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+---
+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.
+
+I thus had 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}
+~~~