Replace ad-hoc isLeft by import from Data.Either

lib/Database/PostgreSQL/Opium.hs

@@ -67,8 +67,9 @@ execParams conn query params = do
         Nothing -> pure result
         Just message -> except $ Left $ ErrorInvalidResult status $ Encoding.decodeUtf8 message
 
+-- The order of the type parameters is important, because it is more common to use type applications for providing the row type.
 fetch
-  :: forall a b. (ToParamList a, FromRow b)
+  :: forall b a. (ToParamList a, FromRow b)
   => Connection
   -> Text
   -> a