Hakyll

Hakyll is a simple static site generator in Haskell. It is mostly inspired by Jekyll, but I like to believe it is simpler.

Configuration

Inspired by xmonad, a small Haskell program is used as configuration file. In this file, you give instructions on how the site should be generated. In the rest of this document, we will examine a small example.

This is our directory layout:

|-- _site
|-- favicon.ico
|-- hakyll.hs
|-- images
|   `-- foo.png
|-- templates
|   |-- default.html
|   `-- sample.html
`-- text.markdown

Static files

Static files can be rendered using the static command. This command ensures the files will copied when you compile the site.

For convenience reasons, there is also a staticDirectory command, which works recursively.

main = do
    static "favicon.ico"
    staticDirectory "images"
    staticDirectory "css"

Pages

Pages can be written in html, markdown, LaTeX, and basically anything pandoc supports. They can also contain metadata, which are always key-value mappings.

---
author: Jasper Van der Jeugt
title: A sample markdown post
---
# A sample markdown post

This is a sample markdown post. It supports pandoc extensions
like code highlighting. For example:

~~~~{.haskell}
main = putStrLn "Hello World!"
~~~~

Metadata is always placed in the beginning of a file, and is delimited by a --- string. The metadata can only contain simple key-value pairs. We can now read in this page using the Text.Hakyll.Page.readPage function. This will return a Page, which is actually just a Map String ByteString. In this example, the map would consist of the following key-value pairs:

author: Jasper Van der Jeugt
title: A sample markdown post
body: The rest of the file (rendered to html).
url: text.html (the original filename was text.markdown, the extension was changed to html).

Templates

In hakyll, there is a strict separation between pages and templates. Templates, for example, cannot contain metadata.

<h2> $title </h2>
by <strong> $author </title>

$body

Templates are rendered using the Haskell Text.Template library. This means that in your template, you can use $identifier, and it will be replaced by the value of identifier.

With this template we could, for example, render the file we saw in the previous section. It would go like this:

page <- readPage "text.markdown"
render <- renderPage "templates/sample.html" page

Now, render will be a Page containing all metadata from page, but the body key would be replaced by the substitution. This means we can combine rendering actions. Given another template templates/default.html:

<html>
    <head>
        <title> $title </title>
    </head>
    <body>
        $body
    </body>
</html>

We can now combine the rendering actions:

page <- readPage "text.markdown"
render <- (renderPage "templates/sample.html" page >>=
            renderPage "templates/default.html")

Of course, you can't really do anything with the render if you don't write it to a file somewhere. That's why the function renderAndWrite exists:

readPage "text.markdown" >>=
    renderPage "templates/sample.html" page >>=
    renderAndWrite "templates/default.html"

Now, where will this file be written? In _site/text.html, of course! That's because the page still contains a key called url, which the renderAndWrite function uses to determine the file destination.

More advanced things

Sometimes, you want to create a Page from scratch, without reading from a file. There are functions to do that for you, and I suggest you read the documentation of Text.Hakyll.Page. As a more advanced example, I will explain the RSS system I wrote for my website.

|-- generate.hs
|-- posts
|   `-- 2009-12-02-a-first-post.markdown
`-- templates
    |-- rss.xml
    `-- rssitem.xml

Our post contains some metadata:

---
title: A first post
date: December 2, 2009
---

# A first post

A first post describing the technical setup of this blog, for that is

The templates/rssitem.xml file is a template for rendering one post to an rss item:

<item>
    <title>$title</title>
    <link>http://jaspervdj.be/$url</link>
    <description>New blogpost: $title</description>
</item>

Now a template for rendering the whole rss feed, templates/rss.xml:

<?xml version="1.0" ?>
<rss version="2.0">
    <channel>
        <title>jaspervdj - a personal blog</title>
        <link>http://jaspervdj.be/</link>
        <description>Personal blog of jaspervdj</description>
        $items
    </channel> 
</rss>

Alright, let's get coding.

-- Find all posts paths.
postPaths <- liftM (L.reverse . L.sort) $ getRecursiveContents "posts"
-- Read and render all posts with the rssitem.xml template
-- Also, only render 5 posts.
pages <- mapM readPage (take 5 postPaths)
items <- mapM (renderPage "templates/rssitem.xml") pages
-- Render the result
renderAndWrite "templates/rss.xml" $ pageFromList [ ("items", concatPages items),
                                                    ("url", "rss.xml") ]

That's that. Now we have a nice rss feed.

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