3 files changed, 36 insertions, 12 deletions
diff --git a/src/Text/Hakyll/Page.hs b/src/Text/Hakyll/Page.hs
index dcb9c97..31e2135 100644
--- a/src/Text/Hakyll/Page.hs
+++ b/src/Text/Hakyll/Page.hs
@@ -2,6 +2,7 @@ module Text.Hakyll.Page
     ( Page,
       PageValue,
       addContext,
+      toURL,
       getURL,
       getBody,
       readPage,
@@ -33,6 +34,10 @@ type PageValue = B.ByteString
 addContext :: String -> String -> Page -> Page
 addContext key value = M.insert key (B.pack value)
 
+-- | Get the url for a given page.
+toURL :: FilePath -> FilePath
+toURL = flip addExtension ".html" . dropExtension
+
 -- | Get the URL for a certain page. This should always be defined. If
 --   not, it will return trash.html.
 getURL :: Page -> String
@@ -88,7 +93,7 @@ cachePage page = do
 readPage :: FilePath -> IO Page
 readPage pagePath = do
     -- Check cache.
-    getFromCache <- isCacheFileValid cacheFile pagePath
+    getFromCache <- isCacheValid cacheFile [pagePath]
     let path = if getFromCache then cacheFile else pagePath
 
     -- Read file.
@@ -108,7 +113,7 @@ readPage pagePath = do
     -- Cache if needed
     if getFromCache then return () else cachePage page
     return page
-    where url = addExtension (dropExtension pagePath) ".html"
+    where url = toURL pagePath
           cacheFile = toCache url
 
 -- | Create a key-value mapping page from an association list.
diff --git a/src/Text/Hakyll/Render.hs b/src/Text/Hakyll/Render.hs
index 79f8764..a554fd5 100644
--- a/src/Text/Hakyll/Render.hs
+++ b/src/Text/Hakyll/Render.hs
@@ -1,7 +1,10 @@
 module Text.Hakyll.Render 
-    ( renderPage,
+    ( depends,
+      renderPage,
       renderAndWrite,
+      writePage,
       renderAndConcat,
+      renderChain,
       static,
       staticDirectory
     ) where
@@ -17,6 +20,12 @@ import System.IO
 import Text.Hakyll.Page
 import Text.Hakyll.Util
 
+depends :: FilePath -> [FilePath] -> IO () -> IO ()
+depends file dependencies action = do
+    valid <- isCacheValid (toDestination file) dependencies
+    if valid then return ()
+             else action
+
 createContext :: Page -> Context
 createContext = M.fromList . map packPair . M.toList
     where packPair (a, b) = (B.pack a, b)
@@ -30,11 +39,14 @@ renderPage templatePath page = do
     return $ M.insert "body" body page
 
 renderAndWrite :: FilePath -> Page -> IO ()
-renderAndWrite templatePath page = do
-    rendered <- renderPage templatePath page
-    let destination = toDestination $ getURL rendered
+renderAndWrite templatePath page =
+    renderPage templatePath page >>= writePage
+
+writePage :: Page -> IO ()
+writePage page = do
+    let destination = toDestination $ getURL page
     makeDirectories destination
-    B.writeFile destination (getBody rendered)
+    B.writeFile destination (getBody page)
 
 renderAndConcat :: FilePath -> [FilePath] -> IO B.ByteString
 renderAndConcat templatePath paths = foldM concatRender' B.empty paths
@@ -45,6 +57,12 @@ renderAndConcat templatePath paths = foldM concatRender' B.empty paths
               let body = getBody rendered
               return $ B.append chunk $ body
 
+renderChain :: FilePath -> [FilePath] -> IO ()
+renderChain pagePath templates = depends (toURL pagePath) (pagePath : templates) $
+    do page <- readPage pagePath
+       result <- foldM (flip renderPage) page templates
+       writePage result
+
 static :: FilePath -> IO ()
 static source = do
     makeDirectories destination
diff --git a/src/Text/Hakyll/Util.hs b/src/Text/Hakyll/Util.hs
index 773e32c..1d79b4a 100644
--- a/src/Text/Hakyll/Util.hs
+++ b/src/Text/Hakyll/Util.hs
@@ -5,7 +5,7 @@ module Text.Hakyll.Util
       getRecursiveContents,
       trim,
       split,
-      isCacheFileValid
+      isCacheValid
     ) where
 
 import System.Directory
@@ -56,8 +56,9 @@ split element = unfoldr splitOnce
                                                     else Just (x, tail xs)
 
 -- | Check is a cache file is still valid.
-isCacheFileValid :: FilePath -> FilePath -> IO Bool
-isCacheFileValid cache file = doesFileExist cache >>= \exists ->
+isCacheValid :: FilePath -> [FilePath] -> IO Bool
+isCacheValid cache depends = doesFileExist cache >>= \exists ->
     if not exists then return False
-                  else liftM2 (<=) (getModificationTime file)
-                                   (getModificationTime cache)
+                  else do dependsModified <- (mapM getModificationTime depends) >>= return . maximum
+                          cacheModified <- getModificationTime cache
+                          return (cacheModified >= dependsModified)