From db70c4f88293c8a7902e132826dba2a7ecfe0a74 Mon Sep 17 00:00:00 2001
From: "Yang, Bo" <pop.atry@gmail.com>
Date: Sat, 18 Jan 2020 01:48:03 -0800
Subject: [PATCH 1/2] Implement MonadError for `ContT e (Codensity m)`

---
 src/Control/Monad/Codensity.hs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/Control/Monad/Codensity.hs b/src/Control/Monad/Codensity.hs
index 3ccc3ea..364299f 100644
--- a/src/Control/Monad/Codensity.hs
+++ b/src/Control/Monad/Codensity.hs
@@ -42,10 +42,12 @@ import Control.Applicative
 import Control.Monad (MonadPlus(..))
 import qualified Control.Monad.Fail as Fail
 import Control.Monad.Free
+import Control.Monad.Error.Class
 import Control.Monad.IO.Class
 import Control.Monad.Reader.Class
 import Control.Monad.State.Class
 import Control.Monad.Trans.Class
+import Control.Monad.Trans.Cont
 import Data.Functor.Adjunction
 import Data.Functor.Apply
 import Data.Functor.Kan.Ran
@@ -248,6 +250,12 @@ instance (Functor f, MonadFree f m) => MonadFree f (Codensity m) where
   wrap t = Codensity (\h -> wrap (fmap (\p -> runCodensity p h) t))
   {-# INLINE wrap #-}
 
+instance MonadError e (ContT e (Codensity m)) where
+  throwError e = ContT (const (pure e))
+  catchError c recover = ContT $ \successHandler -> Codensity $ \errorHandler ->
+    runCodensity (runContT c successHandler)
+      $ \e -> runCodensity (runContT (recover e) successHandler) errorHandler
+
 instance MonadReader r m => MonadState r (Codensity m) where
   get = Codensity (ask >>=)
   {-# INLINE get #-}

From b576025bffa9ab408ecdbe2bbd69c372ae8b3c2e Mon Sep 17 00:00:00 2001
From: "Yang, Bo" <pop.atry@gmail.com>
Date: Sat, 18 Jan 2020 02:36:37 -0800
Subject: [PATCH 2/2] Add a forwarder instance of MonadError

---
 src/Control/Monad/Codensity.hs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/Control/Monad/Codensity.hs b/src/Control/Monad/Codensity.hs
index 364299f..de6c5fd 100644
--- a/src/Control/Monad/Codensity.hs
+++ b/src/Control/Monad/Codensity.hs
@@ -250,12 +250,17 @@ instance (Functor f, MonadFree f m) => MonadFree f (Codensity m) where
   wrap t = Codensity (\h -> wrap (fmap (\p -> runCodensity p h) t))
   {-# INLINE wrap #-}
 
-instance MonadError e (ContT e (Codensity m)) where
+instance {-# OVERLAPPING #-} MonadError e (ContT e (Codensity m)) where
   throwError e = ContT (const (pure e))
   catchError c recover = ContT $ \successHandler -> Codensity $ \errorHandler ->
     runCodensity (runContT c successHandler)
       $ \e -> runCodensity (runContT (recover e) successHandler) errorHandler
 
+instance MonadError e m => MonadError e (Codensity m)  where
+  throwError e = Codensity (const (throwError e))
+  catchError c recover =
+    Codensity $ \k -> catchError (runCodensity c k) $ \e -> runCodensity (recover e) k
+
 instance MonadReader r m => MonadState r (Codensity m) where
   get = Codensity (ask >>=)
   {-# INLINE get #-}