Add Maybe as possible row container type

test/Database/PostgreSQL/OpiumSpec.hs

@@ -131,18 +131,6 @@ spec = do
       rows <- Opium.fetch "SELECT count(*) AS only FROM person WHERE name = $1" (Identity ("paul" :: Text)) conn
       rows `shouldBe` Right [Only (1 :: Int)]
 
-    it "Accepts exactly one row when Identity is the return type" $ \conn -> do
-      row <- Opium.fetch "SELECT ($1 + $2) AS only" (17 :: Int, 25 :: Int) conn
-      row `shouldBe` Right (Identity (Only (42 :: Int)))
-
-    it "Does not accept zero rows when Identity is the return type" $ \conn -> do
-      row <- Opium.fetch @(Only Int) @Identity "SELECT ($1 + $2) AS only WHERE false" (17 :: Int, 25 :: Int) conn
-      row `shouldSatisfy` isLeft
-
-    it "Does not accept two rows when Identity is the return type" $ \conn -> do
-      row <- Opium.fetch @(Only Int) @Identity "SELECT $1 AS only UNION ALL SELECT $2 AS only" (17 :: Int, 25 :: Int) conn
-      row `shouldSatisfy` isLeft
-
   describe "fetch_" $ do
     it "Retrieves a list of rows" $ \conn -> do
       rows <- Opium.fetch_ "SELECT * FROM person" conn
@@ -163,3 +151,27 @@ spec = do
     it "Works for the readme regression example" $ \conn -> do
       rows <- Opium.fetch_ @ScoreByAge @[] "SELECT regr_intercept(score, age) AS t, regr_slope(score, age) AS m FROM person" conn
       rows `shouldSatisfy` \case { (Right [ScoreByAge _ _]) -> True; _ -> False }
+
+    it "Accepts exactly one row when Identity is the row container type" $ \conn -> do
+      row <- Opium.fetch_ "SELECT 42 AS only" conn
+      row `shouldBe` Right (Identity (Only (42 :: Int)))
+
+    it "Does not accept zero rows when Identity is the row container type" $ \conn -> do
+      row <- Opium.fetch_ @(Only Int) @Identity "SELECT 42 AS only WHERE false" conn
+      row `shouldSatisfy` isLeft
+
+    it "Does not accept two rows when Identity is the row container type" $ \conn -> do
+      row <- Opium.fetch_ @(Only Int) @Identity "SELECT 17 AS only UNION ALL SELECT 25 AS only" conn
+      row `shouldSatisfy` isLeft
+
+    it "Accepts zero rows when Maybe is the row container type" $ \conn -> do
+      row <- Opium.fetch_ @(Only Int) @Maybe "SELECT 17 AS only WHERE false" conn
+      row `shouldBe` Right Nothing
+
+    it "Accepts one row when Maybe is the row container type" $ \conn -> do
+      row <- Opium.fetch_ @(Only Int) @Maybe "SELECT 42 AS only" conn
+      row `shouldBe` Right (Just (Only 42))
+
+    it "Does not accept two rows when Maybe is the row container type" $ \conn -> do
+      row <- Opium.fetch_ @(Only Int) @Maybe "SELECT 17 AS only UNION ALL SELECT 25 AS only" conn
+      row `shouldSatisfy` isLeft