Add Monoid instance for Symbol

src/Fcf/Class/Monoid.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE
+    CPP,
     DataKinds,
     PolyKinds,
     TypeFamilies,
@@ -23,6 +24,10 @@ import Fcf.Core (Exp, Eval)
 import Data.Monoid (All(..), Any(..))
 import Data.Type.Bool (type (&&), type (||))
 
+#if __GLASGOW_HASKELL__ >= 802
+import GHC.TypeLits (AppendSymbol)
+#endif
+
 -- $setup
 -- >>> import GHC.TypeLits (Nat)
 
@@ -66,6 +71,12 @@ type instance (<>) ('All a) ('All b) = 'All (a && b)
 -- Any
 type instance (<>) ('Any a) ('Any b) = 'Any (a || b)
 
+#if __GLASGOW_HASKELL__ >= 802
+-- Symbol
+-- | With /base >= 4.10.0.0/.
+type instance (<>) x y = AppendSymbol x y
+#endif
+
 -- | Type-level monoid identity 'Data.Monoid.mempty'.
 --
 -- This is the fcf-encoding of 'MEmpty'.
@@ -112,3 +123,6 @@ type instance MEmpty = 'All 'True
 
 -- Any
 type instance MEmpty = 'Any 'False
+
+-- Symbol
+type instance MEmpty = ""

test/test.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE
+    CPP,
     DataKinds,
     KindSignatures,
     TypeOperators #-}
@@ -64,6 +65,9 @@ _ = Refl :: Eval ('LT .<> 'GT) :~: 'LT
 _ = Refl :: Eval ('EQ .<> 'GT) :~: 'GT
 _ = Refl :: Eval ('Monoid.All 'True .<> 'Monoid.All 'False) :~: 'Monoid.All 'False
 _ = Refl :: Eval ('Monoid.Any 'True .<> 'Monoid.Any 'False) :~: 'Monoid.Any 'True
+#if __GLASGOW_HASKELL__ >= 802
+_ = Refl :: Eval ("a" .<> MEmpty) :~: "a"
+#endif
 
 -- ** Foldable