3 files changed, 31 insertions, 4 deletions
diff --git a/src/Hakyll/Core/Resource/Provider.hs b/src/Hakyll/Core/Resource/Provider.hs
index 377b029..90e93f8 100644
--- a/src/Hakyll/Core/Resource/Provider.hs
+++ b/src/Hakyll/Core/Resource/Provider.hs
@@ -17,8 +17,11 @@ module Hakyll.Core.Resource.Provider
     , resourceModified
     ) where
 
+import Control.Concurrent (MVar, readMVar, modifyMVar_)
 import Control.Monad ((<=<))
 import Data.Word (Word8)
+import Data.Map (Map)
+import qualified Data.Map as M
 
 import qualified Data.ByteString.Lazy as LB
 import OpenSSL.Digest.ByteString.Lazy (digest)
@@ -37,6 +40,8 @@ data ResourceProvider = ResourceProvider
       resourceString         :: Resource -> IO String
     , -- | Retrieve a certain resource as lazy bytestring
       resourceLazyByteString :: Resource -> IO LB.ByteString
+    , -- | Cache keeping track of modified items
+      resourceModifiedCache  :: MVar (Map Resource Bool)
     }
 
 -- | Check if a given identifier has a resource
@@ -53,6 +58,24 @@ resourceDigest provider = digest MD5 <=< resourceLazyByteString provider
 --
 resourceModified :: ResourceProvider -> Resource -> Store -> IO Bool
 resourceModified provider resource store = do
+    cache <- readMVar mvar
+    case M.lookup resource cache of
+        -- Already in the cache
+        Just m  -> return m
+        -- Not yet in the cache, check digests (if it exists)
+        Nothing -> do
+            m <- if resourceExists provider (unResource resource)
+                        then digestModified provider resource store
+                        else return False
+            modifyMVar_ mvar (return . M.insert resource m)
+            return m
+  where
+    mvar = resourceModifiedCache provider
+
+-- | Check if a resource digest was modified
+--
+digestModified :: ResourceProvider -> Resource -> Store -> IO Bool
+digestModified provider resource store = do
     -- Get the latest seen digest from the store
     lastDigest <- storeGet store itemName $ unResource resource
     -- Calculate the digest for the resource
@@ -65,4 +88,4 @@ resourceModified provider resource store = do
         else do storeSet store itemName (unResource resource) newDigest
                 return True
   where
-    itemName = "Hakyll.Core.ResourceProvider.resourceModified"
+    itemName = "Hakyll.Core.ResourceProvider.digestModified"
diff --git a/src/Hakyll/Core/Resource/Provider/File.hs b/src/Hakyll/Core/Resource/Provider/File.hs
index a795fac..953d61c 100644
--- a/src/Hakyll/Core/Resource/Provider/File.hs
+++ b/src/Hakyll/Core/Resource/Provider/File.hs
@@ -5,6 +5,8 @@ module Hakyll.Core.Resource.Provider.File
     ) where
 
 import Control.Applicative ((<$>))
+import Control.Concurrent (newMVar)
+import qualified Data.Map as M
 
 import qualified Data.ByteString.Lazy as LB
 
@@ -22,9 +24,13 @@ fileResourceProvider configuration = do
     list <- map parseIdentifier . filter (not . ignoreFile configuration) <$>
         getRecursiveContents False "."
 
+    -- MVar for the cache
+    mvar <- newMVar M.empty
+
     -- Construct a resource provider
     return ResourceProvider
         { resourceList           = map Resource list
         , resourceString         = readFile . toFilePath . unResource
         , resourceLazyByteString = LB.readFile . toFilePath . unResource
+        , resourceModifiedCache  = mvar
         }
diff --git a/src/Hakyll/Core/Run.hs b/src/Hakyll/Core/Run.hs
index 8e1ba6d..54bb104 100644
--- a/src/Hakyll/Core/Run.hs
+++ b/src/Hakyll/Core/Run.hs
@@ -100,9 +100,7 @@ modified :: ResourceProvider     -- ^ Resource provider
          -> [Identifier]         -- ^ Identifiers to check
          -> IO (Set Identifier)  -- ^ Modified resources
 modified provider store ids = fmap S.fromList $ flip filterM ids $ \id' ->
-    if resourceExists provider id'
-        then resourceModified provider (Resource id') store
-        else return False
+    resourceModified provider (Resource id') store
 
 -- | Add a number of compilers and continue using these compilers
 --