Explicit instance of MonadError for Tx

library/Hasql.hs

@@ -192,8 +192,32 @@ session pool m =
 -- an unpredictable amount of times as well, 
 -- which, chances are, is not what you want.
 newtype Tx c s r =
-  Tx (EitherT (TxError c) (Bknd.Tx c) r)
-  deriving (Functor, Applicative, Monad, MonadError (TxError c))
+  Tx { unwrapTx :: EitherT (TxError c) (Bknd.Tx c) r }
+  deriving (Functor, Applicative, Monad)
+
+instance MonadError (TxError c) (Tx c s) where
+  throwError =
+    \case
+      BackendTxError e ->
+        Tx $ lift $ throwError e
+      e ->
+        Tx $ throwError e
+  catchError m h =
+    Tx $ 
+    let 
+      m1 =
+        let 
+          m2 = 
+            runEitherT $ unwrapTx $ m
+          h2 = 
+            return . Left . BackendTxError
+          in 
+            EitherT $ catchError m2 h2
+      h1 =
+        unwrapTx . h
+      in
+        catchError m1 h1
+
 
 data TxError c =
   -- |