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diff --git a/src/Hakyll/Core/ResourceProvider.hs b/src/Hakyll/Core/ResourceProvider.hs
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+-- | This module provides an API for resource providers. Resource providers
+-- allow Hakyll to get content from resources; the type of resource depends on
+-- the concrete instance.
+--
+-- A resource is represented by the 'Resource' type. This is basically just a
+-- newtype wrapper around 'Identifier' -- but it has an important effect: it
+-- guarantees that a resource with this identifier can be provided by one or
+-- more resource providers.
+--
+-- Therefore, it is not recommended to read files directly -- you should use the
+-- provided 'Resource' methods.
+--
+module Hakyll.Core.ResourceProvider
+    ( Resource (..)
+    , ResourceProvider (..)
+    , resourceExists
+    , resourceDigest
+    , resourceModified
+    ) where
+
+import Control.Monad ((<=<))
+import Data.Word (Word8)
+
+import qualified Data.ByteString.Lazy as LB
+import OpenSSL.Digest.ByteString.Lazy (digest)
+import OpenSSL.Digest (MessageDigest (MD5))
+
+import Hakyll.Core.Identifier
+import Hakyll.Core.Store
+
+-- | A resource
+--
+-- Invariant: the resource specified by the given identifier must exist
+--
+newtype Resource = Resource {unResource :: Identifier}
+                 deriving (Eq, Show, Ord)
+
+-- | A value responsible for retrieving and listing resources
+--
+data ResourceProvider = ResourceProvider
+    { -- | A list of all resources this provider is able to provide
+      resourceList           :: [Resource]
+    , -- | Retrieve a certain resource as string
+      resourceString         :: Resource -> IO String
+    , -- | Retrieve a certain resource as lazy bytestring
+      resourceLazyByteString :: Resource -> IO LB.ByteString
+    }
+
+-- | Check if a given identifier has a resource
+--
+resourceExists :: ResourceProvider -> Identifier -> Bool
+resourceExists provider = flip elem $ map unResource $ resourceList provider
+
+-- | Retrieve a digest for a given resource
+--
+resourceDigest :: ResourceProvider -> Resource -> IO [Word8]
+resourceDigest provider = digest MD5 <=< resourceLazyByteString provider
+
+-- | Check if a resource was modified
+--
+resourceModified :: ResourceProvider -> Resource -> Store -> IO Bool
+resourceModified provider resource store = do
+    -- Get the latest seen digest from the store
+    lastDigest <- storeGet store itemName $ unResource resource
+    -- Calculate the digest for the resource
+    newDigest <- resourceDigest provider resource
+    -- Check digests
+    if Just newDigest == lastDigest
+        -- All is fine, not modified
+        then return False
+        -- Resource modified; store new digest
+        else do storeSet store itemName (unResource resource) newDigest
+                return True
+  where
+    itemName = "Hakyll.Core.ResourceProvider.resourceModified"