-browser-engine-in-haskell-a-lens-interlude.html" style="float:left;">Previous post</a>
</nav>
</article>]]></summary>
</entry>
<entry>
<title>Let's Write a Browser Engine in Haskell: a lens interlude</title>
<link href="http://hrothen.github.com/posts/lets-write-a-browser-engine-in-haskell-a-lens-interlude.html" />
<id>http://hrothen.github.com/posts/lets-write-a-browser-engine-in-haskell-a-lens-interlude.html</id>
<published>2015-01-20T00:00:00Z</published>
<updated>2015-01-20T00:00:00Z</updated>
<summary type="html"><![CDATA[<article>
<div class="info">
Posted on January 20, 2015
</div>
<p>This post doesn’t cover any new functionality, instead, we’ll be revisiting some old code and refactoring it to be less awful. In particular, we’ll be using <a href="http://ekmett.github.io/lens/index.html">lenses</a> make our nested datatypes less annoying to work with. I’ve tried to record every change I made, but it’s possible I missed something, if you have trouble compiling, look at the github changelog to try to find the issue, and also let me know so I can update this article.</p>
<h2 id="a-couple-notes-on-lenses">A couple notes on lenses</h2>
<p>First: I am not going to attempt to explain lenses in any detail. <a href="http://unbui.lt/#!/post/haskell-another-lens-tutorial">This tutorial</a> is a decent introduction to what lenses are, but for our purposes it’s enough to know that lenses look and act a lot like accessors in other languages. Lenses compose “backwards” so for example <code>(content . height)</code> will access the height element of the content element of a <code>Dimensions</code>.</p>
<p>Second: Lenses are a pain to read, partly because they compose backwards but also because the <code>lens</code> library includes operators for every situation Edward Kmett could think of (over 100 operators, including such gems as the squid operator <code>(<<<>~)</code> and the cartoon invective operator <code>(^@!?)</code>). We’ll try to avoid readability issues by using a really minimal set of lens functions and operators, namely: <code>(^.)</code> which gives the value of the field a lens accesses, <code>(.~)</code> which assigns a value to the field a lens accesses, <code>(.=)</code> which is <code>(.~)</code> but for the state inside a <code>State</code> monad, <code>(&)</code> which is just <code>flip $</code> and is used to chain lens operations, <code>(&~)</code> which is <code>(&)</code> inside a <code>State</code> monad, and <code>zoom</code> which “zooms in” on a lens, so if we’re going to perform several operations on the same field, we don’t need to keep writing it.</p>
<h2 id="adding-lenses-to-types">Adding Lenses to Types</h2>
<p>The <code>lens</code> library conveniently provides a template haskell macro to create lenses for a structure, all we need to do is preface all fields we want to derive a lens for with <code>_</code>, and then call <code>makeLenses ''<type name></code>. The ability to automatically derive lenses is the main reason we’re using the <code>lens</code> library instead of just writing our own simple lenses, if you don’t want to include all the dependencies <code>lens</code> has, you can also use <code>lens-family</code> which has all the operators we’ll be using.</p>
<p>For starters, we’ll derive lenses for <code>NTree</code> in <code>Dom.hs</code></p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="co">-- I'm only showing lines that have changed</span>
<span class="co">-- It's not actually mentioned on the page, probably because they thought it</span>
<span class="co">-- was obvious, but you need the TemplateHaskell pragma to use makeLenses</span>
<span class="ot">{-# LANGUAGE OverloadedStrings, FlexibleInstances, TemplateHaskell#-}</span>
<span class="kw">import </span><span class="dt">Control.Lens</span>
<span class="kw">data</span> <span class="dt">NTree</span> a <span class="fu">=</span> <span class="dt">NTree</span> { _<span class="ot">root ::</span> a, _<span class="ot">children ::</span> [<span class="dt">NTree</span> a] }
makeLenses <span class="ch">''</span><span class="dt">NTree</span></code></pre>
<p>Most of our work is going to be in <code>Layout.hs</code> which will look a lot nice by the time we finish.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">{-#LANGUAGE BangPatterns, OverloadedStrings, TemplateHaskell#-}</span>
<span class="kw">import </span><span class="dt">Data.Function</span> (on)
<span class="kw">import </span><span class="dt">Control.Lens</span> <span class="kw">hiding</span> (children)
<span class="kw">data</span> <span class="dt">Rect</span> <span class="fu">=</span> <span class="dt">Rect</span> { _<span class="ot">x ::</span> <span class="dt">Float</span>
, _<span class="ot">y ::</span> <span class="dt">Float</span>
, _<span class="ot">width ::</span> <span class="dt">Float</span>
, _<span class="ot">height ::</span> <span class="dt">Float</span> }
<span class="kw">data</span> <span class="dt">Dimensions</span> <span class="fu">=</span> <span class="dt">Dimensions</span> { _<span class="ot">content ::</span> <span class="dt">Rect</span>
, _<span class="ot">padding ::</span> <span class="dt">EdgeSize</span>
, _<span class="ot">border ::</span> <span class="dt">EdgeSize</span>
, _<span class="ot">margin ::</span> <span class="dt">EdgeSize</span> }
<span class="kw">data</span> <span class="dt">EdgeSize</span> <span class="fu">=</span> <span class="dt">EdgeSize</span> { _<span class="ot">left ::</span> <span class="dt">Float</span>
, _<span class="ot">right ::</span> <span class="dt">Float</span>
, _<span class="ot">top ::</span> <span class="dt">Float</span>
, _<span class="ot">bottom ::</span> <span class="dt">Float</span> }
makeLenses <span class="ch">''</span><span class="dt">Rect</span>
makeLenses <span class="ch">''</span><span class="dt">Dimensions</span>
makeLenses <span class="ch">''</span><span class="dt">EdgeSize</span>
<span class="co">-- here we use (.~) to pass contBlock with its height value set to 0</span>
<span class="ot">layoutTree ::</span> <span class="dt">StyledNode</span> <span class="ot">-></span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
layoutTree root contBlock <span class="fu">=</span> buildLayoutTree root <span class="fu">>>=</span>
flip layout (contBlock <span class="fu">&</span> content<span class="fu">.</span>height<span class="fu">.~</span><span class="dv">0</span>)
<span class="co">-- only one line has changed in buildLayoutTree</span>
<span class="ot">buildLayoutTree ::</span> <span class="dt">StyledNode</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
<span class="fu">...</span>
anonify <span class="fu">=</span> concatMap mergeInlines <span class="fu">.</span> groupBy ((<span class="fu">&&</span>) <span class="ot">`on`</span> isInline)
<span class="fu">...</span>
<span class="co">-- here we use a lens to get rid of the ugly nested pattern match</span>
<span class="ot">layout ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
layout l contBlock <span class="fu">=</span> <span class="kw">case</span> l<span class="fu">^.</span>root<span class="fu">.</span>_2 <span class="kw">of</span>
<span class="dt">BlockNode</span> _ <span class="ot">-></span> layoutBlock contBlock l
<span class="dt">InlineNode</span> _ <span class="ot">-></span> undefined
<span class="dt">AnonymousBlock</span> <span class="ot">-></span> undefined
<span class="co">-- the next few function have changed significantly, and several parts</span>
<span class="co">-- have been pulled out into their own things</span>
auto <span class="fu">=</span> <span class="dt">Keyword</span> <span class="st">"auto"</span>
zero <span class="fu">=</span> <span class="dt">Length</span> <span class="dv">0</span> <span class="dt">Px</span>
<span class="ot">calcWidth ::</span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
calcWidth contBlock rt <span class="fu">=</span> <span class="kw">do</span>
style <span class="ot"><-</span> getStyledElem rt
vals <span class="ot"><-</span> lookupSideVals rt
<span class="kw">let</span> w <span class="fu">=</span> fromMaybe auto <span class="fu">$</span> value style <span class="st">"width"</span>
total <span class="fu">=</span> sum <span class="fu">$</span> map toPx (w<span class="fu">:</span>vals)
underflow <span class="fu">=</span> contBlock<span class="fu">^.</span>content<span class="fu">.</span>width <span class="fu">-</span> total
(margins,vals') <span class="fu">=</span> splitAt <span class="dv">2</span> vals
(w',ml',mr') <span class="fu">=</span> checkUnderflow w underflow <span class="fu">$</span> checkAutoMargins margins w total
[w'',ml,mr,blw,brw,plf,prt] <span class="fu">=</span> map toPx (w'<span class="fu">:</span>ml'<span class="fu">:</span>mr'<span class="fu">:</span>vals')
<span class="co">-- did you know you can use semicolons in haskell?</span>
return <span class="fu">$</span> rt <span class="fu">&~</span> zoom (root <span class="fu">.</span> _1) (<span class="kw">do</span>
content<span class="fu">.</span>width <span class="fu">.=</span> w''
padding<span class="fu">.</span>left <span class="fu">.=</span> plf; padding<span class="fu">.</span>right <span class="fu">.=</span> prt
border<span class="fu">.</span>left <span class="fu">.=</span> blw; border<span class="fu">.</span>right <span class="fu">.=</span> brw
margin<span class="fu">.</span>left <span class="fu">.=</span> ml ; margin<span class="fu">.</span>right <span class="fu">.=</span> mr)
<span class="kw">where</span>
checkAutoMargins [x,y] w total
<span class="fu">|</span> w <span class="fu">/=</span> auto <span class="fu">&&</span> total <span class="fu">></span> contBlock<span class="fu">^.</span>content<span class="fu">.</span>width <span class="fu">=</span> (check x,check y)
<span class="fu">|</span> otherwise <span class="fu">=</span> (x,y)
<span class="kw">where</span> check a <span class="fu">=</span> <span class="kw">if</span> a <span class="fu">==</span> auto <span class="kw">then</span> zero <span class="kw">else</span> a
checkUnderflow w uflow (mlf,mrt) <span class="fu">=</span> <span class="kw">case</span> (w <span class="fu">==</span> auto, mlf <span class="fu">==</span> auto, mrt <span class="fu">==</span> auto) <span class="kw">of</span>
(<span class="dt">False</span>,<span class="dt">False</span>,<span class="dt">False</span>) <span class="ot">-></span> (w , mlf, <span class="dt">Length</span> (toPx mrt <span class="fu">+</span> uflow) <span class="dt">Px</span>)
(<span class="dt">False</span>,<span class="dt">False</span>,<span class="dt">True</span>) <span class="ot">-></span> (w , mlf, <span class="dt">Length</span> uflow <span class="dt">Px</span>)
(<span class="dt">False</span>,<span class="dt">True</span>,<span class="dt">False</span>) <span class="ot">-></span> (w , <span class="dt">Length</span> uflow <span class="dt">Px</span> , mrt)
(<span class="dt">False</span>,<span class="dt">True</span>,<span class="dt">True</span>) <span class="ot">-></span> (w , <span class="dt">Length</span> (uflow<span class="fu">/</span><span class="dv">2</span>) <span class="dt">Px</span>, <span class="dt">Length</span> (uflow<span class="fu">/</span><span class="dv">2</span>) <span class="dt">Px</span>)
(<span class="dt">True</span>,_,_) <span class="ot">-></span>
<span class="kw">let</span> l <span class="fu">=</span> <span class="kw">if</span> mlf <span class="fu">==</span> auto <span class="kw">then</span> zero <span class="kw">else</span> mlf
r <span class="fu">=</span> <span class="kw">if</span> mrt <span class="fu">==</span> auto <span class="kw">then</span> zero <span class="kw">else</span> mrt
<span class="kw">in</span> <span class="kw">if</span> uflow <span class="fu">>=</span> <span class="dv">0</span>
<span class="kw">then</span> (<span class="dt">Length</span> uflow <span class="dt">Px</span>,l,r)
<span class="kw">else</span> (zero,l,<span class="dt">Length</span> (toPx r <span class="fu">+</span> uflow) <span class="dt">Px</span>)
<span class="co">-- I pulled out the lookup functions since they took up a lot of space,</span>
<span class="co">-- and gave them nice descriptive names</span>
<span class="ot">lookupSideVals ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> [<span class="dt">Value</span>]
lookupSideVals rt <span class="fu">=</span> <span class="kw">do</span>
style <span class="ot"><-</span> getStyledElem rt
return <span class="fu">$</span> map (\a <span class="ot">-></span> lookup style a zero)
[ [<span class="st">"margin-left"</span> , <span class="st">"margin"</span>]
, [<span class="st">"margin-right"</span> , <span class="st">"margin"</span>]
, [<span class="st">"border-left-width"</span> , <span class="st">"border-width"</span>]
, [<span class="st">"border-right-width"</span>, <span class="st">"border-width"</span>]
, [<span class="st">"padding-left"</span> , <span class="st">"padding"</span>]
, [<span class="st">"padding-right"</span> , <span class="st">"padding"</span>] ]
<span class="ot">lookupVertVals ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> [<span class="dt">Float</span>]
lookupVertVals rt <span class="fu">=</span> <span class="kw">do</span>
style <span class="ot"><-</span> getStyledElem rt
return <span class="fu">$</span> map (toPx <span class="fu">.</span> (\a <span class="ot">-></span> lookup style a zero))
[ [<span class="st">"margin-top"</span> , <span class="st">"margin"</span>]
, [<span class="st">"margin-bottom"</span> , <span class="st">"margin"</span>]
, [<span class="st">"border-top-width"</span> , <span class="st">"border-width"</span>]
, [<span class="st">"border-bottom-width"</span>, <span class="st">"border-width"</span>]
, [<span class="st">"padding-top"</span> , <span class="st">"padding"</span>]
, [<span class="st">"padding-bottom"</span> , <span class="st">"padding"</span>] ]
<span class="ot">calcPosition ::</span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
calcPosition contBlock rt <span class="fu">=</span> <span class="kw">do</span>
[mt,mb,bt,bb,pt,pb] <span class="ot"><-</span> lookupVertVals rt
<span class="kw">let</span> d <span class="fu">=</span> rt<span class="fu">^.</span>root<span class="fu">.</span>_1
return <span class="fu">$</span> rt <span class="fu">&~</span> zoom (root <span class="fu">.</span> _1) (<span class="kw">do</span>
content<span class="fu">.</span>x<span class="fu">.=</span> contBlock<span class="fu">^.</span>content<span class="fu">.</span>x
<span class="fu">+</span> d<span class="fu">^.</span>margin<span class="fu">.</span>left
<span class="fu">+</span> d<span class="fu">^.</span>border<span class="fu">.</span>left
<span class="fu">+</span> d<span class="fu">^.</span>padding<span class="fu">.</span>left
content<span class="fu">.</span>y<span class="fu">.=</span> contBlock<span class="fu">^.</span>content<span class="fu">.</span>y
<span class="fu">+</span> contBlock<span class="fu">^.</span>content<span class="fu">.</span>height
<span class="fu">+</span> pt <span class="fu">+</span> bt <span class="fu">+</span> mt
padding<span class="fu">.</span>top <span class="fu">.=</span> pt; padding<span class="fu">.</span>bottom <span class="fu">.=</span> pb
border<span class="fu">.</span>top <span class="fu">.=</span> bt; border<span class="fu">.</span>bottom <span class="fu">.=</span> bb
margin<span class="fu">.</span>top <span class="fu">.=</span> mt; margin<span class="fu">.</span>bottom <span class="fu">.=</span> mb)
<span class="ot">layoutChildren ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
layoutChildren rt <span class="fu">=</span> <span class="kw">do</span>
(dim,cs) <span class="ot"><-</span> foldM foo (rt<span class="fu">^.</span>root<span class="fu">.</span>_1,[]) <span class="fu">$</span> rt<span class="fu">^.</span>children
return <span class="fu">$</span> rt <span class="fu">&~</span> root<span class="fu">.</span>_1<span class="fu">.=</span> dim <span class="fu">&~</span> children<span class="fu">.=</span> cs
<span class="kw">where</span>
<span class="ot"> foo ::</span> (<span class="dt">Dimensions</span>,[<span class="dt">LayoutBox</span>]) <span class="ot">-></span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> (<span class="dt">Dimensions</span>,[<span class="dt">LayoutBox</span>])
foo (d,acc) c <span class="fu">=</span> <span class="kw">do</span>
c' <span class="ot"><-</span> layout c d
return (d <span class="fu">&</span> content<span class="fu">.</span>height<span class="fu">+~</span> marginBoxHeight (c'<span class="fu">^.</span>root<span class="fu">.</span>_1), acc <span class="fu">++</span> [c'])
<span class="ot">calcHeight ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
calcHeight rt <span class="fu">=</span> <span class="kw">do</span>
s <span class="ot"><-</span> getStyledElem rt
<span class="kw">case</span> value s <span class="st">"height"</span> <span class="kw">of</span>
<span class="dt">Just</span> (<span class="dt">Length</span> h <span class="dt">Px</span>) <span class="ot">-></span> return <span class="fu">$</span> rt <span class="fu">&</span> root<span class="fu">.</span>_1<span class="fu">.</span>content<span class="fu">.</span>height<span class="fu">.~</span> h
<span class="dt">Nothing</span> <span class="ot">-></span> return rt
<span class="ot">marginBoxHeight ::</span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">Float</span>
marginBoxHeight dim <span class="fu">=</span> (marginBox dim)<span class="fu">^.</span>height
<span class="ot">getStyledElem ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">StyledNode</span>
getStyledElem rt <span class="fu">=</span> <span class="kw">case</span> rt<span class="fu">^.</span>root<span class="fu">.</span>_2 <span class="kw">of</span>
<span class="dt">BlockNode</span> s <span class="ot">-></span> <span class="dt">Right</span> <span class="fu">$</span> <span class="dt">NTree</span> s []
<span class="dt">InlineNode</span> s <span class="ot">-></span> <span class="dt">Right</span> <span class="fu">$</span> <span class="dt">NTree</span> s []
<span class="dt">AnonymousBlock</span> <span class="ot">-></span> <span class="dt">Left</span> <span class="st">"Error: attempted to access the nonexistant\</span>
<span class="st"> \ StyleNode of an AnonymousBlock"</span>
<span class="ot">expandedBy ::</span> <span class="dt">Rect</span> <span class="ot">-></span> <span class="dt">EdgeSize</span> <span class="ot">-></span> <span class="dt">Rect</span>
expandedBy rec edge <span class="fu">=</span> rec <span class="fu">&~</span> <span class="kw">do</span>
x <span class="fu">-=</span> edge<span class="fu">^.</span>left
y <span class="fu">-=</span> edge<span class="fu">^.</span>top
width <span class="fu">+=</span> (edge<span class="fu">^.</span>left <span class="fu">+</span> edge<span class="fu">^.</span>right)
height <span class="fu">+=</span> (edge<span class="fu">^.</span>top <span class="fu">+</span> edge<span class="fu">^.</span>bottom)
<span class="ot">paddingBox ::</span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">Rect</span>
paddingBox d <span class="fu">=</span> (d<span class="fu">^.</span>content) <span class="ot">`expandedBy`</span> (d<span class="fu">^.</span>padding)
<span class="ot">marginBox ::</span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">Rect</span>
marginBox d <span class="fu">=</span> borderBox d <span class="ot">`expandedBy`</span> (d<span class="fu">^.</span>margin)
<span class="ot">borderBox ::</span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">Rect</span>
borderBox d <span class="fu">=</span> paddingBox d <span class="ot">`expandedBy`</span> (d<span class="fu">^.</span>margin)</code></pre>
<p>Now that we’re using lenses throughout our layout tree, we’ll need to update <code>Painting.hs</code> and <code>tests.hs</code> as well.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="co">-- Painting.hs</span>
<span class="kw">import </span><span class="dt">Control.Lens</span>
<span class="ot">paint ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Rect</span> <span class="ot">-></span> <span class="dt">Canvas</span>
paint root bounds <span class="fu">=</span> <span class="kw">let</span> dlist <span class="fu">=</span> buildDisplayList root
canvas <span class="fu">=</span> newCanvas w h
w <span class="fu">=</span> fromInteger <span class="fu">.</span> floor <span class="fu">$</span> bounds<span class="fu">^.</span>width
h <span class="fu">=</span> fromInteger <span class="fu">.</span> floor <span class="fu">$</span> bounds<span class="fu">^.</span>height
<span class="kw">in</span> F.foldl' paintItem canvas dlist
<span class="ot">buildDisplayList ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">DisplayList</span>
buildDisplayList <span class="fu">=</span> F.foldMap renderLayoutBox
<span class="ot">renderBorders ::</span> (<span class="dt">Dimensions</span>,<span class="dt">BoxType</span>) <span class="ot">-></span> <span class="dt">DisplayList</span>
renderBorders (dim,ty) <span class="fu">=</span> maybe mempty renderBorders' (getColor ty <span class="st">"border-color"</span>)
<span class="kw">where</span>
renderBorders' color <span class="fu">=</span> V.fromList <span class="fu">$</span> map (<span class="dt">SolidColor</span> color) [l, r, t, b]
bbox <span class="fu">=</span> borderBox dim
bdr <span class="fu">=</span> dim<span class="fu">^.</span>border
l <span class="fu">=</span> bbox <span class="fu">&</span> width<span class="fu">.~</span> bdr<span class="fu">^.</span>left
r <span class="fu">=</span> bbox <span class="fu">&</span> x<span class="fu">+~</span> bbox<span class="fu">^.</span>width <span class="fu">-</span> bdr<span class="fu">^.</span>right
<span class="fu">&</span> width<span class="fu">.~</span> bdr<span class="fu">^.</span>right
t <span class="fu">=</span> bbox <span class="fu">&</span> height<span class="fu">.~</span> bdr<span class="fu">^.</span>top
b <span class="fu">=</span> bbox <span class="fu">&</span> y<span class="fu">+~</span> bbox<span class="fu">^.</span>height <span class="fu">-</span> bdr<span class="fu">^.</span>bottom
<span class="fu">&</span> height<span class="fu">.~</span> bdr<span class="fu">^.</span>bottom
<span class="ot">paintItem ::</span> <span class="dt">Canvas</span> <span class="ot">-></span> <span class="dt">DisplayCommand</span> <span class="ot">-></span> <span class="dt">Canvas</span>
paintItem cs (<span class="dt">SolidColor</span> color rect) <span class="fu">=</span> updateChunk cs (x0,x1) (y0,y1) color
<span class="kw">where</span>
x0 <span class="fu">=</span> clampInt <span class="dv">0</span> (w<span class="fu">-</span><span class="dv">1</span>) (rect<span class="fu">^.</span>x)
y0 <span class="fu">=</span> clampInt <span class="dv">0</span> (h<span class="fu">-</span><span class="dv">1</span>) (rect<span class="fu">^.</span>y)
x1 <span class="fu">=</span> clampInt <span class="dv">0</span> (w<span class="fu">-</span><span class="dv">1</span>) (rect<span class="fu">^.</span>x <span class="fu">+</span> rect<span class="fu">^.</span>width <span class="fu">-</span> <span class="dv">1</span>)
y1 <span class="fu">=</span> clampInt <span class="dv">0</span> (h<span class="fu">-</span><span class="dv">1</span>) (rect<span class="fu">^.</span>y <span class="fu">+</span> rect<span class="fu">^.</span>height <span class="fu">-</span> <span class="dv">1</span>)
w <span class="fu">=</span> asFloat <span class="fu">$</span> wdth cs
h <span class="fu">=</span> asFloat <span class="fu">$</span> hght cs
asFloat <span class="fu">=</span> fromInteger <span class="fu">.</span> toInteger
<span class="co">-- tests.hs</span>
<span class="kw">import </span><span class="dt">Control.Lens</span>
contBlock <span class="fu">=</span> defaultDim <span class="fu">&</span> content<span class="fu">.</span>width<span class="fu">.~</span><span class="dv">800</span> <span class="fu">&</span> content<span class="fu">.</span>height<span class="fu">.~</span><span class="dv">168</span>
paintpng <span class="fu">=</span> paintpng' s d
<span class="kw">where</span>
(<span class="dt">Right</span> d) <span class="fu">=</span> PS.parseHtml pnghtml
(<span class="dt">Right</span> s) <span class="fu">=</span> parseCSS pngcss
paintpng' s d <span class="fu">=</span> <span class="kw">do</span>
<span class="kw">let</span> st <span class="fu">=</span> styleTree d s
lyt <span class="ot"><-</span> layoutTree st contBlock
<span class="kw">let</span> vec <span class="fu">=</span> pixels <span class="fu">$</span> paint lyt (contBlock<span class="fu">^.</span>content)
return <span class="fu">$</span> generateImage (\x y<span class="ot">-></span> c2px <span class="fu">$</span> vec <span class="fu">V.!</span> (x <span class="fu">+</span> (y <span class="fu">*</span> <span class="dv">800</span>))) <span class="dv">800</span> <span class="dv">168</span>
c2px (<span class="dt">Color</span> r g b _) <span class="fu">=</span> <span class="dt">PixelRGB8</span> r g b</code></pre>
<p>Finally, update <code>hubert.cabal</code> to add our lens dependency</p>
<pre><code>executable hubert
main-is: Main.hs
other-modules: Dom,
HTML.Parser,
HTML.Parsec,
CSS,
Style,
Layout
-- other-extensions:
build-depends: base >=4.7 && <5,
unordered-containers >=0.2 && <0.3,
mtl >= 2.2.1,
text >= 1.1.0.0,
parsec == 3.1.*,
lens >= 4.6
Test-Suite hunit-tests
type: exitcode-stdio-1.0
main-is: tests.hs
other-modules: Dom,
HTML.Parser,
HTML.Parsec,
CSS,
Style,
Layout,
Painting
build-depends: base >= 4.7 && < 5,
unordered-containers >=0.2 && <0.3,
mtl >= 2.2.1,
text >= 1.1.0.0,
HUnit >= 1.2.5.0,
parsec == 3.1.*,
vector >= 0.10.9.1,
JuicyPixels >= 3.1.7.1,
lens >= 4.6
hs-source-dirs: tests,
src
default-language: Haskell2010</code></pre>
<p>At this point, everything should compile and run correctly. We’ve used lenses to make the deeply nested updates in <code>Layout.hs</code> somewhat less awful to read, and we should be set until Robinson updates again.</p>
<p>As usual, you can find the source for this post <a href="https://github.com/Hrothen/Hubert/tree/master/src">here</a> and the source for Robinson <a href="https://github.com/mbrubeck/robinson">here</a>.</p>
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</entry>
<entry>
<title>Let's Build a Browser Engine in Haskell: part 6</title>
<link href="http://hrothen.github.com/posts/lets-build-a-browser-engine-in-haskell-part-6.html" />
<id>http://hrothen.github.com/posts/lets-build-a-browser-engine-in-haskell-part-6.html</id>
<published>2014-11-19T00:00:00Z</published>
<updated>2014-11-19T00:00:00Z</updated>
<summary type="html"><![CDATA[<article>
<div class="info">
Posted on November 19, 2014
</div>
<p>Welcome back, today we’re going to implement painting of the layout tree to an image (but not to a real window, yet). The actual painting code is fairly simple, but before that we’re going to want to backtrack and modify some older code.</p>
<p>First we’ll add a <code>Foldable</code> instance to our <code>NTree</code> in Dom.hs.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="kw">import </span><span class="dt">Data.Monoid</span> ((<>)) <span class="co">-- (<>) is an infix version of mappend</span>
<span class="kw">import </span><span class="dt">Data.Foldable</span>
<span class="kw">instance</span> <span class="dt">Foldable</span> <span class="dt">NTree</span> <span class="kw">where</span>
foldMap f (<span class="dt">NTree</span> n []) <span class="fu">=</span> f n
foldMap f (<span class="dt">NTree</span> n ns) <span class="fu">=</span> f n <span class="fu"><></span> foldMap (foldMap f) ns</code></pre>
<p>This will let us easily convert the layout tree into an intermediate representation.</p>
<p>Next we’ll refactor a couple data structures to match how Robinson is laying things out. We’ll pull the first four fields of the <code>Dimensions</code> struct out into their own <code>Rect</code> struct and write a couple helpers for it. We’ll also pull the color data out of the <code>Value</code> type, so that later we can add other representations if we want to. (If this next bit seems hard to follow, the <a href="https://github.com/Hrothen/Hubert/commits/master">changelog</a> has a full diff)</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell">
<span class="fu">----------------------</span> Layout.hs <span class="fu">--------------------</span>
<span class="kw">data</span> <span class="dt">Rect</span> <span class="fu">=</span> <span class="dt">Rect</span> {<span class="ot"> x ::</span> <span class="dt">Float</span>
,<span class="ot"> y ::</span> <span class="dt">Float</span>
,<span class="ot"> width ::</span> <span class="dt">Float</span>
,<span class="ot"> height ::</span> <span class="dt">Float</span> }
<span class="kw">data</span> <span class="dt">Dimensions</span> <span class="fu">=</span> <span class="dt">Dimensions</span> {<span class="ot"> content ::</span> <span class="dt">Rect</span>
,<span class="ot"> padding ::</span> <span class="dt">EdgeSize</span>
,<span class="ot"> border ::</span> <span class="dt">EdgeSize</span>
,<span class="ot"> margin ::</span> <span class="dt">EdgeSize</span> }
emptyRect <span class="fu">=</span> <span class="dt">Rect</span> <span class="dv">0</span> <span class="dv">0</span> <span class="dv">0</span> <span class="dv">0</span>
defaultDim <span class="fu">=</span> <span class="dt">Dimensions</span> emptyRect emptyEdge emptyEdge emptyEdge
<span class="co">-- Rect and Dimensions helpers</span>
<span class="ot">expandedBy ::</span> <span class="dt">EdgeSize</span> <span class="ot">-></span> <span class="dt">Rect</span> <span class="ot">-></span> <span class="dt">Rect</span>
expandedBy edge rec <span class="fu">=</span> <span class="dt">Rect</span>{ x <span class="fu">=</span> x rec <span class="fu">-</span> left edge
, y <span class="fu">=</span> y rec <span class="fu">-</span> top edge
, width <span class="fu">=</span> width rec <span class="fu">+</span> left edge <span class="fu">+</span> right edge
, height <span class="fu">=</span> height rec <span class="fu">+</span> top edge <span class="fu">+</span> bottom edge }
<span class="ot">paddingBox ::</span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">Rect</span>
paddingBox d <span class="fu">=</span> expandedBy (padding d) <span class="fu">$</span> content d
<span class="ot">marginBox ::</span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">Rect</span>
marginBox d <span class="fu">=</span> expandedBy (margin d) <span class="fu">$</span> borderBox d
<span class="ot">borderBox ::</span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">Rect</span>
borderBox d <span class="fu">=</span> expandedBy (border d) <span class="fu">$</span> paddingBox d
<span class="ot">layoutTree ::</span> <span class="dt">StyledNode</span> <span class="ot">-></span> <span class="dt">Dimensions</span> <span class="ot">-></span><span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
layoutTree root contBlock <span class="fu">=</span> buildLayoutTree root <span class="fu">>>=</span>
flip layout contBlock{content <span class="fu">=</span> (content contBlock){height<span class="fu">=</span><span class="dv">0</span>}}
<span class="co">-- updateDim in calcWidth</span>
updateDim d <span class="fu">=</span> <span class="kw">let</span> pad <span class="fu">=</span> padding d
mar <span class="fu">=</span> margin d
bor <span class="fu">=</span> border d
rec <span class="fu">=</span> content d
<span class="kw">in</span> d{ content <span class="fu">=</span> rec{ width <span class="fu">=</span> w'' }
, padding <span class="fu">=</span> pad{ left <span class="fu">=</span> plf, right <span class="fu">=</span> prt }
, border <span class="fu">=</span> bor{ left <span class="fu">=</span> blw, right <span class="fu">=</span> brw }
, margin <span class="fu">=</span> mar{ left <span class="fu">=</span> ml, right <span class="fu">=</span> mr } }
<span class="co">-- updateDim in calcPosition</span>
updateDim d [mt,mb,bt,bb,pt,pb] <span class="fu">=</span>
<span class="kw">let</span> pad <span class="fu">=</span> padding d
mar <span class="fu">=</span> margin d
bor <span class="fu">=</span> border d
brec <span class="fu">=</span> content contBlock
drec <span class="fu">=</span> content d
x' <span class="fu">=</span> x brec
<span class="fu">+</span> left (margin d)
<span class="fu">+</span> left (border d)
<span class="fu">+</span> left (padding d)
y' <span class="fu">=</span> y brec <span class="fu">+</span> height brec <span class="fu">+</span> pt <span class="fu">+</span> bt <span class="fu">+</span> mt
<span class="kw">in</span> d{ content <span class="fu">=</span> drec{ x <span class="fu">=</span> x', y <span class="fu">=</span> y' }
, padding <span class="fu">=</span> pad{ top <span class="fu">=</span> pt, bottom <span class="fu">=</span> pb }
, border <span class="fu">=</span> bor{ top <span class="fu">=</span> bt, bottom <span class="fu">=</span> bb }
, margin <span class="fu">=</span> mar{ top <span class="fu">=</span> mt, bottom <span class="fu">=</span> mb } }
layoutChildren (<span class="dt">NTree</span> (dim,x) cs) <span class="fu">=</span> <span class="kw">do</span>
(dim',cs') <span class="ot"><-</span> foldM foo (dim,[]) cs
return <span class="fu">$</span> <span class="dt">NTree</span> (dim',x) cs'
<span class="kw">where</span>
foo (d,acc) c<span class="fu">@</span>(<span class="dt">NTree</span> (cdim,_) _) <span class="fu">=</span> <span class="kw">do</span>
c'<span class="fu">@</span>(<span class="dt">NTree</span> (cdim',_)_) <span class="ot"><-</span> layout c d
<span class="kw">let</span> rec <span class="fu">=</span> content d
return (d{ content <span class="fu">=</span>
rec{height <span class="fu">=</span> height rec <span class="fu">+</span> marginBoxHeight cdim'}}, acc <span class="fu">++</span> [c'])
<span class="ot">calcHeight ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
calcHeight root<span class="fu">@</span>(<span class="dt">NTree</span> (d,x)y) <span class="fu">=</span> <span class="kw">do</span>
s <span class="ot"><-</span> getStyledElem root
<span class="kw">let</span> d' <span class="fu">=</span> <span class="kw">case</span> value s <span class="st">"height"</span> <span class="kw">of</span>
<span class="dt">Just</span> (<span class="dt">Length</span> h <span class="dt">Px</span>) <span class="ot">-></span> d{content <span class="fu">=</span> (content d){height<span class="fu">=</span>h}}
<span class="dt">Nothing</span> <span class="ot">-></span> d
return <span class="fu">$</span> <span class="dt">NTree</span> (d',x) y
<span class="fu">-----------------------------</span> Css.hs <span class="fu">------------------------------</span>
<span class="kw">data</span> <span class="dt">Value</span> <span class="fu">=</span> <span class="dt">Keyword</span> <span class="dt">T.Text</span>
<span class="fu">|</span> <span class="dt">ColorValue</span> <span class="dt">Color</span>
<span class="fu">|</span> <span class="dt">Length</span> <span class="dt">Float</span> <span class="dt">Unit</span>
<span class="kw">deriving</span> (<span class="dt">Show</span>, <span class="dt">Eq</span>)
<span class="kw">data</span> <span class="dt">Color</span> <span class="fu">=</span> <span class="dt">Color</span> <span class="dt">Word8</span> <span class="dt">Word8</span> <span class="dt">Word8</span> <span class="dt">Word8</span>
<span class="kw">deriving</span> (<span class="dt">Show</span>, <span class="dt">Eq</span>)
color <span class="fu">=</span> <span class="kw">do</span>
char <span class="ch">'#'</span>
cs <span class="ot"><-</span> count <span class="dv">3</span> (count <span class="dv">2</span> hexDigit)
<span class="kw">let</span> [r,g,b] <span class="fu">=</span> map (fst <span class="fu">.</span> head <span class="fu">.</span> readHex) cs
return <span class="fu">$</span> <span class="dt">ColorValue</span> (<span class="dt">Color</span> r g b <span class="dv">255</span>) </code></pre>
<p>There are a couple other places where you’ll need to replace <code>foo rect</code> with <code>foo (content rect)</code> but the compiler will point them out.</p>
<p>Now we’re ready to implement painting.</p>
<p>Painting will be a two-phase process: first we construct a <code>DisplayList</code> from our layout tree, and then we write all the entries in the <code>DisplayList</code> to a canvas.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">{-# LANGUAGE OverloadedStrings #-}</span>
<span class="kw">module</span> <span class="dt">Painting</span>
( <span class="dt">Canvas</span> (<span class="fu">..</span>)
, newCanvas
, paint
) <span class="kw">where</span>
<span class="kw">import </span><span class="dt">Data.Monoid</span> ((<>),mempty)
<span class="kw">import </span><span class="dt">Data.Word</span>
<span class="kw">import qualified</span> <span class="dt">Data.Foldable</span> <span class="kw">as</span> <span class="dt">F</span>
<span class="kw">import qualified</span> <span class="dt">Data.Vector</span> <span class="kw">as</span> <span class="dt">V</span>
<span class="kw">import qualified</span> <span class="dt">Data.Text</span> <span class="kw">as</span> <span class="dt">T</span>
<span class="kw">import </span><span class="dt">Dom</span>
<span class="kw">import </span><span class="dt">Layout</span>
<span class="kw">import </span><span class="dt">Style</span>
<span class="kw">import </span><span class="dt">CSS</span> (<span class="dt">Value</span>(<span class="dt">ColorValue</span>), <span class="dt">Color</span>(..))
<span class="kw">type</span> <span class="dt">DisplayList</span> <span class="fu">=</span> <span class="dt">V.Vector</span> <span class="dt">DisplayCommand</span>
<span class="kw">data</span> <span class="dt">DisplayCommand</span> <span class="fu">=</span> <span class="dt">SolidColor</span> <span class="dt">Color</span> <span class="dt">Rect</span>
<span class="kw">data</span> <span class="dt">Canvas</span> <span class="fu">=</span> <span class="dt">Canvas</span> {<span class="ot"> pixels ::</span> <span class="dt">V.Vector</span> <span class="dt">Color</span>
,<span class="ot"> wdth ::</span> <span class="dt">Word</span> <span class="co">-- we need to abbreviate these because they conflict</span>
,<span class="ot"> hght ::</span> <span class="dt">Word</span> } <span class="co">-- with Rect.width and Rect.height, GHC 7.10 fixes this</span>
<span class="ot">newCanvas ::</span> <span class="dt">Word</span> <span class="ot">-></span> <span class="dt">Word</span> <span class="ot">-></span> <span class="dt">Canvas</span>
newCanvas w h <span class="fu">=</span> <span class="kw">let</span> white <span class="fu">=</span> <span class="dt">Color</span> <span class="dv">255</span> <span class="dv">255</span> <span class="dv">255</span> <span class="dv">255</span> <span class="kw">in</span>
<span class="dt">Canvas</span> (V.replicate (fromIntegral(w <span class="fu">*</span> h)) white) w h</code></pre>
<p>The <code>DisplayList</code> is an intermediate representation we can use to modify the data we draw, for instance by culling boxes that are entirely outside the screen. I suspect that a <code>Data.Vector</code> will be a good choice for this structure, but I’d like to minimize the amount of code we need to change if we later want to use something like <code>Data.Sequence</code> instead, so we’ll try to use functions from <code>Data.Foldable</code> where possible. Right now we’re only drawing boxes, so our <code>DisplayCommand</code> is just a solid color and a rectangle.</p>
<p>Building a <code>DisplayList</code> is a simple fold:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">buildDisplayList ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">DisplayList</span>
buildDisplayList lbox <span class="fu">=</span> F.foldMap renderLayoutBox lbox</code></pre>
<p>Rendering a layout box consists of drawing the background rectangle, and then a rect for each border. If a color isn’t specified for the background or borders, we don’t add a <code>DisplayCommand</code> for that part.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">renderLayoutBox ::</span> (<span class="dt">Dimensions</span>,<span class="dt">BoxType</span>) <span class="ot">-></span> <span class="dt">DisplayList</span>
renderLayoutBox box <span class="fu">=</span> renderBackgroud box <span class="fu"><></span> renderBorders box
<span class="ot">renderBackgroud ::</span> (<span class="dt">Dimensions</span>,<span class="dt">BoxType</span>) <span class="ot">-></span> <span class="dt">DisplayList</span>
renderBackgroud (dim,ty) <span class="fu">=</span> maybe mempty
(return <span class="fu">.</span> flip <span class="dt">SolidColor</span> (borderBox dim)) (getColor ty <span class="st">"background"</span>)
<span class="ot">renderBorders ::</span> (<span class="dt">Dimensions</span>,<span class="dt">BoxType</span>) <span class="ot">-></span> <span class="dt">DisplayList</span>
renderBorders (dim,ty) <span class="fu">=</span> maybe mempty renderBorders' (getColor ty <span class="st">"border-color"</span>)
<span class="kw">where</span>
renderBorders' color <span class="fu">=</span> V.fromList <span class="fu">$</span> map (<span class="dt">SolidColor</span> color) [l, r, t, b]
bbox <span class="fu">=</span> borderBox dim
bdr <span class="fu">=</span> border dim
l <span class="fu">=</span> bbox{ width <span class="fu">=</span> left bdr }
r <span class="fu">=</span> bbox{ x <span class="fu">=</span> x bbox <span class="fu">+</span> width bbox <span class="fu">-</span> right bdr
, width <span class="fu">=</span> right bdr }
t <span class="fu">=</span> bbox{ height <span class="fu">=</span> top bdr }
b <span class="fu">=</span> bbox{ y <span class="fu">=</span> y bbox <span class="fu">+</span> height bbox <span class="fu">-</span> bottom bdr
, height <span class="fu">=</span> bottom bdr }
<span class="ot">getColor ::</span> <span class="dt">BoxType</span> <span class="ot">-></span> <span class="dt">T.Text</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">Color</span>
getColor (<span class="dt">BlockNode</span> style) name <span class="fu">=</span> getColor' style name
getColor (<span class="dt">InlineNode</span> style) name <span class="fu">=</span> getColor' style name
getColor <span class="dt">AnonymousBlock</span> _ <span class="fu">=</span> <span class="dt">Nothing</span>
getColor' style name <span class="fu">=</span> <span class="kw">case</span> value (<span class="dt">NTree</span> style []) name <span class="kw">of</span>
<span class="dt">Just</span> (<span class="dt">ColorValue</span> (<span class="dt">Color</span> r g b a)) <span class="ot">-></span> <span class="dt">Just</span> (<span class="dt">Color</span> r g b a)
_ <span class="ot">-></span> <span class="dt">Nothing</span></code></pre>
<p>We’re not actually implementing any functions to modify the <code>DisplayList</code> right now, so all that’s left is painting. We draw each command on top of the previous one.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">paint ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Rect</span> <span class="ot">-></span> <span class="dt">Canvas</span>
paint root bounds <span class="fu">=</span> <span class="kw">let</span> dlist <span class="fu">=</span> buildDisplayList root
canvas <span class="fu">=</span> newCanvas w h
w <span class="fu">=</span> fromInteger <span class="fu">.</span> floor <span class="fu">$</span> width bounds
h <span class="fu">=</span> fromInteger <span class="fu">.</span> floor <span class="fu">$</span> height bounds
<span class="kw">in</span> F.foldl' paintItem canvas dlist
<span class="ot">paintItem ::</span> <span class="dt">Canvas</span> <span class="ot">-></span> <span class="dt">DisplayCommand</span> <span class="ot">-></span> <span class="dt">Canvas</span>
paintItem cs (<span class="dt">SolidColor</span> color rect) <span class="fu">=</span> updateChunk cs (x0,x1) (y0,y1) color
<span class="kw">where</span>
x0 <span class="fu">=</span> clampInt <span class="dv">0</span> (w<span class="fu">-</span><span class="dv">1</span>) (x rect)
y0 <span class="fu">=</span> clampInt <span class="dv">0</span> (h<span class="fu">-</span><span class="dv">1</span>) (y rect)
x1 <span class="fu">=</span> clampInt <span class="dv">0</span> (w<span class="fu">-</span><span class="dv">1</span>) (x rect <span class="fu">+</span> width rect <span class="fu">-</span> <span class="dv">1</span>)
y1 <span class="fu">=</span> clampInt <span class="dv">0</span> (h<span class="fu">-</span><span class="dv">1</span>) (y rect <span class="fu">+</span> height rect <span class="fu">-</span> <span class="dv">1</span>)
w <span class="fu">=</span> asFloat <span class="fu">$</span> wdth cs
h <span class="fu">=</span> asFloat <span class="fu">$</span> hght cs
asFloat <span class="fu">=</span> fromInteger <span class="fu">.</span> toInteger
<span class="co">-- this probably modifies the pixel vector in-place, if I'm reading the</span>
<span class="co">-- Data.Vector source correctly</span>
<span class="ot">updateChunk ::</span> <span class="dt">Canvas</span> <span class="ot">-></span> (<span class="dt">Integer</span>,<span class="dt">Integer</span>) <span class="ot">-></span> (<span class="dt">Integer</span>,<span class="dt">Integer</span>) <span class="ot">-></span> <span class="dt">Color</span> <span class="ot">-></span> <span class="dt">Canvas</span>
updateChunk cs (x0,x1) (y0,y1) c <span class="fu">=</span> <span class="kw">let</span> pxs <span class="fu">=</span> V.update (pixels cs) chunk <span class="kw">in</span>
cs{ pixels <span class="fu">=</span> pxs}
<span class="kw">where</span>
chunk <span class="fu">=</span> V.map (\a<span class="ot">-></span>(fromIntegral a,c)) indicies
indicies <span class="fu">=</span> V.fromList [ y <span class="fu">*</span> (toInteger <span class="fu">$</span> wdth cs) <span class="fu">+</span> x <span class="fu">|</span> x <span class="ot"><-</span> [x0<span class="fu">..</span>x1], y <span class="ot"><-</span> [y0<span class="fu">..</span>y1] ]
<span class="ot">clampInt ::</span> <span class="dt">Float</span> <span class="ot">-></span> <span class="dt">Float</span> <span class="ot">-></span> <span class="dt">Float</span> <span class="ot">-></span> <span class="dt">Integer</span>
clampInt f c <span class="fu">=</span> floor <span class="fu">.</span> min c <span class="fu">.</span> max f</code></pre>
<p>Note that this code differs slightly from the equivalent Rust code in Robinson: clamping to [0,height] and [0,width] is a runtime error (you’ll try to write one row/column of pixels too far, I’m not sure why Rust allows it), and straight up adding the dimensions to the coordinates will also result in boxes that are 1 pixel too large in both dimensions.</p>
<p>That’s all our painting code, we just need to add a quick test to make sure it works. We’ll need a library to load images for this, and sadly there isn’t one included in the Haskell Platform, so we’ll need to install it. Add the following lines to build-depends in your .cabal file:</p>
<pre><code>vector >= 0.10.9.1,
JuicyPixels >= 3.1.7.1</code></pre>
<p>And then in a command prompt run:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="kw">cabal</span> update
<span class="kw">cabal</span> install --enable-tests --only-dependencies</code></pre>
<p>This will install the JuicyPixels library, which has good support for dealing with images in the most common formats.</p>
<p>We’ll add one test (you’ll need to grab rainbow.png from the github repo)</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="co">-- add these at the top of tests.hs</span>
<span class="kw">import </span><span class="dt">Data.Maybe</span>
<span class="kw">import qualified</span> <span class="dt">Data.Vector</span> <span class="kw">as</span> <span class="dt">V</span>
<span class="kw">import </span><span class="dt">Codec.Picture</span>
<span class="kw">import </span><span class="dt">Codec.Picture.Types</span>
<span class="kw">import </span><span class="dt">Painting</span>
<span class="fu">-----------------------------</span> <span class="dt">PAINT</span> <span class="dt">TESTS</span> <span class="fu">---------------------------</span>
testPaint <span class="fu">=</span> <span class="dt">TestCase</span> <span class="fu">$</span> <span class="kw">do</span>
testpng <span class="ot"><-</span> readPng <span class="st">"tests/rainbow.png"</span>
either (\_<span class="ot">-></span>assertFailure <span class="st">"missing png image"</span>)
(compareImage paintpng)
testpng
compareImage (<span class="dt">Left</span> e) _ <span class="fu">=</span> assertFailure <span class="fu">$</span> T.unpack e
compareImage (<span class="dt">Right</span> i1) (<span class="dt">ImageRGB8</span> i2) <span class="fu">=</span> <span class="kw">do</span>
assertEqual <span class="st">"height"</span> (imageHeight i1) (imageHeight i2)
assertEqual <span class="st">"width"</span> (imageWidth i1) (imageWidth i2)
assertEqual <span class="st">"pixels"</span> (imageData i1) (imageData i2)
contBlock <span class="fu">=</span> defaultDim{content <span class="fu">=</span> (content defaultDim){ width<span class="fu">=</span><span class="dv">800</span>, height<span class="fu">=</span><span class="dv">168</span> } }
paintpng <span class="fu">=</span> paintpng' s d
<span class="kw">where</span>
(<span class="dt">Right</span> d) <span class="fu">=</span> PS.parseHtml pnghtml
(<span class="dt">Right</span> s) <span class="fu">=</span> parseCSS pngcss
paintpng' s d <span class="fu">=</span> <span class="kw">do</span>
<span class="kw">let</span> st <span class="fu">=</span> styleTree d s
lyt <span class="ot"><-</span> layoutTree st contBlock
<span class="kw">let</span> vec <span class="fu">=</span> pixels <span class="fu">$</span> paint lyt (content contBlock)
return <span class="fu">$</span> generateImage (\x y<span class="ot">-></span> c2px <span class="fu">$</span> vec <span class="fu">V.!</span> (x <span class="fu">+</span> (y <span class="fu">*</span> <span class="dv">800</span>))) <span class="dv">800</span> <span class="dv">168</span>
c2px (<span class="dt">Color</span> r g b _) <span class="fu">=</span> <span class="dt">PixelRGB8</span> r g b
pnghtml <span class="fu">=</span> <span class="st">"<div class=\"a\">\</span>
<span class="st">\ <div class=\"b\">\</span>
<span class="st">\ <div class=\"c\">\</span>
<span class="st">\ <div class=\"d\">\</span>
<span class="st">\ <div class=\"e\">\</span>
<span class="st">\ <div class=\"f\">\</span>
<span class="st">\ <div class=\"g\">\</span>
<span class="st">\ </div>\</span>
<span class="st">\ </div>\</span>
<span class="st">\ </div>\</span>
<span class="st">\ </div>\</span>
<span class="st">\ </div>\</span>
<span class="st">\ </div>\</span>
<span class="st">\</div>"</span>
pngcss <span class="fu">=</span> <span class="st">"* { display: block; padding: 12px; }\</span>
<span class="st">\.a { background: #ff0000; }\</span>
<span class="st">\.b { background: #ffa500; }\</span>
<span class="st">\.c { background: #ffff00; }\</span>
<span class="st">\.d { background: #008000; }\</span>
<span class="st">\.e { background: #0000ff; }\</span>
<span class="st">\.f { background: #4b0082; }\</span>
<span class="st">\.g { background: #800080; }"</span></code></pre>
<p>Don’t forget to add <code>testPaint</code> to the test list.</p>
<p>That’s everything for painting. I might put up another post covering some refactorings and additions to the current code before the next Robinson update comes out, or I might work on something completely different for a bit. We’ll see.</p>
<p>As usual, you can find the source for this post <a href="https://github.com/Hrothen/Hubert/blob/master/src/Painting.hs">here</a> and the source for Robinson <a href="https://github.com/mbrubeck/robinson">here</a>.</p>
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</article>]]></summary>
</entry>
<entry>
<title>Let's Build a Browser Engine in Haskell: part 5</title>
<link href="http://hrothen.github.com/posts/lets-build-a-browser-engine-in-haskell-part-5.html" />
<id>http://hrothen.github.com/posts/lets-build-a-browser-engine-in-haskell-part-5.html</id>
<published>2014-10-01T00:00:00Z</published>
<updated>2014-10-01T00:00:00Z</updated>
<summary type="html"><![CDATA[<article>
<div class="info">
Posted on October 1, 2014
</div>
<p>Welcome back, today we’re going to implement Boxes and also the Block Layout, because Matt’s post on Boxes was mostly talking about how they’re set up and I don’t have to do that since he already has. To start with we’re going to want to set up our imports and define some new types.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">{-#LANGUAGE BangPatterns, OverloadedStrings#-}</span>
<span class="kw">module</span> <span class="dt">Layout</span> <span class="kw">where</span>
<span class="kw">import </span><span class="dt">Prelude</span> <span class="kw">hiding</span> (lookup)
<span class="kw">import </span><span class="dt">Control.Applicative</span> ((<$>))
<span class="kw">import </span><span class="dt">Control.Monad</span> (foldM)
<span class="kw">import </span><span class="dt">Data.List</span> (foldl', groupBy)
<span class="kw">import </span><span class="dt">Data.Maybe</span> (fromMaybe)
<span class="kw">import qualified</span> <span class="dt">Data.Text</span> <span class="kw">as</span> <span class="dt">T</span>
<span class="kw">import </span><span class="dt">Dom</span>
<span class="kw">import </span><span class="dt">CSS</span>
<span class="kw">import </span><span class="dt">Style</span>
<span class="kw">data</span> <span class="dt">Dimensions</span> <span class="fu">=</span> <span class="dt">Dimensions</span> {<span class="ot"> x ::</span> <span class="dt">Float</span>
,<span class="ot"> y ::</span> <span class="dt">Float</span>
,<span class="ot"> width ::</span> <span class="dt">Float</span>
,<span class="ot"> height ::</span> <span class="dt">Float</span>
,<span class="ot"> padding ::</span> <span class="dt">EdgeSize</span>
,<span class="ot"> border ::</span> <span class="dt">EdgeSize</span>
,<span class="ot"> margin ::</span> <span class="dt">EdgeSize</span> }
<span class="kw">data</span> <span class="dt">EdgeSize</span> <span class="fu">=</span> <span class="dt">EdgeSize</span> {<span class="ot"> left ::</span> <span class="dt">Float</span>
,<span class="ot"> right ::</span> <span class="dt">Float</span>
,<span class="ot"> top ::</span> <span class="dt">Float</span>
,<span class="ot"> bottom ::</span> <span class="dt">Float</span> }
<span class="kw">type</span> <span class="dt">LayoutBox</span> <span class="fu">=</span> <span class="dt">NTree</span> (<span class="dt">Dimensions</span>,<span class="dt">BoxType</span>)
<span class="kw">type</span> <span class="dt">StyledElement</span> <span class="fu">=</span> (<span class="dt">NodeType</span>,<span class="dt">PropertyMap</span>)
<span class="kw">data</span> <span class="dt">BoxType</span> <span class="fu">=</span> <span class="dt">BlockNode</span> <span class="dt">StyledElement</span> <span class="fu">|</span> <span class="dt">InlineNode</span> <span class="dt">StyledElement</span> <span class="fu">|</span> <span class="dt">AnonymousBlock</span>
emptyEdge <span class="fu">=</span> <span class="dt">EdgeSize</span> <span class="dv">0</span> <span class="dv">0</span> <span class="dv">0</span> <span class="dv">0</span>
defaultDim <span class="fu">=</span> <span class="dt">Dimensions</span> <span class="dv">0</span> <span class="dv">0</span> <span class="dv">0</span> <span class="dv">0</span> emptyEdge emptyEdge emptyEdge</code></pre>
<p>We’ll also want to add some code to the style module:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="kw">data</span> <span class="dt">Display</span> <span class="fu">=</span> <span class="dt">Inline</span> <span class="fu">|</span> <span class="dt">Block</span> <span class="fu">|</span> <span class="dt">DisplayNone</span>
<span class="kw">deriving</span> (<span class="dt">Eq</span>)
<span class="co">-- if name exists, return its specified value</span>
<span class="ot">value ::</span> <span class="dt">StyledNode</span> <span class="ot">-></span> <span class="dt">T.Text</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">Value</span>
value (<span class="dt">NTree</span> node _) name <span class="fu">=</span> HM.lookup name (snd node)
<span class="co">-- look up the display value of a node</span>
<span class="ot">display ::</span> <span class="dt">StyledNode</span> <span class="ot">-></span> <span class="dt">Display</span>
display n <span class="fu">=</span> <span class="kw">case</span> value n <span class="st">"display"</span> <span class="kw">of</span>
<span class="dt">Just</span> (<span class="dt">Keyword</span> <span class="st">"block"</span>) <span class="ot">-></span> <span class="dt">Block</span>
<span class="dt">Just</span> (<span class="dt">Keyword</span> <span class="st">"none"</span>) <span class="ot">-></span> <span class="dt">DisplayNone</span>
_ <span class="ot">-></span> <span class="dt">Inline</span>
<span class="co">-- this lookup is different than the robinson one, it supports</span>
<span class="co">-- an arbitrary number of possible keywords</span>
<span class="co">-- return the specified value of the first property in ks to exist</span>
<span class="co">-- or def if no properties match</span>
lookup<span class="ot"> ::</span> <span class="dt">StyledNode</span> <span class="ot">-></span> [<span class="dt">T.Text</span>] <span class="ot">-></span> <span class="dt">Value</span> <span class="ot">-></span> <span class="dt">Value</span>
lookup s ks def <span class="fu">=</span> maybe def (fromJust <span class="fu">.</span> value s) (find (isJust <span class="fu">.</span> value s) ks)</code></pre>
<p>And a single helper function to CSS</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">toPx ::</span> <span class="dt">Value</span> <span class="ot">-></span> <span class="dt">Float</span>
toPx (<span class="dt">Length</span> len <span class="dt">Px</span>) <span class="fu">=</span> len
toPx _ <span class="fu">=</span> <span class="dv">0</span></code></pre>
<p>Now we can start to work on the layout tree. Our first function traverses the style tree and builds a box for each node. Nodes with <code>display="none"</code> (and their children) are filtered from the tree, and adjacent Inline boxes that are children of a Block node are grouped together beneath Anonymous boxes. We also give every node a default initial set of dimensions.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">buildLayoutTree ::</span> <span class="dt">StyledNode</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
buildLayoutTree root <span class="fu">=</span> <span class="kw">case</span> display root <span class="kw">of</span>
<span class="dt">Block</span> <span class="ot">-></span> <span class="dt">Right</span> <span class="fu">$</span> addDim <span class="fu"><$></span> blt root
<span class="dt">Inline</span> <span class="ot">-></span> <span class="dt">Right</span> <span class="fu">$</span> addDim <span class="fu"><$></span> blt root
<span class="dt">DisplayNone</span> <span class="ot">-></span> <span class="dt">Left</span> <span class="st">"error: root node has display:none"</span>
<span class="kw">where</span>
addDim x <span class="fu">=</span> (defaultDim,x)
blt rt<span class="fu">@</span>(<span class="dt">NTree</span> nd cs) <span class="fu">=</span> <span class="dt">NTree</span> n ns
<span class="kw">where</span>
(<span class="fu">!</span>n, <span class="fu">!</span>ns) <span class="fu">=</span> <span class="kw">case</span> display rt <span class="kw">of</span>
<span class="dt">Block</span> <span class="ot">-></span> (<span class="dt">BlockNode</span> nd, anonify ns')
<span class="dt">Inline</span> <span class="ot">-></span> (<span class="dt">InlineNode</span> nd, ns')
<span class="co">-- won't ever hit DisplayNone, it's filtered out</span>
anonify <span class="fu">=</span> concatMap mergeInlines <span class="fu">.</span> groupBy (\x y <span class="ot">-></span> isInline x <span class="fu">&&</span> isInline y)
mergeInlines x <span class="fu">=</span> <span class="kw">if</span> isInline <span class="fu">$</span> head x <span class="kw">then</span> [<span class="dt">NTree</span> <span class="dt">AnonymousBlock</span> x] <span class="kw">else</span> x
isInline (<span class="dt">NTree</span> <span class="dt">InlineNode</span>{} _) <span class="fu">=</span> <span class="dt">True</span>
isInline _ <span class="fu">=</span> <span class="dt">False</span>
ns' <span class="fu">=</span> map blt <span class="fu">$</span> filter ((<span class="fu">/=</span><span class="dt">DisplayNone</span>) <span class="fu">.</span> display) cs</code></pre>
<p>I’m a little worried about the space performance of this function, for now I’ve used the <code>BangPatterns</code> pragma which allows me to mark <code>n</code> and <code>ns</code> as “strict” (really just evaluated to <a href="http://www.haskell.org/haskellwiki/Weak_head_normal_form">weak head normal form</a>) by prefixing them with an exclamation point. This should hopefully prevent the program from building up a huge set of thunks while traversing the tree (reasoning about strictness/laziness and space usage are areas I need to improve on).</p>
<p>We now add some code to set the dimensions on the layout tree, at the moment we’ll only support Block nodes, so anything without <code>display="block"</code> set will give us an error (I won’t be adding a test for this part yet).</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="co">-- walk a layout tree, setting the dimensions of each node</span>
<span class="ot">layout ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
layout l<span class="fu">@</span>(<span class="dt">NTree</span> (_,box)_) contBlock <span class="fu">=</span> <span class="kw">case</span> box <span class="kw">of</span>
<span class="dt">BlockNode</span> _ <span class="ot">-></span> layoutBlock contBlock l
<span class="dt">InlineNode</span> _ <span class="ot">-></span> undefined
<span class="dt">AnonymousBlock</span> <span class="ot">-></span> undefined
layoutBlock dim root <span class="fu">=</span> calcWidth dim root <span class="fu">>>=</span>
calcPosition dim <span class="fu">>>=</span>
layoutChildren <span class="fu">>>=</span> <span class="co">-- you know what? this might leak</span>
calcHeight </code></pre>
<p><code>layoutBlock</code> starts at the root, computes the width and position of the node, then does the same for all its children. Once the whole tree has been set out, we walk back up it from the leaves calculating the height of each node. It’s possible that recursing in the middle of the function could cause problems for large pages, although that is also how the Rust implementation is written. This will probably get refactored later.</p>
<p>Calculating the width is fairly ugly, nothing is complicated, but there are a bunch of boundary checks to make.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">calcWidth ::</span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
calcWidth contBlock root<span class="fu">@</span>(<span class="dt">NTree</span> (dim,x) y) <span class="fu">=</span> <span class="kw">do</span>
style <span class="ot"><-</span> getStyledElem root
<span class="kw">let</span>
auto <span class="fu">=</span> <span class="dt">Keyword</span> <span class="st">"auto"</span>
zero <span class="fu">=</span> <span class="dt">Length</span> <span class="dv">0</span> <span class="dt">Px</span>
w <span class="fu">=</span> fromMaybe auto <span class="fu">$</span> value style <span class="st">"width"</span>
vals <span class="fu">=</span> map (\a <span class="ot">-></span> lookup style a zero) [
[<span class="st">"margin-left"</span> , <span class="st">"margin"</span>]
, [<span class="st">"margin-right"</span> , <span class="st">"margin"</span>]
, [<span class="st">"border-left-width"</span> , <span class="st">"border-width"</span>]
, [<span class="st">"border-right-width"</span>, <span class="st">"border-width"</span>]
, [<span class="st">"padding-left"</span> , <span class="st">"padding"</span>]
, [<span class="st">"padding-right"</span> , <span class="st">"padding"</span>] ]
total <span class="fu">=</span> sum <span class="fu">$</span> map toPx (w<span class="fu">:</span>vals)
underflow <span class="fu">=</span> width contBlock <span class="fu">-</span> total
([ml'',mr''],vals') <span class="fu">=</span> splitAt <span class="dv">2</span> vals
(w',ml',mr') <span class="fu">=</span> checkUnderflow w <span class="fu">$</span> checkAutoMargins (ml'',mr'')
checkAutoMargins (x,y)
<span class="fu">|</span> w <span class="fu">/=</span> auto <span class="fu">&&</span> total <span class="fu">></span> width contBlock <span class="fu">=</span> (check x,check y)
<span class="fu">|</span> otherwise <span class="fu">=</span> (x,y)
<span class="kw">where</span> check a <span class="fu">=</span> <span class="kw">if</span> a <span class="fu">==</span> auto <span class="kw">then</span> zero <span class="kw">else</span> a
checkUnderflow w (mlf,mrt) <span class="fu">=</span> <span class="kw">case</span> (w <span class="fu">==</span> auto, mlf <span class="fu">==</span> auto, mrt <span class="fu">==</span> auto) <span class="kw">of</span>
(<span class="dt">False</span>,<span class="dt">False</span>,<span class="dt">False</span>) <span class="ot">-></span> (w , mlf, <span class="dt">Length</span> (toPx mrt <span class="fu">+</span> underflow) <span class="dt">Px</span>)
(<span class="dt">False</span>,<span class="dt">False</span>,<span class="dt">True</span>) <span class="ot">-></span> (w , mlf, <span class="dt">Length</span> underflow <span class="dt">Px</span>)
(<span class="dt">False</span>,<span class="dt">True</span>,<span class="dt">False</span>) <span class="ot">-></span> (w , <span class="dt">Length</span> underflow <span class="dt">Px</span> , mrt)
(<span class="dt">False</span>,<span class="dt">True</span>,<span class="dt">True</span>) <span class="ot">-></span> (w , <span class="dt">Length</span> (underflow<span class="fu">/</span><span class="dv">2</span>) <span class="dt">Px</span>, <span class="dt">Length</span> (underflow<span class="fu">/</span><span class="dv">2</span>) <span class="dt">Px</span>)
(<span class="dt">True</span>,_,_) <span class="ot">-></span>
<span class="kw">let</span> l <span class="fu">=</span> <span class="kw">if</span> mlf <span class="fu">==</span> auto <span class="kw">then</span> zero <span class="kw">else</span> mlf
r <span class="fu">=</span> <span class="kw">if</span> mrt <span class="fu">==</span> auto <span class="kw">then</span> zero <span class="kw">else</span> mrt
<span class="kw">in</span> <span class="kw">if</span> underflow <span class="fu">>=</span> <span class="dv">0</span> <span class="kw">then</span> (<span class="dt">Length</span> underflow <span class="dt">Px</span>,l,r)
<span class="kw">else</span> (zero,l,<span class="dt">Length</span> (toPx r <span class="fu">+</span> underflow) <span class="dt">Px</span>)
[w'',ml,mr,blw,brw,plf,prt] <span class="fu">=</span> map toPx (w'<span class="fu">:</span>ml'<span class="fu">:</span>mr'<span class="fu">:</span>vals')
updateDim d <span class="fu">=</span> <span class="kw">let</span> pad <span class="fu">=</span> padding d
mar <span class="fu">=</span> margin d
bor <span class="fu">=</span> border d
<span class="kw">in</span> d{ width <span class="fu">=</span> w''
, padding <span class="fu">=</span> pad{ left <span class="fu">=</span> plf, right <span class="fu">=</span> prt}
, border <span class="fu">=</span> bor{ left <span class="fu">=</span> blw, right <span class="fu">=</span> brw}
, margin <span class="fu">=</span> mar{ left <span class="fu">=</span> ml, right <span class="fu">=</span> mr} }
return <span class="fu">$</span> <span class="dt">NTree</span> (updateDim dim,x) y
<span class="ot">getStyledElem ::</span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">StyledNode</span>
getStyledElem (<span class="dt">NTree</span> (_,box) _) <span class="fu">=</span> <span class="kw">case</span> box <span class="kw">of</span>
<span class="dt">BlockNode</span> s <span class="ot">-></span> <span class="dt">Right</span> <span class="fu">$</span> <span class="dt">NTree</span> s []
<span class="dt">InlineNode</span> s <span class="ot">-></span> <span class="dt">Right</span> <span class="fu">$</span> <span class="dt">NTree</span> s []
<span class="dt">AnonymousBlock</span> <span class="ot">-></span> <span class="dt">Left</span> <span class="st">"Error: attempted to access the nonexistant\</span>
<span class="st"> \ StyleNode of an AnonymousBlock"</span></code></pre>
<p>This will probably be refactored to look a bit nicer, something like</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">calcWidth ::</span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
calcWidth contBlock root<span class="fu">@</span>(<span class="dt">NTree</span> (dim,x) y) <span class="fu">=</span> checkBounds <span class="fu"><$></span> getStyledElem rt
<span class="kw">where</span>
checkBounds <span class="fu">=</span> updateDim <span class="fu">.</span> checkUnderflow <span class="fu">.</span> checkAutoMargins <span class="fu">.</span> computeVals</code></pre>
<p>The resulting function there won’t be any nicer, but at least you get a quick idea of what it’s doing.</p>
<p>The remaining functions aren’t as bad.</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">calcPosition ::</span> <span class="dt">Dimensions</span> <span class="ot">-></span> <span class="dt">LayoutBox</span> <span class="ot">-></span> <span class="dt">Either</span> <span class="dt">T.Text</span> <span class="dt">LayoutBox</span>
calcPosition contBlock root<span class="fu">@</span>(<span class="dt">NTree</span> (dim,a)b) <span class="fu">=</span> <span class="kw">do</span>
style <span class="ot"><-</span> getStyledElem root
<span class="kw">let</span>
zero <span class="fu">=</span> <span class="dt">Length</span> <span class="dv">0</span> <span class="dt">Px</span>
vals <span class="fu">=</span> map (toPx <span class="fu">.</span> (\a <span class="ot">-></span> lookup style a zero)) [
[<span class="st">"margin-top"</span> , <span class="st">"margin"</span>]
, [<span class="st">"margin-bottom"</span> , <span class="st">"margin"</span>]
, [<span class="st">"border-top-width"</span> , <span class="st">"border-width"</span>]
, [<span class="st">"border-bottom-width"</span>, <span class="st">"border-width"</span>]
, [<span class="st">"padding-top"</span> , <span class="st">"padding"</span>]
, [<span class="st">"padding-bottom"</span> , <span class="st">"padding"</span>] ]
updateDim d [mt,mb,bt,bb,pt,pb] <span class="fu">=</span>
<span class="kw">let</span> pad <span class="fu">=</span> padding d
mar <span class="fu">=</span> margin d
bor <span class="fu">=</span> border d
x' <span class="fu">=</span> x contBlock
<span class="fu">+</span> left (margin d)
<span class="fu">+</span> left (border d)
<span class="fu">+</span> left (padding d)
y' <span class="fu">=</span> y contBlock <span class="fu">+</span> height contBlock <span class="fu">+</span> pt <span class="fu">+</span> bt <span class="fu">+</span> mt
<span class="kw">in</span> d{ x <span class="fu">=</span> x'
, y <span class="fu">=</span> y'
, padding <span class="fu">=</span> pad{ top <span class="fu">=</span> pt, bottom <span class="fu">=</span> pb }
, border <span class="fu">=</span> bor{ top <span class="fu">=</span> bt, bottom <span class="fu">=</span> bb }
, margin <span class="fu">=</span> mar{ top <span class="fu">=</span> mt, bottom <span class="fu">=</span> mb } }
return <span class="fu">$</span> <span class="dt">NTree</span> (updateDim dim vals,a) b
<span class="co">-- recursively lay out the children of a node</span>