Make the container type for fetch customizable

test/Database/PostgreSQL/OpiumSpec.hs

@@ -131,23 +131,35 @@ 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
       rows `shouldBe` Right [Person "paul" 25, Person "albus" 103]
 
     it "Fails for invalid queries" $ \conn -> do
-      rows <- Opium.fetch_ @Person "MRTLBRNFT" conn
+      rows <- Opium.fetch_ @Person @[] "MRTLBRNFT" conn
       rows `shouldSatisfy` isLeft
 
     it "Fails for unexpected NULLs" $ \conn -> do
-      rows <- Opium.fetch_ @Person "SELECT NULL AS name, 0 AS age" conn
+      rows <- Opium.fetch_ @Person @[] "SELECT NULL AS name, 0 AS age" conn
       rows `shouldBe` Left (Opium.ErrorUnexpectedNull (Opium.ErrorPosition 0 "name"))
 
     it "Fails for the wrong column type" $ \conn -> do
-      rows <- Opium.fetch_ @Person "SELECT 'quby' AS name, 'indeterminate' AS age" conn
+      rows <- Opium.fetch_ @Person @[] "SELECT 'quby' AS name, 'indeterminate' AS age" conn
       rows `shouldBe` Left (Opium.ErrorInvalidOid "age" $ LibPQ.Oid 25)
 
     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 <- 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 }