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Tutorial: Snapshots, and how to produce an RSS/Atom feed

Basic feed configuration

Hakyll has built-in support for two types of feeds: RSS and Atom. This tutorial explains how you can add these to your blog or website. The first step is to define a FeedConfiguration to set some basic options. For example, a cooking blog may have the following declaration:

myFeedConfiguration :: FeedConfiguration
myFeedConfiguration = FeedConfiguration
    { feedTitle       = "Healthy cooking: latest recipes"
    , feedDescription = "This feed provides fresh recipes for fresh food!"
    , feedAuthorName  = "John Doe"
    , feedAuthorEmail = "test@example.com"
    , feedRoot        = "http://healthycooking.example.com"
    }

Simple feed rendering

Now, let’s look at how we actually create a feed. Two functions are available:

renderAtom :: FeedConfiguration
           -> Context String
           -> [Item String]
           -> Compiler (Item String)
renderRss :: FeedConfiguration
          -> Context String
          -> [Item String]
          -> Compiler (Item String)

As you can see, they have exactly the same signature: we’re going to use renderAtom in this tutorial, but it’s trivial to change this to an RSS feed.

create ["atom.xml"] $ do
    route idRoute
    compile $ do
        let feedCtx = postCtx `mappend`
                constField "description" "This is the post description"

        posts <- fmap (take 10) . recentFirst =<< loadAll "posts/*"
        renderAtom myFeedConfiguration feedCtx posts

There we go! We simply take the 10 last posts and pass them to renderAtom, with our configuration and a Context.

It’s a bit of a problem that we don’t have a description for our posts, and the Atom/RSS feed renderer requires this. One option is to add a description: Foo header to our all posts. However, the description is the body text as it appears in most RSS readers, so we would prefer to include the entire content of the posts here.

Snapshots

This poses a problem: if we just load all posts and take their content: we get the finished, processed content. This means all templates have been applied at that point (including templates/default.html). We don’t want to include our entire site with navigation in the RSS feed, but rather just the post HTML.

Snapshots provide a solution to this problem. They allow you to save an Item at any point during its compilation, so you can load it later. Let’s apply this to our concrete problem.

match "posts/*" $ do
    route $ setExtension "html"
    compile $ pandocCompiler
        >>= loadAndApplyTemplate "templates/post.html"    postCtx
        >>= loadAndApplyTemplate "templates/default.html" postCtx
        >>= relativizeUrls

now becomes:

match "posts/*" $ do
    route $ setExtension "html"
    compile $ pandocCompiler
        >>= loadAndApplyTemplate "templates/post.html"    postCtx
        >>= saveSnapshot "content"
        >>= loadAndApplyTemplate "templates/default.html" postCtx
        >>= relativizeUrls

The saveSnapshot function is really simple: it takes an item and returns the same item, after saving it. This return value makes it easier to use saveSnapshot in a chain of compilers as we did in the above example, but you can discard it if you want.

type Snapshot = String

saveSnapshot :: (Typeable a, Binary a)
             => Snapshot -> Item a -> Compiler (Item a)

Including the post body

With this modification, we can update our Atom code. Instead of loading the compiled posts, we just load their content (i.e. the snapshot we just took).

We update the Context to map $description$ to the post body, and we’re done!

create ["atom.xml"] $ do
    route idRoute
    compile $ do
        let feedCtx = postCtx `mappend` bodyField "description"
        posts <- fmap (take 10) . recentFirst =<<
            loadAllSnapshots "posts/*" "content"
        renderAtom myFeedConfiguration feedCtx posts

Other tutorials

The other tutorials can be found here.

Documentation inaccurate or out-of-date? Found a typo?

Hakyll is an open source project, and one of the hardest parts is writing correct, up-to-date, and understandable documentation. Therefore, the authors would really appreciate it if you would give feedback about the tutorials, and especially report errors or difficulties you encountered. If you have a github account, you can use the issue system. Thanks! If you run into any problems, all questions are welcome in the above google group, or you could try the IRC channel, #hakyll on freenode.