Add Semigroup instances for existing Monoids

Ensures forwards compatibility with future Haskell/GHC releases
as the Semigroup/Monoid Proposal is gradually implemented.

Closes #525 and #536.

1 files changed, 14 insertions, 0 deletions

diff --git a/lib/Hakyll/Core/Compiler/Internal.hs b/lib/Hakyll/Core/Compiler/Internal.hs
index 7b1df83..5b6d1aa 100644
--- a/lib/Hakyll/Core/Compiler/Internal.hs
+++ b/lib/Hakyll/Core/Compiler/Internal.hs
@@ -1,5 +1,6 @@
 --------------------------------------------------------------------------------
 -- | Internally used compiler module
+{-# LANGUAGE CPP                        #-}
 {-# LANGUAGE FlexibleInstances          #-}
 {-# LANGUAGE GADTs                      #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
@@ -32,6 +33,9 @@ import           Control.Applicative            (Alternative (..))
 import           Control.Exception              (SomeException, handle)
 import           Control.Monad                  (forM_)
 import           Control.Monad.Except            (MonadError (..))
+#if MIN_VERSION_base(4,9,0)
+import           Data.Semigroup                 (Semigroup (..))
+#endif
 import           Data.Set                       (Set)
 import qualified Data.Set                       as S
 
@@ -83,10 +87,20 @@ data CompilerWrite = CompilerWrite
 
 
 --------------------------------------------------------------------------------
+#if MIN_VERSION_base(4,9,0)
+instance Semigroup CompilerWrite where
+    (<>) (CompilerWrite d1 h1) (CompilerWrite d2 h2) =
+        CompilerWrite (d1 ++ d2) (h1 + h2)
+
+instance Monoid CompilerWrite where
+    mempty  = CompilerWrite [] 0
+    mappend = (<>)
+#else
 instance Monoid CompilerWrite where
     mempty = CompilerWrite [] 0
     mappend (CompilerWrite d1 h1) (CompilerWrite d2 h2) =
         CompilerWrite (d1 ++ d2) (h1 + h2)
+#endif
 
 
 --------------------------------------------------------------------------------