Hakyll - automatic sitemap generation

Search engines like sitemaps. Hakyll doesn’t have this ability out of the box, but it is quite trivial to add. Here is my approach.

The Sitemaps protocol is an easy one. Just create a XML file with a bunch of elements, put it on a website and tell search engines (Bing, Google) that it exists. Here is a sample file, used on this site:

<?xml version='1.0' ?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">

Most of blog engines have it built-in. Hakyll does not, which isn’t great, but because modifying a build process of a site is fairly easy, we can generate sitemap.xml ourselves.


Let’s start with two types.

data SitemapConfiguration = SitemapConfiguration {
    sitemapExtensions :: [String]                    -- defaults to [".html"]
  , sitemapChangeFreq :: FilePath -> ChangeFrequency -- defaults to const Weekly
  , sitemapPriority   :: FilePath -> Double          -- defaults to const 0.5
  , sitemapBase       :: String
  , sitemapRewriter   :: FilePath -> FilePath        -- defaults to ('/':)

data ChangeFrequency = Always | Hourly | Daily | Weekly | Monthly | Yearly | Never

My aim is to make it a little bit elastic, so that everyone could use it on their site with simple copy-and-paste approach. Exposing SitemapConfiguration allows us to separate configuration from logic and do not hardcode every decision.

So what does this type control? sitemapExtension is used as a filter, specifying which files should be included (extension must be in this list), sitemapChangeFreq and sitemapPriority allow to control changefreq and priority fields respectively, sitemapBase is prepended to every loc and sitemapRewriter controls rewriting of URLs if we want to have nice links on our website (e.g. without extension, as I have).

Generate XML

Although the format is very simple, I didn’t want to generate XML files by hand. I used xml package, as I found it easy to use and not full of needless (for this task) features. Unluckily, it does not have a nice way to build XML documents, but we may create a small DSL for it. Two simple functions should suffice for our purposes - one will generate element with text as a content (elementString), and one with children (element). They are obvious to write, so I’ll skip them for brevity.

With these, creating our document is as easy as making a couple of one/two-liners for each element:

xmlUrlSet config = add_attr xmlns . element "urlset" . map (xmlUrl config)
    where xmlns = Attr (unqual "xmlns") "http://www.sitemaps.org/schemas/sitemap/0.9"
xmlUrl config r = element "url" [f config r | f <- sub]
    where sub = [xmlLoc, xmlLastMod, xmlChangeFreq, xmlPriority]

-- snip

Putting it all together

Now, we just have to list all pages, get modification time, filter them, generate XML and save the final sitemap to a file. ‘Listing pages’ and ‘saving to file’ is Hakyll’s business, so we’ll just use Compiler monad and (over)use its capabilities.

Listing all pages means getting all Identifiers. We have getMatches (from MonadMetadata) and it supports recursive glob patterns, so getMatches "**" will return every file that Hakyll knows of.

ids <- getMatches "**"

Next, we have to get route (destination path) and its modification time. Retrieving route is easy, but modification time is not trivial to extract from Hakyll - resourceModificationTime from Hakyll.Core.Provider.Internal is, well, internal. We could access it with modificationTimeField from Hakyll.Web.Template.Context, but this will require some gruesome code. I decided to go a different way and use modification time of source file, using normal Haskell function - getModificationTime. We need to convert Hakyll’s Identifier to FilePath and handle errors, but the code is simple.

mapM getRouteWithModTime ids -- We'll use this later
-- ...
getRouteWithModTime i = do
    mtime <- itemModTime i
    rt <- getRoute i
    return $ maybe Nothing (Just.(,mtime)) rt

Now we can filter out unnecessary pages. Some won’t have route (e.g. templates), some won’t be needed (e.g. images, CSS). This can be achieved with this simple lines:

urls <- filter extFilter . catMaybes <$> mapM getRouteWithModTime ids
-- ...
extFilter = flip elem (sitemapExtensions config) . takeExtensions

And now we can generate XML document and pretty print it as an Item:

let urlset = xmlUrlSet config urls
makeItem $ ppcTopElement prettyConfigPP urlset


To sum up, even though Hakyll lacks this feature (and some other ;) ), it is quite easy (and pleasant) to extend, thanks to Haskell terseness and simplicity of Hakyll itself. Generating sitemap is an easy task, but I think that even more complex won’t require much code.

Entire source code used in this post is available on GitHub as part of my site. It should be ready to copy-and-paste, if you want to use it. ;)