Put link into Meta, add prev and next (fix #5)

src/Build.hs

@@ -30,11 +30,11 @@ generateNavbarMeta navItems = MetaList $ map f navItems where
 
 -- | Reads in a single document to a Pandoc value using an extra Meta.
 read :: (PandocMonad m, MonadIO m) => Meta -> (Path Rel File, Text) -> m (Path Rel File, Pandoc)
-read extraMeta (path, content) = do
+read (Meta extraMetaMap) (path, content) = do
     contentR <- read' content
     -- TODO: This should really be done without IO...
     newPath <- liftIO $ setFileExtension ".html" path
-    return (newPath, addMeta extraMeta contentR)
+    return (newPath, addMeta (Meta $ Map.insert "link" (MetaString $ unpack $ toFilePath newPath) extraMetaMap) contentR)
     where read' = readMarkdown $ def {readerExtensions = pandocExtensions}
 
 -- | Writes a single Pandoc document to HTML5 output using a template.
@@ -62,8 +62,12 @@ buildArticles navbar inputs = do
     templateArticle <- readFileUtf8 "template/article.html"
     templateIndex <- readFileUtf8 "template/index.html"
 
-    articlesInput <- read extraMeta `mapM` inputs
-    let indexInput = generateIndex extraMeta articlesInput
+    articlesInput' <- read extraMeta `mapM` inputs
+    let neighbors = getNeighbors $ snd <$> articlesInput'
+    -- TODO Refactor!
+    let articlesInput = map (\((prevM, nextM), (path, art)) -> (path, addMeta (Meta $ Map.fromList [("prev", fromMaybe (MetaString "") prevM), ("next", fromMaybe (MetaString "") nextM)]) art)) (zip neighbors articlesInput')
+
+    let indexInput = generateIndex extraMeta (snd <$> articlesInput)
 
     articlesOutput <- write templateArticle `mapMT` articlesInput
     indexOutput <- write templateIndex indexInput
@@ -72,20 +76,35 @@ buildArticles navbar inputs = do
 
     where extraMeta = Meta $ Map.fromList [("navbar", navbar)]
 
+getNeighbors :: [Pandoc] -> [(Maybe MetaValue, Maybe MetaValue)]
+getNeighbors articles = f <$> [0..(length articles - 1)] where
+    f n = (getLink =<< articles `safeIndex` (n-1), getLink =<< articles `safeIndex` (n+1))
+
+getLink :: Pandoc -> Maybe MetaValue
+getLink (Pandoc meta _) = lookupMeta "link" meta
+
+
+safeIndex :: [a] -> Int -> Maybe a
+safeIndex [] _ = Nothing
+safeIndex _ n | n < 0 = Nothing
+safeIndex (x:_) 0 = Just x
+safeIndex (_:xs) n = safeIndex xs (n-1)
+
 -- | Generate an index page from a list of Articles.
-generateIndex :: Meta -> [(Path Rel File, Pandoc)] -> Pandoc
+-- Expects each article to contain a meta value "link" with its link path.
+generateIndex :: Meta -> [Pandoc] -> Pandoc
 generateIndex extraMeta = generateIndex' . sort where
-    sort = sortOn (Down . (\(_, Pandoc meta _) -> lookupMeta "date" meta))
+    sort = sortOn (Down . (\(Pandoc meta _) -> lookupMeta "date" meta))
     generateIndex' articles = Pandoc metaOutput [] where
         metaOutput = extraMeta <> Meta (Map.fromList
             [("pagetitle", MetaString "Home"), ("articles", MetaList articlesOutput)])
-        articlesOutput = uncurry generateIndexMeta <$> articles
+        articlesOutput = generateIndexMeta <$> articles
 
 -- | Generates the Pandoc MetaMap for a document for the index page.
 -- It contains the meta of the source, the link and the body.
-generateIndexMeta :: Path Rel File -> Pandoc -> MetaValue
-generateIndexMeta path (Pandoc (Meta metaMap) body) = MetaMap $ metaMap
-    <> Map.fromList [("link", MetaString $ toFilePath path), ("body", MetaBlocks body)]
+generateIndexMeta :: Pandoc -> MetaValue
+generateIndexMeta (Pandoc (Meta metaMap) body) = MetaMap $ metaMap
+    <> Map.fromList [("body", MetaBlocks body)]
 
 -- | Adds a Meta block to a pandocument. Already known keys are overwritten.
 addMeta :: Meta -> Pandoc -> Pandoc