1 files changed, 25 insertions, 3 deletions
diff --git a/src/Text/Hakyll/RenderAction.hs b/src/Text/Hakyll/RenderAction.hs
index 9fb9477..b84a3c5 100644
--- a/src/Text/Hakyll/RenderAction.hs
+++ b/src/Text/Hakyll/RenderAction.hs
@@ -2,17 +2,22 @@ module Text.Hakyll.RenderAction
     ( RenderAction (..)
     , createRenderAction
     , createSimpleRenderAction
+    , createFileRenderAction
     , createManipulationAction
     , chain
     , runRenderAction
+    , runRenderActionIfNeeded
     ) where
 
-import Prelude hiding ((.), id)
 import Control.Category
-import Control.Monad ((<=<), mplus)
+import Control.Monad ((<=<), mplus, unless)
+import Control.Monad.Reader (liftIO)
+import Prelude hiding ((.), id)
+import System.IO (hPutStrLn, stderr)
 
-import Text.Hakyll.Hakyll
 import Text.Hakyll.Context
+import Text.Hakyll.File (toDestination, isFileMoreRecent)
+import Text.Hakyll.Hakyll
 
 data RenderAction a b = RenderAction
     { actionDependencies :: [FilePath]
@@ -26,6 +31,13 @@ createRenderAction f = id { actionFunction = f }
 createSimpleRenderAction :: Hakyll b -> RenderAction () b
 createSimpleRenderAction = createRenderAction . const
 
+createFileRenderAction :: FilePath -> Hakyll b -> RenderAction () b
+createFileRenderAction path action = RenderAction
+    { actionDependencies = [path]
+    , actionUrl          = Just $ return path
+    , actionFunction     = const action
+    }
+
 instance Category RenderAction where
     id = RenderAction
         { actionDependencies = []
@@ -47,3 +59,13 @@ chain = foldl1 (>>>)
 
 runRenderAction :: RenderAction () a -> Hakyll a
 runRenderAction action = actionFunction action ()
+
+runRenderActionIfNeeded :: RenderAction () () -> Hakyll ()
+runRenderActionIfNeeded action = do
+    url <- case actionUrl action of
+        (Just u) -> u
+        Nothing  -> error "No url when checking dependencies."
+    destination <- toDestination url
+    valid <- isFileMoreRecent destination $ actionDependencies action
+    unless valid $ do liftIO $ hPutStrLn stderr $ "Rendering " ++ destination
+                      runRenderAction action