added Monoid instance for Css

src/Clay/Stylesheet.hs

@@ -1,4 +1,6 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE TypeSynonymInstances #-}
+{-# LANGUAGE FlexibleInstances #-}
 module Clay.Stylesheet where
 
 import Control.Applicative
@@ -61,6 +63,10 @@ rule a = S (tell [a])
 
 type Css = StyleM ()
 
+instance Monoid Css where
+  mempty = pure ()
+  mappend = liftA2 mappend
+
 -- | Add a new style property to the stylesheet with the specified `Key` and
 -- value. The value can be any type that is in the `Val' typeclass, with other
 -- words: can be converted to a `Value`.

src/Test.hs

@@ -6,8 +6,9 @@ import Test.Framework.Providers.HUnit (testCase)
 import Test.HUnit.Base ((@?=))
 
 import Clay.Render (renderWith, compact)
+import Data.Monoid
 
 main :: IO ()
 main = defaultMain
-  [ testCase "empty Clay produces empty compact CSS" $ renderWith compact [] (return ()) @?= ""
+  [ testCase "mempty produces empty compact CSS" $ renderWith compact [] mempty @?= ""
   ]