lib/Database/PostgreSQL/Opium.hs

@@ -26,10 +26,9 @@ module Database.PostgreSQL.Opium
   )
   where
 
-import Control.Monad (unless, void)
+import Control.Monad (void)
 import Control.Monad.IO.Class (liftIO)
-import Control.Monad.Trans.Except (ExceptT (..), except, runExceptT, throwE)
+import Control.Monad.Trans.Except (ExceptT (..), except, runExceptT)
-import Data.Functor.Identity (Identity (..))
 import Data.Proxy (Proxy (..))
 import Data.Text (Text)
 import Database.PostgreSQL.LibPQ
@@ -42,42 +41,24 @@ import qualified Database.PostgreSQL.LibPQ as LibPQ
 
 import Database.PostgreSQL.Opium.Error (Error (..), ErrorPosition (..))
 import Database.PostgreSQL.Opium.FromField (FromField (..), RawField (..))
-import Database.PostgreSQL.Opium.FromRow (FromRow (..), ColumnTable)
+import Database.PostgreSQL.Opium.FromRow (FromRow (..))
 import Database.PostgreSQL.Opium.ToField (ToField (..))
 import Database.PostgreSQL.Opium.ToParamList (ToParamList (..))
 
-class RowContainer c where
+-- The order of the type parameters is important, because it is more common to use type applications for providing the row type.
-  extract :: FromRow a => Result -> LibPQ.Row -> ColumnTable -> ExceptT Error IO (c a)
-
-instance RowContainer [] where
-  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 nRows columnTable = do
-    unless (nRows == 1) $ throwE ErrorNotExactlyOneRow
-    Identity <$> ExceptT (fromRow result columnTable 0)
-
--- The order of the type parameters is important, because it is more common to use type applications for providing the row type and row container type.
 fetch
-  :: forall a b c. (ToParamList c, FromRow a, RowContainer b)
+  :: forall a b. (ToParamList b, FromRow a)
   => Text
-  -> c
+  -> b
   -> Connection
-  -> IO (Either Error (b a))
+  -> IO (Either Error [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 nRows columnTable
+  nRows <- liftIO $ LibPQ.ntuples result
+  mapM (ExceptT . fromRow result columnTable) [0..nRows - 1]
 
-fetch_ :: forall a c. (FromRow a, RowContainer c) => Text -> Connection -> IO (Either Error (c a))
+fetch_ :: forall a. FromRow a => Text -> Connection -> IO (Either Error [a])
 fetch_ query = fetch query ()
 
 execute

lib/Database/PostgreSQL/Opium/Error.hs

@@ -17,8 +17,6 @@ data Error
   | ErrorInvalidOid Text Oid
   | ErrorUnexpectedNull ErrorPosition
   | ErrorInvalidField ErrorPosition Oid ByteString String
-  | ErrorNotExactlyOneRow
-  | ErrorMoreThanOneRow
   deriving (Eq, Show)
 
 instance Exception Error where

lib/Database/PostgreSQL/Opium/FromRow.hs

@@ -14,7 +14,6 @@ module Database.PostgreSQL.Opium.FromRow
   -- * FromRow
   ( FromRow (..)
   -- * Internal
-  , ColumnTable
   , toListColumnTable
   ) where
 

test/Database/PostgreSQL/OpiumSpec.hs

@@ -137,41 +137,17 @@ spec = do
       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 }
-
-    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