Previewing a post in Hakyll

Posted on August 7, 2021

I still don’t have a great idea of what I want to do, aesthetically, with this blog. That’s not so bad, it gives me room to mess around and try things, and one of the things I wanted to try is a fairly compact front page with a preview of the latest post, a few links to recent posts, and pointers into the archives. I implemented the first this afternoon, and I thought I’d write about it.

Yes, I’m aware that Hakyll supports teaser text with <!-- more --> comments. Where’s the fun in that?

My first stop was Rebecca Skinner’s blog repo, because I browsed it while I was first trying to figure out Hakyll and remembered that she’d done something similar. I started with her approach of decorating each post with a field holding the preview. Here’s where I ended up, which isn’t far off from what she wrote:

postCtx :: Context String
postCtx = mconcat
    [ dateField "date" "%B %e, %Y"
    , field "description" previewBody
    , defaultContext

previewBody :: Item String -> Compiler String
previewBody Item {..} = do
    rawBody <- loadSnapshotBody itemIdentifier "postPreTemplate"
    pure $ unwords . take 100 . words $ rawBody

I’m a bit fuzzy yet about how intermediate results are organized, but here we’ve taken a snapshot of each post mid-compliation - this one’s called "postPreTemplate" - and we’re pulling it off an Item (such as a blog post) and taking the first hundred words. Well, we’re telling Hakyll how to do that to make a field named "description", given an Item, by providing a function previewBody to do that.

That snapshot gets taken while we’re compiling the posts:

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

Actually we have a couple of snapshots here: One right after running the Pandoc compiler (where we get our post’s text) and one after applying the post.html template (which we pipe to an RSS feed later - did you know this blog has an RSS feed now? check the footer). That gives us different intermediate representations of the final page for different contexts.

So this is all well and good, but how does it get used? I made a number of different attempts at pulling the first post off the list when compiling the index page, pulling this attribute off and putting it over here while that attribute had to go over there, and generally playing type tetris. In the process I learned a fair bit about Hakyll. I also had a couple of the promised moments where it didn’t matter what Compiler or Item meant, I just needed the String inside and fmap works just fine.

In the end, though, I’m not showing you any of that code because I threw it all away. I decided it was better to make processing the preview post and the other listed posts as similar as possible. So I did this in the index path:

match "index.markdown" $ do
    route $ setExtension "html"
    compile $ do
        posts <- recentFirst =<< loadAll "posts/*"

        let indexCtx = mconcat
                [ listField "posts" postCtx
                    (pure $ tail posts)
                , listField "firstPosts" postCtx 
                    (pure $ take 1 posts)
                , defaultContext

            >>= applyAsTemplate indexCtx
            >>= loadAndApplyTemplate "templates/default.html"
            >>= relativizeUrls

Pretty good, although the names might need work. Believe me when I say this looks a lot cleaner than some of my intermediate attempts.

All that’s left is to do something with that "firstPosts" list. My index markdown now has a couple partial templates in it:

## Latest:

## Other Posts

The last piece of the puzzle is the post-preview.html partial, which works a lot like post-list.html except it uses the "description" field and adds the “read more” link

<div class="post-summary pl-8">
<h2><a href="$url$">$title$</a></h2>
<div class="postDescription">
  $description$ ...<a href="$url$"><strong><em>read more</em></strong></a>

For a finishing touch, I struggled a bit getting Pandoc’s syntax highlighting to play nicely with Tailwind before borrowing from Leo Orpilla III’s Hakyll repo - another source of reference and inspiration since he’s also using Tailwind.

Next, I’ll probably play around with the colour scheme, and add tags as I post more. Longer-term I’d like to add short-name linking (e.g. linking to this post as "previewing-a-post" rather than the full path).

Writing code for a Hakyll blog is half the fun!