From beeac9b1011dd57bcc12f316bf9483faa5c38744 Mon Sep 17 00:00:00 2001
From: Jasper Van der Jeugt <jaspervdj@gmail.com>
Date: Sat, 26 Dec 2009 11:53:25 +0100
Subject: Added module to deal with tags.

---
 hakyll.cabal            |  1 +
 src/Text/Hakyll/Tags.hs | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+)
 create mode 100644 src/Text/Hakyll/Tags.hs

diff --git a/hakyll.cabal b/hakyll.cabal
index 3312dc7..0f6d98e 100644
--- a/hakyll.cabal
+++ b/hakyll.cabal
@@ -27,3 +27,4 @@ library
                    Text.Hakyll.File
                    Text.Hakyll.Page
                    Text.Hakyll.Util
+                   Text.Hakyll.Tags
diff --git a/src/Text/Hakyll/Tags.hs b/src/Text/Hakyll/Tags.hs
new file mode 100644
index 0000000..98fda86
--- /dev/null
+++ b/src/Text/Hakyll/Tags.hs
@@ -0,0 +1,21 @@
+-- | Module containing some specialized functions to deal with tags.
+module Text.Hakyll.Tags
+    ( readTagMap
+    ) where
+
+import qualified Data.Map as M
+import qualified Data.ByteString.Lazy.Char8 as B
+import Control.Monad
+
+import Text.Hakyll.Util
+import Text.Hakyll.Page
+
+-- | Read a tag map. This creates a map from tags to page paths. This function
+--   assumes the tags are located in the `tags` metadata field, separated by
+--   commas.
+readTagMap :: [FilePath] -> IO (M.Map String [FilePath])
+readTagMap paths = foldM addPaths M.empty paths
+    where addPaths current path = do
+            page <- readPage path
+            let tags = map trim $ split "," $ B.unpack $ getValue ("tags") page
+            return $ foldr (\t -> M.insertWith (++) t [path]) current tags
-- 
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