diff --git a/LICENSE b/LICENSE
--- a/LICENSE
+++ b/LICENSE
@@ -1,4 +1,4 @@
-Copyright (c) 2012 Ian-Woo Kim
+Copyright (c) 2012-2013 Ian-Woo Kim
 
 All rights reserved.
 
diff --git a/coroutine-object.cabal b/coroutine-object.cabal
--- a/coroutine-object.cabal
+++ b/coroutine-object.cabal
@@ -1,5 +1,5 @@
 Name:		coroutine-object
-Version:	0.1.0
+Version:	0.2
 Synopsis:       Object-oriented programming realization using coroutine 
 Description: 	Object-oriented programming realization using coroutine 
 License: 	BSD3
@@ -15,14 +15,16 @@
   ghc-prof-options: -caf-all -auto-all
   Build-Depends:   base >4 && <5, 
                    mtl>2, 
-                   lens >= 2.4,
+                   lens >= 2.5,
                    containers >= 0.4,
                    transformers == 0.3.*, 
                    transformers-free == 1.0.*, 
                    bytestring >= 0.9,
                    safecopy >= 0.6,
                    uuid == 1.2.*,
-                   cereal == 0.3.*
+                   cereal == 0.3.*, 
+                   either >= 3 
+                   -- errors >= 1.3
   Other-Modules:   
   exposed-modules:
                    Control.Monad.Trans.Crtn
diff --git a/src/Control/Monad/Trans/Crtn.hs b/src/Control/Monad/Trans/Crtn.hs
--- a/src/Control/Monad/Trans/Crtn.hs
+++ b/src/Control/Monad/Trans/Crtn.hs
@@ -17,7 +17,7 @@
 
 module Control.Monad.Trans.Crtn where 
 
-import Control.Monad.Error
+-- import Control.Monad.Error
 import Control.Monad.Reader 
 import Control.Monad.State 
 import Control.Monad.Trans.Free
@@ -98,17 +98,11 @@
 type CliT req ans = CrtnT req ans  
 
 -- | type for coroutine status after execution
-data CrtnErr r = NoError 
-               | ServerFinished r 
-               | Other String 
-
--- | 
-deriving instance (Show r) => Show (CrtnErr r) 
+data CrtnErr = ServerFinished  
+             | Other String 
 
 -- | 
-instance Error (CrtnErr r) where 
-  noMsg = NoError 
-  strMsg str = Other str 
+deriving instance Show CrtnErr 
 
 -----------------------------
 -- communication combinator 
@@ -118,24 +112,16 @@
 (<==|) :: Monad m => 
           SrvT req ans m r'    -- ^ server coroutine
        -> CliT req ans m r  -- ^ client coroutine
-       -> ErrorT (CrtnErr r') m (SrvT req ans m r', r)
+       -> m (Either CrtnErr (SrvT req ans m r', r)) 
 s <==| c = do 
-    y <- lift (runFreeT c)
+    y <- runFreeT c 
     case y of
-      Pure r -> return (s,r)
+      Pure r -> return (Right (s,r))
       Free (Rqst rq af) -> do 
-        x <- lift (runFreeT (runReaderT s rq))
+        x <- runFreeT (runReaderT s rq) 
         case x of 
-          Pure r' -> throwError (ServerFinished r')
+          Pure _r' -> return (Left ServerFinished) 
           Free (Rqst ans rf) -> (ReaderT rf) <==| (af ans)
-
-{-
--- | synonym of connectE
-(<==>) :: Monad m => SrvT req ans m r' -> CliT req ans m r
-          -> ErrorT (CrtnErr r') m (SrvT req ans m r', r)
-(<==>) = connectE
- -}         
-
 
 ----------------------
 -- some utility 
diff --git a/src/Control/Monad/Trans/Crtn/Driver.hs b/src/Control/Monad/Trans/Crtn/Driver.hs
--- a/src/Control/Monad/Trans/Crtn/Driver.hs
+++ b/src/Control/Monad/Trans/Crtn/Driver.hs
@@ -7,8 +7,9 @@
 
 module Control.Monad.Trans.Crtn.Driver where 
 
-import Control.Monad.Error
+-- import Control.Monad.Error
 import Control.Monad.Reader
+import Control.Monad.Trans.Either 
 import Data.Foldable
 -- 
 import Control.Monad.Trans.Crtn 
@@ -63,12 +64,16 @@
                     , World e (SObjBT (DrvOp e) m) ()
                     , [EvOrAct e])
 singleDispatch (Right ev) (logobj,worldobj,evacc) = do
-    Right (logobj',worldobj',events) <- 
-      runErrorT $ do (worldobj1,_)  <- worldobj  <==| giveEvent ev
-                     (worldobj2,logobj1) <- worldobj1 <==| flushLog logobj
-                     (worldobj3,events) <- worldobj2 <==| flushQueue 
-                     return (logobj1,worldobj3,events)
-    return (logobj',worldobj',evacc++events) 
+    -- Right (logobj',worldobj',events) <- 
+    r <- 
+      runEitherT $ do (worldobj1,_)  <- EitherT (worldobj  <==| giveEvent ev)
+                      (worldobj2,logobj1) <- EitherT (worldobj1 <==| flushLog logobj)
+                      (worldobj3,events) <- EitherT (worldobj2 <==| flushQueue)
+                      return (logobj1,worldobj3,events)
+    case r of 
+      Left _ -> -- resuming original (this must be refined. resume point must be defined ) 
+                return (logobj,worldobj,evacc)
+      Right (logobj',worldobj',events) -> return (logobj',worldobj',evacc++events) 
 singleDispatch (Left act) (logobj,worldobj,evacc) = do 
     Arg Dispatch ev <- request (Res Dispatch (Just act))
     return (logobj,worldobj,evacc++[Right ev]) 
diff --git a/src/Control/Monad/Trans/Crtn/EventHandler.hs b/src/Control/Monad/Trans/Crtn/EventHandler.hs
--- a/src/Control/Monad/Trans/Crtn/EventHandler.hs
+++ b/src/Control/Monad/Trans/Crtn/EventHandler.hs
@@ -16,7 +16,8 @@
   
 import Control.Concurrent.MVar 
 import Control.Monad.State 
-import Control.Monad.Error
+-- import Control.Monad.Error
+import Control.Monad.Trans.Either
 -- from this package 
 -- import Control.Monad.Trans.Crtn
 import Control.Monad.Trans.Crtn.Event 
@@ -37,5 +38,5 @@
           Right Nothing -> return () 
           Right (Just (ActionOrder act)) -> 
             act (eventHandler evar) >>= eventHandler evar 
-  where eaction = runStateT (runErrorT $ fire ev) 
+  where eaction = runStateT (runEitherT $ fire ev) 
                   
diff --git a/src/Control/Monad/Trans/Crtn/Object.hs b/src/Control/Monad/Trans/Crtn/Object.hs
--- a/src/Control/Monad/Trans/Crtn/Object.hs
+++ b/src/Control/Monad/Trans/Crtn/Object.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE GADTs, RankNTypes #-}
+{-# LANGUAGE GADTs, RankNTypes, ScopedTypeVariables #-}
 
 -----------------------------------------------------------------------------
 -- |
@@ -18,7 +18,8 @@
 
 -- from other packages
 import Control.Monad.State 
-import Control.Monad.Error 
+-- import Control.Monad.Error 
+import Control.Monad.Trans.Either 
 -- from this package
 import Control.Monad.Trans.Crtn
 
@@ -50,13 +51,17 @@
 
 
 -- | convenient error state monad for object  
-type EStT s m = ErrorT (CrtnErr ()) (StateT s m)  
+type EStT s m = EitherT CrtnErr (StateT s m)  
 
 -- |  
-query :: (Monad m) => CObjT s m r -> EStT (SObjT s m ()) m r  
+query :: forall m s r. (Monad m) => CObjT s m r -> EStT (SObjT s m ()) m r  
 query cli = do 
   qserv <- lift get 
-  (qserv',r) <- mapErrorT lift (qserv <==| cli )
+  let result :: m (Either CrtnErr (SrvT (Arg s) (Res s) m (), r))
+      result = qserv <==| cli
+      r2 :: StateT (SObjT s m ()) m (Either CrtnErr (SrvT (Arg s) (Res s) m (), r))
+      r2 = lift result
+  (qserv',r) <- EitherT r2 
   lift (put qserv')
   return r
   
diff --git a/src/Control/Monad/Trans/Crtn/World.hs b/src/Control/Monad/Trans/Crtn/World.hs
--- a/src/Control/Monad/Trans/Crtn/World.hs
+++ b/src/Control/Monad/Trans/Crtn/World.hs
@@ -16,7 +16,8 @@
 
 module Control.Monad.Trans.Crtn.World where 
 
-import Control.Monad.Error 
+-- import Control.Monad.Error 
+-- import Control.Monad.Trans.Either 
 -- 
 import Control.Monad.Trans.Crtn
 import Control.Monad.Trans.Crtn.Event