Add Semigroup instances for Map and Set

This fixes the build on GHC 8.4.1.

CHANGES

@@ -1,3 +1,7 @@
+= Version next, ????-??-?? =
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+* Add Semigroup instances for Map and Set
+
 = Version 1.3.2, 2016-11-30 =
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 * Replace internal use of deprecated containers functions

Data/HashMap.hs

@@ -122,6 +122,9 @@ import Data.Hashable
 import Data.Foldable (Foldable(foldMap))
 import Data.List (foldl')
 import Data.Monoid (Monoid(..))
+#if MIN_VERSION_base(4,9,0)
+import Data.Semigroup (Semigroup((<>), stimes), stimesIdempotentMonoid)
+#endif
 import Data.Traversable (Traversable(traverse))
 import Data.Typeable
 
@@ -178,8 +181,16 @@ instance Functor (Map k) where
 
 instance Ord k => Monoid (Map k a) where
   mempty  = empty
-  mappend = union
   mconcat = unions
+#if !(MIN_VERSION_base(4,9,0))
+  mappend = union
+#else
+  mappend = (<>)
+
+instance Ord k => Semigroup (Map k a) where
+  (<>)   = union
+  stimes = stimesIdempotentMonoid
+#endif
 
 instance Foldable (Map k) where
   foldMap f (Map m) = foldMap some_fold m

Data/HashSet.hs

@@ -73,6 +73,9 @@ import Control.DeepSeq
 import Data.Hashable
 import Data.List (foldl')
 import Data.Monoid (Monoid(..))
+#if MIN_VERSION_base(4,9,0)
+import Data.Semigroup (Semigroup((<>), stimes), stimesIdempotentMonoid)
+#endif
 import Data.Typeable
 
 #if __GLASGOW_HASKELL__
@@ -117,8 +120,16 @@ instance NFData a => NFData (Set a) where
 
 instance Ord a => Monoid (Set a) where
   mempty  = empty
-  mappend = union
   mconcat = unions
+#if !(MIN_VERSION_base(4,9,0))
+  mappend = union
+#else
+  mappend = (<>)
+
+instance Ord a => Semigroup (Set a) where
+  (<>)   = union
+  stimes = stimesIdempotentMonoid
+#endif
 
 instance Show a => Show (Set a) where
   showsPrec d m   = showParen (d > 10) $