--- 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](git://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} ~~~