3 files changed, 119 insertions, 23 deletions
diff --git a/src/Hakyll/Web/Page.hs b/src/Hakyll/Web/Page.hs
index 00d143e..35a58ff 100644
--- a/src/Hakyll/Web/Page.hs
+++ b/src/Hakyll/Web/Page.hs
@@ -5,8 +5,6 @@
 {-# LANGUAGE DeriveDataTypeable #-}
 module Hakyll.Web.Page
     ( Page (..)
-    , getField
-    , addField
     , toMap
     , pageRead
     , addDefaultFields
@@ -16,7 +14,6 @@ import Prelude hiding (id)
 import Control.Category (id)
 import Control.Arrow ((>>^), (&&&), (>>>))
 import System.FilePath (takeBaseName, takeDirectory)
-import Data.Maybe (fromMaybe)
 import Data.Map (Map)
 import qualified Data.Map as M
 
@@ -24,24 +21,9 @@ import Hakyll.Core.Identifier
 import Hakyll.Core.Compiler
 import Hakyll.Web.Page.Internal
 import Hakyll.Web.Page.Read
+import Hakyll.Web.Page.Metadata
 import Hakyll.Web.Util.String
 
--- | Get a metadata field. If the field does not exist, the empty string is
--- returned.
---
-getField :: String  -- ^ Key
-         -> Page a  -- ^ Page
-         -> String  -- ^ Value
-getField key = fromMaybe "" . M.lookup key . pageMetadata
-
--- | Add a metadata field. If the field already exists, it is not overwritten.
---
-addField :: String  -- ^ Key
-         -> String  -- ^ Value
-         -> Page a  -- ^ Page to add it to
-         -> Page a  -- ^ Resulting page
-addField k v (Page m b) = Page (M.insertWith (flip const) k v m) b
-
 -- | Convert a page to a map. The body will be placed in the @body@ key.
 --
 toMap :: Page String -> Map String String
@@ -56,20 +38,23 @@ pageRead = getResourceString >>^ readPage
 --
 -- * @$url@
 --
--- * @$root@
+-- * @$category@
 --
 -- * @$title@
 --
+-- * @$path@
+--
 addDefaultFields :: Compiler (Page a) (Page a)
 addDefaultFields =   (getRoute &&& id >>^ uncurry addRoute)
                  >>> (getIdentifier &&& id >>^ uncurry addIdentifier)
   where
     -- Add root and url, based on route
     addRoute Nothing  = id
-    addRoute (Just r) = addField "url" (toUrl r)
+    addRoute (Just r) = setField "url" (toUrl r)
 
     -- Add title and category, based on identifier
-    addIdentifier i = addField "title" (takeBaseName p)
-                    . addField "category" (takeBaseName $ takeDirectory p)
+    addIdentifier i = setField "title" (takeBaseName p)
+                    . setField "category" (takeBaseName $ takeDirectory p)
+                    . setField "path" p
       where
         p = toFilePath i
diff --git a/src/Hakyll/Web/Page/Metadata.hs b/src/Hakyll/Web/Page/Metadata.hs
new file mode 100644
index 0000000..28be7d5
--- /dev/null
+++ b/src/Hakyll/Web/Page/Metadata.hs
@@ -0,0 +1,110 @@
+-- | Provides various functions to manipulate the metadata fields of a page
+--
+module Hakyll.Web.Page.Metadata
+    ( getField
+    , setField
+    , renderField
+    , changeField
+    , copyField
+    , renderDateField
+    , renderDateFieldWith
+    ) where
+
+import Data.List (intercalate)
+import Data.Maybe (fromMaybe)
+import Data.Time.Clock (UTCTime)
+import Data.Time.Format (parseTime, formatTime)
+import qualified Data.Map as M
+import System.FilePath (takeFileName)
+import System.Locale (TimeLocale, defaultTimeLocale)
+
+import Hakyll.Web.Page.Internal
+import Hakyll.Web.Util.String
+
+-- | Get a metadata field. If the field does not exist, the empty string is
+-- returned.
+--
+getField :: String  -- ^ Key
+         -> Page a  -- ^ Page
+         -> String  -- ^ Value
+getField key = fromMaybe "" . M.lookup key . pageMetadata
+
+-- | Add a metadata field. If the field already exists, it is not overwritten.
+--
+setField :: String  -- ^ Key
+         -> String  -- ^ Value
+         -> Page a  -- ^ Page to add it to
+         -> Page a  -- ^ Resulting page
+setField k v (Page m b) = Page (M.insertWith (flip const) k v m) b
+
+-- | Do something with a metadata value, but keep the old value as well. If the
+-- key given is not present in the metadata, nothing will happen. If the source
+-- and destination keys are the same, the value will be changed (but you should
+-- use 'changeField' for this purpose).
+--
+renderField :: String              -- ^ Key of which the value should be copied
+            -> String              -- ^ Key the value should be copied to
+            -> (String -> String)  -- ^ Function to apply on the value
+            -> Page a              -- ^ Page on which this should be applied
+            -> Page a              -- ^ Resulting page
+renderField src dst f page = case M.lookup src (pageMetadata page) of
+    Nothing      -> page
+    (Just value) -> setField dst (f value) page
+
+-- | Change a metadata value.
+--
+-- > import Data.Char (toUpper)
+-- > changeField "title" (map toUpper)
+--
+-- Will put the title in UPPERCASE.
+--
+changeField :: String              -- ^ Key to change.
+            -> (String -> String)  -- ^ Function to apply on the value.
+            -> Page a              -- ^ Page to change
+            -> Page a              -- ^ Resulting page
+changeField key = renderField key key
+
+-- | Make a copy of a metadata field (put the value belonging to a certain key
+-- under some other key as well)
+--
+copyField :: String  -- ^ Key to copy
+          -> String  -- ^ Destination to copy to
+          -> Page a  -- ^ Page on which this should be applied
+          -> Page a  -- ^ Resulting page
+copyField src dst = renderField src dst id
+
+-- | When the metadata has a field called @path@ in a
+-- @folder/yyyy-mm-dd-title.extension@ format (the convention for pages),
+-- this function can render the date.
+--
+-- > renderDate "date" "%B %e, %Y" "Date unknown"
+--
+-- Will render something like @January 32, 2010@.
+--
+renderDateField :: String  -- ^ Key in which the rendered date should be placed
+                -> String  -- ^ Format to use on the date
+                -> String  -- ^ Default value, in case the date cannot be parsed
+                -> Page a  -- ^ Page on which this should be applied
+                -> Page a  -- ^ Resulting page
+renderDateField = renderDateFieldWith defaultTimeLocale
+
+-- | This is an extended version of 'renderDateField' that allows you to
+-- specify a time locale that is used for outputting the date. For more
+-- details, see 'renderDateField'.
+--
+renderDateFieldWith :: TimeLocale  -- ^ Output time locale
+                    -> String      -- ^ Destination key
+                    -> String      -- ^ Format to use on the date
+                    -> String      -- ^ Default value
+                    -> Page a      -- ^ Target page
+                    -> Page a      -- ^ Resulting page
+renderDateFieldWith locale key format defaultValue =
+    renderField "path" key renderDate'
+  where
+    renderDate' filePath = fromMaybe defaultValue $ do
+        let dateString = intercalate "-" $ take 3
+                       $ splitAll "-" $ takeFileName filePath
+        time <- parseTime defaultTimeLocale
+                          "%Y-%m-%d"
+                          dateString :: Maybe UTCTime
+        return $ formatTime locale format time
diff --git a/src/Hakyll/Web/Tags.hs b/src/Hakyll/Web/Tags.hs
index 14aaab5..62a99fc 100644
--- a/src/Hakyll/Web/Tags.hs
+++ b/src/Hakyll/Web/Tags.hs
@@ -49,6 +49,7 @@ import qualified Text.Blaze.Html5 as H
 import qualified Text.Blaze.Html5.Attributes as A
 
 import Hakyll.Web.Page
+import Hakyll.Web.Page.Metadata
 import Hakyll.Web.Util.String
 import Hakyll.Core.Writable