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Generating abstracts for Hakyll articles

Suppose you have a list of recent posts and want to include an abstract for each one. Or maybe you want to include brief article summaries in metadata about your content. In this post I demonstrate several ways to declare or generate abstracts for content on your Hakyll site.

Objective §

The goal is to include an $abstract$ field in each article’s context. The field value should be a brief abstract or description of the article. What to actually do with the value is outside the scope of this post. But it is fair to include an example, so here’s how you could use it in a “recent posts” list:

    <li><a href="$url$">$title$</a>: $abstract$</li>

I will discuss a more interesting use case in a future post.

Metadata §

Hakyll processes optional metadata at the top of the article source. The format is YAML. Fields in the YAML map are available via metadataField, which is also part of the defaultContext.

So you can define an abstract in the metadata, like so:

tags: hakyll, pandoc
abstract: >
  In this post I demonstrate several ways to generate
  abstracts for articles in your Hakyll site.

# Generating abstracts for Hakyll articles

Be careful of including HTML special characters (&, <, >, ", ') in the metadata. These will not be escaped automatically, and could break the page. I avoid this pitfall by escaping all values that come from metadataField:

context :: Context String
context =
  mapContext escapeHtml metadataField 

Markup §

I don’t like repeating myself. If I were to use metadataField, the abstract I write would often be a repeat the article’s introduction or some part thereof. Wouldn’t it be nice if I could just indicate—inline—a portion of the article to use as the abstract? For example:

# Generating abstracts for Hakyll articles

Suppose you have … [In this post I demonstrate several
ways to generate abstracts for articles in your Hakyll

The example above uses Pandoc’s bracketed_spans extension. You could achieve the same with explicit <span> tags. Other input formats may or may not provide a way to do it.

On the Hakyll side, we first need a function to locate a span with the abstract class in the Pandoc AST:

abstract :: Pandoc -> Maybe [Inline]
abstract (Pandoc _ blocks) =
  removeFormatting <$> findSpan blocks
  findSpan = fmap getFirst . query $ \inl -> case inl of
    Span (_id, cls, _attrs) inls
        | "abstract" `elem` cls -> First (Just inls)
    _                           -> mempty

In the unlikely event that there are multiple spans with class abstract, the First [Inline] monoid keeps only the first. I strip all formatting via removeFormatting, which I described in a previous post.

The next step is to update the compiler to save a snapshot of the abstract. pandocCompilerWithTransformM gives access to the Pandoc AST, and allows arbitrary Compiler actions including saveSnapshot.

match "posts/*" $ do
  route $ setExtension "html"
  compile $
      (\pandoc -> do
        let render = fmap writePandoc . makeItem
                     . Pandoc mempty . pure . Plain
            (pure ())
            (void . (saveSnapshot "abstract" <=< render))
            (abstract pandoc)
        pure pandoc
    >>= loadAndApplyTemplate "templates/post.html" context

Finally we define a new kind of context field that can read snapshots:

snapshotField :: String -> Snapshot -> Context String
snapshotField key snap = field key $ \item ->
  loadSnapshotBody item snap

and add the field to the context:

context :: Context String
context =
  snapshotField "abstract" "abstract"

Autogeneration §

Consider the following heuristic for autogenerating an abstract: Take the first paragraph that immediately precedes a heading; that is the abstract.

This is a very basic heuristic. But absent other data it’s probably better than nothing. So let’s implement it:

abstract :: Pandoc -> Maybe [Inline]
abstract (Pandoc _ blocks) =
  removeFormatting <$> fallback
  fallback (Para inlines : Header _ _ _ : _) = Just inlines
  fallback (_h : t) = fallback t
  fallback [] = Nothing

This version of abstract scans the list of block elements at the top level of the Pandoc AST. The first time it sees a Para preceding a Header, it returns the paragraph content.

Putting it all together §

For my sites, I want to use all three methods described above. An abstract specified in the metadata is preferred. Explicit markup is my second preference and the autogeneration heuristic is a last resort. This will provide a good user experience for me. With a tiny bit of markup I can avoid repeating myself most of the time. But if it is warranted, I can use the metadata to write something different. Sometimes I’ll get a fair result without doing anything.

Combining the two versions of abstract is left as an exercise for the reader (hint: Control.Applicative.<|>).

Take care when composing the context. The metadataField has to come before the snapshotField (if that’s the priority you want):

context :: Context String
context =
  mapContext escapeHtml metadataField
  <> snapshotField "abstract" "abstract"

Now I have a nice way to generate abstracts for my articles. I will explore an interesting use case in an upcoming post.

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