Add Maybe as possible row container type

lib/Database/PostgreSQL/Opium.hs

@@ -47,16 +47,20 @@ import Database.PostgreSQL.Opium.ToField (ToField (..))
 import Database.PostgreSQL.Opium.ToParamList (ToParamList (..))
 
 class RowContainer c where
-  extract :: FromRow a => Result -> ColumnTable -> ExceptT Error IO (c a)
+  extract :: FromRow a => Result -> LibPQ.Row -> ColumnTable -> ExceptT Error IO (c a)
 
 instance RowContainer [] where
-  extract result columnTable = do
-    nRows <- liftIO $ LibPQ.ntuples result
+  extract result nRows columnTable = do
     mapM (ExceptT . fromRow result columnTable) [0..nRows - 1]
 
+instance RowContainer Maybe where
+  extract result nRows columnTable
+    | nRows == 0 = pure Nothing
+    | nRows == 1 = Just <$> ExceptT (fromRow result columnTable 0)
+    | otherwise = throwE ErrorMoreThanOneRow
+
 instance RowContainer Identity where
-  extract result columnTable = do
-    nRows <- liftIO $ LibPQ.ntuples result
+  extract result nRows columnTable = do
     unless (nRows == 1) $ throwE ErrorNotExactlyOneRow
     Identity <$> ExceptT (fromRow result columnTable 0)
 
@@ -69,8 +73,9 @@ fetch
   -> IO (Either Error (b a))
 fetch query params conn = runExceptT $ do
   result <- execParams conn query params
+  nRows <- liftIO $ LibPQ.ntuples result
   columnTable <- ExceptT $ getColumnTable @a Proxy result
-  extract result columnTable
+  extract result nRows columnTable
 
 fetch_ :: forall a c. (FromRow a, RowContainer c) => Text -> Connection -> IO (Either Error (c a))
 fetch_ query = fetch query ()

lib/Database/PostgreSQL/Opium/Error.hs

@@ -18,6 +18,7 @@ data Error
   | ErrorUnexpectedNull ErrorPosition
   | ErrorInvalidField ErrorPosition Oid ByteString String
   | ErrorNotExactlyOneRow
+  | ErrorMoreThanOneRow
   deriving (Eq, Show)
 
 instance Exception Error where