---
title: Switch to Hakyll
published: 2015-08-03
tags: Blog Software
---

I stopped using the software suite inherited from
[math.kleen.org](http://math.kleen.org) and switched over to using
[hakyll](http://jaspervdj.be/hakyll/) instead, since I realised that the two
were doing essentially the same job and keeping my mess in one haskell file
(`src/Site.hs`, for those of you who are willing to checkout the
[git repo](https://git.yggdrasil.li/dirty-haskell.org)) instead of spread over a
large number of interlocking zsh and haskell scripts.

I expect nothing to be seriously broken (Only the filepaths of lists have
changed), but some feed readers might have stopped working (hakyll´s
deceptively named `renderRss` actually renders atom).

## Implementation Details

I´m using this post to document some of the more involved things I had to do
during migration in no particular order.

### Lists → Tags

I´m using hakyll´s implementation of
[tags](http://hackage.haskell.org/package/hakyll-4.7.2.2/docs/Hakyll-Web-Tags.html)
instead of the [math.kleen.org](http://math.kleen.org) concept of lists, now.

This required some tweaking.

In order to retain the [All Posts](/tags/all-posts.html) list I introduced a
function to add new tags to an already existing
[Tags](http://hackage.haskell.org/package/hakyll-4.7.2.2/docs/Hakyll-Web-Tags.html#t:Tags)
structure and used it to add my desired pseudo-tag.

~~~ {.haskell}
main = hakyllWith config $ do
  …
  tags <- buildTags "posts/*" tagTranslation' >>= addTag "All Posts" "posts/*"
  …

addTag :: MonadMetadata m => String -> Pattern -> Tags -> m Tags
addTag name pattern tags = do
  ids <- getMatches pattern
  return $ tags { tagsMap = tagsMap tags ++ [(name, ids)] }
~~~

### Printing lists is an involved affair

I wanted to keep the layout of the site including the lists of posts on the
[index page](/).

Generating those lists turned out to be a hassle.

The `Rule` for `index.md` adds to the context of the templates used in it´s
creation a list field which contains verbatim HTML as produced by renderTag for
each tag.
A trick I used to implement the desired behaviour of replacing old posts with
"…" is to introduce a pseudo post-item which has a flag in it´s context to tell
the corresponding template to only print "…".
Trimming the list of posts is straightforward.

~~~ {.haskell}
renderTag :: String -- ^ Tag name
          -> Tags
          -> Compiler (Item String)
renderTag tag tags = do
  ellipsisItem <- makeItem ""
  let
    ids = fromMaybe [] $ lookup tag $ tagsMap tags
    postCtx = mconcat [ listField "posts" (ellipsisContext ellipsisItem) $
                          liftM (withEllipsis ellipsisItem) $ chronological =<< mapM load ids
                      , constField "title" tag
                      , constField "rss" ("tags/" ++ tagTranslation tag ++ ".rss")
                      , constField "url" ("tags/" ++ tagTranslation tag ++ ".html")
                      , defaultContext
                      ]
  makeItem ""
    >>= loadAndApplyTemplate "templates/post-list.html" postCtx
    >>= loadAndApplyTemplate "templates/tag.html" postCtx
  where
    ellipsisContext item = mconcat [ boolField "ellipsis" (== item)
                                   , defaultContext
                                   ]
    boolField name f = field name (\i -> if f i
                                         then pure (error $ unwords ["no string value for bool field:",name])
                                         else empty)
    withEllipsis ellipsisItem xs
      | length xs > max = [ellipsisItem] ++ takeEnd (max - 1) xs
      | otherwise = xs
    takeEnd i = reverse . take i . reverse
    max = 4
~~~

### Everything needs a [Rule](http://hackage.haskell.org/package/hakyll-4.7.2.2/docs/Hakyll-Core-Rules.html#t:Rules)

I was stumped for a while when my templates wouldn´t
[load](http://hackage.haskell.org/package/hakyll-4.7.2.2/docs/Hakyll-Web-Template.html#v:loadAndApplyTemplate).

This was easily rectified by realising, that even templates need (of course) a
declaration of how to compile them:

~~~ {.haskell}
main = hakyllWith config $ do
  match "templates/*" $ compile templateCompiler
  …
~~~

### Duplicate Rules are duplicate

Hakyll tracks dependencies.
Therefore it seems to keep a list of
[Identifier](http://hackage.haskell.org/package/hakyll-4.7.2.2/docs/Hakyll-Core-Identifier.html#t:Identifier)s
it has encountered with priority given to the more early ones.

It was thus necessary to tweak the function that does `Identifier`/`String`
conversion for tags contained within a
[Tags](http://hackage.haskell.org/package/hakyll-4.7.2.2/docs/Hakyll-Web-Tags.html#v:Tags)
structure if I wanted to use (the very convenient)
[tagsRules](http://hackage.haskell.org/package/hakyll-4.7.2.2/docs/Hakyll-Web-Tags.html#v:tagsRules)
twice.

So I did:

~~~ {.haskell}
main = hakyllWith config $ do
  tags <- buildTags "posts/*" tagTranslation' …
  let
    tags' = tags { tagsMakeId = fromFilePath . (\b -> "rss" </> b <.> "rss") . takeBaseName . toFilePath . tagsMakeId tags}
~~~