Monoid instance for page

src/Hakyll/Web/Page/Internal.hs

@@ -6,10 +6,12 @@ module Hakyll.Web.Page.Internal
     ) where
 
 import Control.Applicative ((<$>), (<*>))
+import Data.Monoid (Monoid, mempty, mappend)
 
 import Data.Map (Map)
 import Data.Binary (Binary, get, put)
 import Data.Typeable (Typeable)
+import qualified Data.Map as M
 
 import Hakyll.Core.Writable
 
@@ -20,6 +22,11 @@ data Page a = Page
     , pageBody     :: a
     } deriving (Show, Typeable)
 
+instance Monoid a => Monoid (Page a) where
+    mempty = Page M.empty mempty
+    mappend (Page m1 b1) (Page m2 b2) =
+        Page (M.union m1 m2) (mappend b1 b2)
+
 instance Functor Page where
     fmap f (Page m b) = Page m (f b)