add Data.Empty typeclass

semantic.cabal

@@ -70,6 +70,7 @@ library
                      , Data.AST
                      , Data.Blob
                      , Data.Diff
+                     , Data.Empty
                      , Data.Error
                      , Data.Functor.Both
                      , Data.Functor.Classes.Generic

src/Data/Empty.hs

@@ -0,0 +1,9 @@
+{-# LANGUAGE UndecidableInstances #-}
+
+module Data.Empty where
+
+class Empty a where
+  empty :: a
+
+instance {-# OVERLAPS #-} Monoid a => Empty a where
+  empty = mempty