Make the container type for fetch customizable

lib/Database/PostgreSQL/Opium.hs

@@ -26,9 +26,10 @@ module Database.PostgreSQL.Opium
   )
   where
 
-import Control.Monad (void)
+import Control.Monad (unless, void)
 import Control.Monad.IO.Class (liftIO)
-import Control.Monad.Trans.Except (ExceptT (..), except, runExceptT)
+import Control.Monad.Trans.Except (ExceptT (..), except, runExceptT, throwE)
+import Data.Functor.Identity (Identity (..))
 import Data.Proxy (Proxy (..))
 import Data.Text (Text)
 import Database.PostgreSQL.LibPQ
@@ -41,24 +42,37 @@ 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 (..))
+import Database.PostgreSQL.Opium.FromRow (FromRow (..), ColumnTable)
 import Database.PostgreSQL.Opium.ToField (ToField (..))
 import Database.PostgreSQL.Opium.ToParamList (ToParamList (..))
 
--- The order of the type parameters is important, because it is more common to use type applications for providing the row type.
+class RowContainer c where
+  extract :: FromRow a => Result -> ColumnTable -> ExceptT Error IO (c a)
+
+instance RowContainer [] where
+  extract result columnTable = do
+    nRows <- liftIO $ LibPQ.ntuples result
+    mapM (ExceptT . fromRow result columnTable) [0..nRows - 1]
+
+instance RowContainer Identity where
+  extract result columnTable = do
+    nRows <- liftIO $ LibPQ.ntuples result
+    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. (ToParamList b, FromRow a)
+  :: forall a b c. (ToParamList c, FromRow a, RowContainer b)
   => Text
-  -> b
+  -> c
   -> Connection
-  -> IO (Either Error [a])
+  -> IO (Either Error (b a))
 fetch query params conn = runExceptT $ do
   result <- execParams conn query params
   columnTable <- ExceptT $ getColumnTable @a Proxy result
-  nRows <- liftIO $ LibPQ.ntuples result
-  mapM (ExceptT . fromRow result columnTable) [0..nRows - 1]
+  extract result columnTable
 
-fetch_ :: forall a. FromRow a => Text -> Connection -> IO (Either Error [a])
+fetch_ :: forall a c. (FromRow a, RowContainer c) => Text -> Connection -> IO (Either Error (c a))
 fetch_ query = fetch query ()
 
 execute

lib/Database/PostgreSQL/Opium/Error.hs

@@ -17,6 +17,7 @@ data Error
   | ErrorInvalidOid Text Oid
   | ErrorUnexpectedNull ErrorPosition
   | ErrorInvalidField ErrorPosition Oid ByteString String
+  | ErrorNotExactlyOneRow
   deriving (Eq, Show)
 
 instance Exception Error where

lib/Database/PostgreSQL/Opium/FromRow.hs

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