--- a/src/OptionalConfiguration.hs
+++ b/src/OptionalConfiguration.hs
@@ -27,3 +27,3 @@ import System.FilePath ( (</>) )
 import System.IO.Error ( catchIOError )
-
+import qualified Data.Semigroup as S
 
@@ -65,2 +65,16 @@ merge_maybes (Just _) (Just y) = Just y
 --
+
+instance S.Semigroup OptionalConfiguration where
+  -- | Combine @cfg1@ and @cfg2@, giving precedence to @cfg2@.
+  cfg1 <> cfg2 =
+    OptionalConfiguration
+      (merge_maybes (database cfg1) (database cfg2))
+      (merge_maybes (detail cfg1) (detail cfg2))
+      (merge_maybes (detail_query cfg1) (detail_query cfg2))
+      (merge_maybes (host cfg1) (host cfg2))
+      (merge_maybes (password cfg1) (password cfg2))
+      (merge_maybes (port cfg1) (port cfg2))
+      (merge_maybes (summary_query cfg1) (summary_query cfg2))
+      (merge_maybes (username cfg1) (username cfg2))
+
 instance Monoid OptionalConfiguration where