diff --git a/cabal.project b/cabal.project
index 9f96719..bbd0a58 100644
--- a/cabal.project
+++ b/cabal.project
@@ -13,7 +13,7 @@ allow-newer: eff:primitive
 source-repository-package
     type: git
     location: https://github.com/sayo-hs/data-effects
-    tag: 9fd125aecb96c7d56db7c464abd31056bc17c921
+    tag: 1fbb3f73e7ba4e9e7e69cd09f563416cab032fb5
     subdir: data-effects-core
     subdir: data-effects-th
     subdir: data-effects
diff --git a/heftia-effects/Example/Stream/Main.hs b/heftia-effects/Example/Stream/Main.hs
index a6d5fac..b0282e0 100644
--- a/heftia-effects/Example/Stream/Main.hs
+++ b/heftia-effects/Example/Stream/Main.hs
@@ -10,14 +10,16 @@ import Control.Monad.Hefty (
     Eff,
     interpret,
     liftIO,
+    raise,
     (&),
     type (<:),
     type (<<|),
     type (<|),
     type (~>),
  )
-import Control.Monad.Hefty.Concurrent.Parallel (runParallelIO)
+import Control.Monad.Hefty.Concurrent.Async (runAsyncIO, runAsyncSeq)
 import Control.Monad.Hefty.Concurrent.Stream (connect)
+import Control.Monad.Hefty.Concurrent.Timer (Timer, runTimerIO, sleep)
 import Control.Monad.Hefty.Except (runThrow, throw)
 import Control.Monad.Hefty.Input (Input, input)
 import Control.Monad.Hefty.Output (Output, output)
@@ -49,24 +51,37 @@ This function is equivalent to the following (as a result of reducing 'runThrow'
 
 @
 produce = void do
-    for_ [1 .. 2] \(i :: Int) -> do
+    for_ [1 .. 4] \(i :: Int) -> do
         output i
+        sleep 0.5
 @
 -}
-produce :: (Output Int <| ef) => Eff '[] ef ()
-produce = void . runThrow @() $ do
-    for_ [1 .. 3] \(i :: Int) -> do
-        when (i >= 3) $ throw ()
-        output i
+produce :: (Output Int <| ef, Timer <| ef) => Int -> Eff '[] ef ()
+produce n = void . runThrow @() $ do
+    for_ [1 ..] \(i :: Int) -> do
+        when (i == 5) $ throw ()
+        output $ n + i
+
+-- sleep 0.5
 
 consume :: (Input Int <: m, MonadIO m) => m ()
 consume = forever do
     liftIO . print =<< input @Int
 
+plus100 :: (Input Int <: m, Output Int <: m, MonadIO m) => m ()
+plus100 = forever do
+    i <- input @Int
+    liftIO $ print i
+    output $ i + 100
+
 main :: IO ()
-main = runUnliftIO . runResourceIO . runParallelIO $ do
-    _ <- runSomeResource $ connect @Int produce consume
-    _ <- runSomeResource $ connect @Int produce consume
+main = runUnliftIO . runTimerIO . runResourceIO $ do
+    -- _ <- runSomeResource $ runAsyncIO $ connect @Int (produce 0) consume
+    -- _ <- runSomeResource $ runAsyncIO $ connect @Int (connect @Int (produce 0) plus100) consume
+    _ <-
+        runAsyncIO do
+            let m = connect @Int (raise (produce 1000)) consume
+            connect @Int m consume
     pure ()
 
 {-
diff --git a/heftia-effects/bench/BenchParallel.hs b/heftia-effects/bench/BenchParallel.hs
deleted file mode 100644
index c65111e..0000000
--- a/heftia-effects/bench/BenchParallel.hs
+++ /dev/null
@@ -1,33 +0,0 @@
-{-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-}
-
--- SPDX-License-Identifier: MPL-2.0
-
-module BenchParallel where
-
-import Control.Monad (liftM2)
-import Control.Monad.Hefty (Eff, type (<:), type (<|))
-import Control.Monad.Hefty.Concurrent.Parallel (runParallelIO)
-import Control.Monad.Hefty.Concurrent.Stream (closing, connect)
-import Control.Monad.Hefty.Input (Input, input)
-import Control.Monad.Hefty.Output (Output, output)
-import Control.Monad.Hefty.Unlift (runUnliftIO)
-import Control.Monad.IO.Class (MonadIO)
-import Data.Foldable (for_)
-import Data.Maybe (fromJust)
-import Data.These.Combinators (justThese)
-import System.IO.Unsafe (unsafePerformIO)
-
-produce :: (Output Int <| ef) => Int -> Eff '[] ef ()
-produce n =
-    for_ [1 .. n] \(i :: Int) -> do
-        output i
-
-consume :: (Input Int <: m, MonadIO m) => Int -> m [Int]
-consume 0 = pure []
-consume n = liftM2 (:) (input @Int) (consume (n - 1))
-
-parallel :: Int -> [Int]
-parallel n = unsafePerformIO . runUnliftIO . runParallelIO $ do
-    stat <- connect @Int (produce n) (consume n)
-    pure $ snd . fromJust $ justThese $ closing stat
-{-# NOINLINE parallel #-}
diff --git a/heftia-effects/bench/Main.hs b/heftia-effects/bench/Main.hs
index 9a6bb3c..cb73a46 100644
--- a/heftia-effects/bench/Main.hs
+++ b/heftia-effects/bench/Main.hs
@@ -7,7 +7,6 @@ import BenchCatch
 import BenchCoroutine
 import BenchCountdown
 import BenchLocal
-import BenchParallel
 import BenchPyth
 import Data.Functor ((<&>))
 import Test.Tasty.Bench
@@ -141,9 +140,4 @@ main =
                     , bench "eff.5+5" $ nf coroutineEffDeep x
                     , bench "mp.5+5" $ nf coroutineMpDeep x
                     ]
-        , bgroup "parallel" $
-            [10000] <&> \x ->
-                bgroup
-                    (show x)
-                    [bench "parallel" $ nf parallel x]
         ]
diff --git a/heftia-effects/heftia-effects.cabal b/heftia-effects/heftia-effects.cabal
index 0bca852..5fd031a 100644
--- a/heftia-effects/heftia-effects.cabal
+++ b/heftia-effects/heftia-effects.cabal
@@ -64,6 +64,7 @@ common common-base
         containers > 0.6.5 && < 0.8,
         these,
         co-log-core,
+        random,
 
     ghc-options: -Wall
 
@@ -87,6 +88,7 @@ library
         Control.Monad.Hefty.Fresh
         Control.Monad.Hefty.Fail
         Control.Monad.Hefty.Concurrent.Parallel
+        Control.Monad.Hefty.Concurrent.Async
         Control.Monad.Hefty.Concurrent.Stream
         Control.Monad.Hefty.Concurrent.Timer
         Control.Monad.Hefty.Log
@@ -121,6 +123,7 @@ library
         Data.Effect.Fresh,
         Data.Effect.Fail,
         Data.Effect.Concurrent.Parallel,
+        Data.Effect.Concurrent.Async,
         Data.Effect.Concurrent.Timer,
         Data.Effect.Log,
 
@@ -142,6 +145,7 @@ test-suite test
         Test.Writer
         Test.Pyth
         Test.Coroutine
+        Test.Async
 
     hs-source-dirs: test
 
@@ -272,4 +276,3 @@ benchmark heftia-bench
         BenchLocal
         BenchCoroutine
         BenchPyth
-        BenchParallel
diff --git a/heftia-effects/src/Control/Monad/Hefty/Concurrent/Async.hs b/heftia-effects/src/Control/Monad/Hefty/Concurrent/Async.hs
new file mode 100644
index 0000000..be8e627
--- /dev/null
+++ b/heftia-effects/src/Control/Monad/Hefty/Concurrent/Async.hs
@@ -0,0 +1,164 @@
+{-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-}
+
+-- SPDX-License-Identifier: MPL-2.0
+
+module Control.Monad.Hefty.Concurrent.Async (
+    module Control.Monad.Hefty.Concurrent.Async,
+    module Data.Effect.Concurrent.Async,
+    module Data.Effect.NonDet,
+)
+where
+
+import Control.Arrow ((>>>))
+import Control.Exception (AsyncException (ThreadKilled), BlockedIndefinitelyOnSTM (BlockedIndefinitelyOnSTM), SomeException, fromException)
+import Control.Monad (void)
+import Control.Monad.Hefty (
+    Eff,
+    MemberBy,
+    bundleN,
+    interpretH,
+    nil,
+    unkey,
+    (!+),
+    (&),
+    type (+),
+    type (<<|),
+    type (<|),
+    type (~>),
+ )
+import Control.Monad.Hefty.Interpret (reinterpretWith)
+import Control.Monad.Hefty.Unlift (UnliftIO)
+import Data.Effect.Concurrent.Async
+import Data.Effect.Concurrent.Parallel (Parallel)
+import Data.Effect.NonDet
+import Data.Effect.Unlift (withRunInIO)
+import Data.Functor.Const (Const (Const))
+import Data.Functor.Identity (Identity (Identity))
+import Data.Set qualified as Set
+import Data.Void (Void, absurd)
+import Data.Word (Word8)
+import Debug.Trace (trace)
+import GHC.Conc (retry)
+import GHC.Generics (type (:+:) (L1, R1))
+import System.Random (randomIO)
+import UnliftIO (
+    atomically,
+    catch,
+    catchJust,
+    finally,
+    liftIO,
+    mask,
+    modifyTVar,
+    newEmptyTMVarIO,
+    newTVarIO,
+    putTMVar,
+    readTMVar,
+    readTVarIO,
+    throwIO,
+    tryReadTMVar,
+    uninterruptibleMask_,
+ )
+import UnliftIO.Concurrent (ThreadId, forkIO, killThread)
+
+type HasAsync ef f = MemberBy AsyncKey (Async' f) ef
+
+runAsyncSeq :: Eff '[] (Async (Const (Eff eh ef ans) + Identity) ': ef) ans -> Eff eh ef ans
+runAsyncSeq =
+    unkey >>> reinterpretWith \case
+        Fork -> \k -> k $ Left \x ->
+            L1 . Const . k . Right $
+                Future
+                    (R1 $ Identity x)
+                    (R1 $ Identity $ Just x)
+                    (R1 $ Identity ())
+        Perform (L1 (Const k)) -> const k
+        Perform (R1 (Identity x)) -> ($ x)
+
+runAsyncIO :: (IO <| ef, UnliftIO <<| eh) => Eff '[] (Async (Const (IO Void) + IO) ': ef) ~> Eff eh ef
+runAsyncIO m = do
+    zombieThreads <- newTVarIO Set.empty
+
+    r <-
+        m
+            & unkey
+            & reinterpretWith \case
+                Fork -> \k ->
+                    withRunInIO \run -> do
+                        chan <- newEmptyTMVarIO
+
+                        mask \restore -> do
+                            t <-
+                                forkIO do
+                                    uid <- randomIO @Word8
+                                    liftIO $ putStrLn $ "Start thread " <> show uid
+                                    catchJust
+                                        (\e -> if fromException e == Just ThreadKilled then Nothing else Just e)
+                                        ( void . restore . run $ k $ Left \s ->
+                                            L1 $ Const $ do
+                                                liftIO $ putStrLn $ "Performed write on " <> show uid
+                                                atomically $ putTMVar chan . Right $ s
+                                                atomically retry
+                                        )
+                                        (atomically . putTMVar chan . Left)
+
+                            liftIO $ putStrLn $ "START " <> show t
+
+                            atomically $ modifyTVar zombieThreads $ Set.insert t
+
+                            let kill = do
+                                    liftIO $ putStrLn $ "KILL " <> show t
+                                    killThread t
+                                    atomically (modifyTVar zombieThreads $ Set.delete t)
+
+                            restore . run . k . Right $
+                                Future
+                                    ( R1 $
+                                        traceSTMBlock t (atomically (readTMVar chan)) `finally` kill >>= \case
+                                            Right r -> pure r
+                                            Left (e :: SomeException) -> do
+                                                throwIO e
+                                    )
+                                    (R1 $ atomically $ (either (const Nothing) Just =<<) <$> tryReadTMVar chan)
+                                    (R1 kill)
+                Perform (L1 (Const send)) -> const $ liftIO $ absurd <$> send
+                Perform (R1 a) -> (liftIO a >>=)
+
+    mapM_
+        ( \t -> do
+            liftIO $ putStrLn $ "KILL ZOMBIE " <> show t
+            killThread t
+        )
+        =<< readTVarIO zombieThreads
+
+    pure r
+
+traceSTMBlock :: ThreadId -> IO a -> IO a
+traceSTMBlock t m = do
+    putStrLn $ "read " <> show t
+    m `catch` \BlockedIndefinitelyOnSTM -> throwIO $ userError $ "STM blocked for read async thread " <> show t
+
+runParallelAsync :: (HasAsync ef f) => Eff (Parallel ': eh) ef ~> Eff eh ef
+runParallelAsync = interpretH parallelToAsync
+
+runNonDetRaceIO
+    :: (IO <| ef, UnliftIO <<| eh)
+    => Eff '[] (Choose ': Empty ': ef) ~> Eff eh ef
+runNonDetRaceIO =
+    bundleN @2
+        >>> reinterpretWith
+            ( ( \Choose k -> withRunInIO \run -> do
+                    var <- newEmptyTMVarIO
+                    mask \restore -> do
+                        let runThread b = forkIO $ do
+                                x <- restore $ run $ k b
+                                atomically $ putTMVar var x
+
+                        t1 <- runThread False
+                        t2 <- runThread True
+
+                        atomically (readTMVar var)
+                            <* uninterruptibleMask_ (killThread t1 *> killThread t2)
+              )
+                !+ (\Empty _ -> liftIO $ atomically retry)
+                !+ nil
+            )
diff --git a/heftia-effects/src/Control/Monad/Hefty/Concurrent/Stream.hs b/heftia-effects/src/Control/Monad/Hefty/Concurrent/Stream.hs
index 07a4e4c..22940be 100644
--- a/heftia-effects/src/Control/Monad/Hefty/Concurrent/Stream.hs
+++ b/heftia-effects/src/Control/Monad/Hefty/Concurrent/Stream.hs
@@ -2,8 +2,17 @@
 
 module Control.Monad.Hefty.Concurrent.Stream where
 
-import Control.Monad.Hefty (Eff, interpretBy, raiseAllH, (&), type (<<|))
-import Control.Monad.Hefty.Concurrent.Parallel (Parallel, liftP2)
+import Control.Monad.Hefty (
+    Eff,
+    interpretBy,
+    raiseAllH,
+    unkey,
+    untag,
+    (&),
+    type (#),
+    type (#>),
+ )
+import Control.Monad.Hefty.Concurrent.Async (HasAsync, liftAsync2)
 import Control.Monad.Hefty.Coroutine (Status (Continue, Done))
 import Control.Monad.Hefty.Input (Input (Input))
 import Control.Monad.Hefty.Output (Output (Output))
@@ -11,8 +20,8 @@ import Data.Bifunctor (Bifunctor, bimap)
 import Data.These (These (That, These, This))
 
 connect
-    :: forall v a b eh ef
-     . (Parallel <<| eh)
+    :: forall v a b eh ef f
+     . (HasAsync ef f)
     => Eff '[] (Output v ': ef) a
     -> Eff '[] (Input v ': ef) b
     -> Eff eh ef (StreamStatus (Eff '[] ef) v a b)
@@ -21,14 +30,32 @@ connect a b =
         (a & interpretBy (pure . Done) \(Output v) k -> pure $ Continue v k)
         (b & interpretBy (pure . Done) \Input k -> pure $ Continue () k)
 
+connect'
+    :: forall o i v a b eh ef f
+     . (HasAsync ef f)
+    => Eff '[] (Output v # o ': ef) a
+    -> Eff '[] (Input v # i ': ef) b
+    -> Eff eh ef (StreamStatus (Eff '[] ef) v a b)
+connect' a b = connect (untag a) (untag b)
+{-# INLINE connect' #-}
+
+connect''
+    :: forall o i v a b eh ef f
+     . (HasAsync ef f)
+    => Eff '[] (o #> Output v ': ef) a
+    -> Eff '[] (i #> Input v ': ef) b
+    -> Eff eh ef (StreamStatus (Eff '[] ef) v a b)
+connect'' a b = connect (unkey a) (unkey b)
+{-# INLINE connect'' #-}
+
 runStream
-    :: forall v a b eh ef
-     . (Parallel <<| eh)
+    :: forall v a b eh ef f
+     . (HasAsync ef f)
     => Eff '[] ef (Status (Eff '[] ef) v () a)
     -> Eff '[] ef (Status (Eff '[] ef) () v b)
     -> Eff eh ef (StreamStatus (Eff '[] ef) v a b)
 runStream a b = do
-    (a', b') <- liftP2 (,) (raiseAllH a) (raiseAllH b)
+    (a', b') <- liftAsync2 (,) (raiseAllH a) (raiseAllH b)
 
     case (a', b') of
         (Done x, Done y) -> pure $ Equilibrium x y
diff --git a/heftia-effects/src/Control/Monad/Hefty/Concurrent/Timer.hs b/heftia-effects/src/Control/Monad/Hefty/Concurrent/Timer.hs
index 616c3d5..ecc61b3 100644
--- a/heftia-effects/src/Control/Monad/Hefty/Concurrent/Timer.hs
+++ b/heftia-effects/src/Control/Monad/Hefty/Concurrent/Timer.hs
@@ -23,7 +23,7 @@ import Control.Monad.Hefty (
  )
 import Control.Monad.Hefty.Coroutine (runCoroutine)
 import Control.Monad.Hefty.State (evalState)
-import Data.Effect.Concurrent.Timer (CyclicTimer (Wait), Timer (Clock, Sleep), clock, cyclicTimer)
+import Data.Effect.Concurrent.Timer
 import Data.Effect.Coroutine (Status (Continue, Done))
 import Data.Effect.State (get, put)
 import Data.Time (DiffTime)
diff --git a/heftia-effects/src/Control/Monad/Hefty/NonDet.hs b/heftia-effects/src/Control/Monad/Hefty/NonDet.hs
index a1cb01a..9a5b4a0 100644
--- a/heftia-effects/src/Control/Monad/Hefty/NonDet.hs
+++ b/heftia-effects/src/Control/Monad/Hefty/NonDet.hs
@@ -115,3 +115,5 @@ branch a b = do
     world <- choose
     bool a b world
 {-# INLINE branch #-}
+
+infixl 3 `branch`
diff --git a/heftia-effects/src/Control/Monad/Hefty/ShiftReset.hs b/heftia-effects/src/Control/Monad/Hefty/ShiftReset.hs
index 8e3f330..4c2811a 100644
--- a/heftia-effects/src/Control/Monad/Hefty/ShiftReset.hs
+++ b/heftia-effects/src/Control/Monad/Hefty/ShiftReset.hs
@@ -10,11 +10,16 @@ where
 
 import Control.Monad.Hefty (
     Eff,
+    MemberHBy,
+    interpret,
+    interpretBy,
     interpretH,
     interpretHBy,
     interpretRecHWith,
     raiseH,
     runEff,
+    send0,
+    sendH,
     type (~>),
  )
 import Data.Effect.Key (KeyH (KeyH))
@@ -33,13 +38,27 @@ runShift :: (a -> Eff '[] ef ans) -> Eff '[ShiftFix ans '[] ef] ef a -> Eff '[]
 runShift f =
     interpretHBy f \e k ->
         evalShift $ case e of
-            KeyH (Shift g) -> unShiftBase $ g (ShiftBase . raiseH . k) ShiftBase
+            KeyH (Shift initiate) -> unShiftBase $ initiate (ShiftBase . raiseH . k) ShiftBase
 
 withShift :: Eff '[ShiftFix ans '[] '[Eff eh ef]] '[Eff eh ef] ans -> Eff eh ef ans
 withShift = runEff . evalShift
 
-runShift_ :: forall r ef. Eff (Shift_ (Eff r ef) ': r) ef ~> Eff r ef
-runShift_ = interpretRecHWith \(KeyH (Shift_' f)) k -> f k id
+runShift_ :: forall eh ef. Eff (Shift_ (Eff eh ef) ': eh) ef ~> Eff eh ef
+runShift_ = interpretRecHWith \(KeyH (Shift_' initiate)) k -> initiate k id
 
-runReset :: forall r ef. Eff (Reset ': r) ef ~> Eff r ef
+runReset :: forall eh ef. Eff (Reset ': eh) ef ~> Eff eh ef
 runReset = interpretH \(Reset a) -> a
+
+runShiftF :: Eff '[] (ShiftF (Eff '[] ef ans) ': ef) ans -> Eff '[] ef ans
+runShiftF = interpretBy pure \(ShiftF initiate) resume -> initiate resume
+
+runShiftEff :: (Monad n) => (a -> n ans) -> Eff '[] '[ShiftF (n ans), n] a -> n ans
+runShiftEff f =
+    runEff
+        . interpretBy (send0 . f) \(ShiftF initiate) resume ->
+            send0 $ initiate $ runEff . resume
+
+runShiftAsF
+    :: (MemberHBy ShiftKey (Shift' ans n) eh)
+    => Eff eh (ShiftF (n ans) ': ef) ~> Eff eh ef
+runShiftAsF = interpret $ sendH . fromShiftF
diff --git a/heftia-effects/test/Test/Async.hs b/heftia-effects/test/Test/Async.hs
new file mode 100644
index 0000000..09b1851
--- /dev/null
+++ b/heftia-effects/test/Test/Async.hs
@@ -0,0 +1,95 @@
+{-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-}
+
+-- SPDX-License-Identifier: MPL-2.0
+
+module Test.Async where
+
+import Control.Monad.Hefty (Eff, liftIO, type (<|))
+import Control.Monad.Hefty.Concurrent.Async (
+    Future (await, cancel, poll),
+    HasAsync,
+    async,
+    perform,
+    runAsyncIO,
+    runAsyncSeq,
+ )
+import Control.Monad.Hefty.Concurrent.Parallel (halt, runHaltIO)
+import Control.Monad.Hefty.Concurrent.Timer (runTimerIO)
+import Control.Monad.Hefty.State (get, modify, runStateIORef)
+import Control.Monad.Hefty.Unlift (runUnliftIO)
+import Data.Effect.Concurrent.Timer (sleep)
+import Data.Functor ((<&>))
+import Test.Hspec (Spec, it, shouldBe, shouldReturn, shouldThrow)
+import UnliftIO (throwIO)
+
+spec_Async :: Spec
+spec_Async = do
+    let
+        prog :: (HasAsync ef f, IO <| ef) => Eff eh ef (String, String)
+        prog = runTimerIO . runStateIORef "" $ do
+            fu <- async do
+                sleep 0.001
+                modify (<> "B")
+                get @String <&> (<> "C")
+            modify (<> "A")
+            perform $ await fu
+
+    it "sequential" do
+        (s, a) <- runUnliftIO . runAsyncSeq $ prog
+        s `shouldBe` "BA"
+        a `shouldBe` "BC"
+
+    it "await" do
+        (s, a) <- runUnliftIO . runAsyncIO $ prog
+        s `shouldBe` "AB"
+        a `shouldBe` "ABC"
+
+    it "cancel" do
+        (s, ()) <- runUnliftIO . runTimerIO . runStateIORef "" . runAsyncIO $ do
+            fu <- async do
+                sleep 0.001
+                modify (<> "B")
+            modify (<> "A")
+            perform $ cancel fu
+            sleep 0.002
+        s `shouldBe` "A"
+
+    it "poll" do
+        (s, ()) <- runUnliftIO . runTimerIO . runStateIORef "" . runAsyncIO $ do
+            fu <- async do
+                sleep 0.001
+                modify (<> "B")
+                get @String <&> (<> "C")
+            modify (<> "A")
+
+            x <- perform $ poll fu
+            liftIO $ x `shouldBe` Nothing
+
+            sleep 0.002
+
+            x' <- perform $ poll fu
+            liftIO $ x' `shouldBe` Just "ABC"
+
+        s `shouldBe` "AB"
+
+    it "catch IO exception" $
+        do
+            runUnliftIO . runAsyncIO $ do
+                fu <- async $ throwIO $ userError "Test IO Exception"
+                perform $ await fu
+            `shouldThrow` (== userError "Test IO Exception")
+
+    it "suppress IO exception" $
+        do
+            runUnliftIO . runTimerIO . runAsyncIO $ do
+                _ <- async $ throwIO $ userError "Test IO Exception"
+                sleep 0.001
+                pure ()
+            `shouldReturn` ()
+
+    it "early exit" $
+        do
+            runUnliftIO . runAsyncIO . runHaltIO $ do
+                _ <- async halt
+                pure ()
+            `shouldReturn` ()
diff --git a/heftia/src/Control/Monad/Hefty.hs b/heftia/src/Control/Monad/Hefty.hs
index bebee37..a99c95f 100644
--- a/heftia/src/Control/Monad/Hefty.hs
+++ b/heftia/src/Control/Monad/Hefty.hs
@@ -506,9 +506,10 @@ module Control.Monad.Hefty (
     -- *** For first-order effects
     reinterpret,
     reinterpretN,
-    reinterpretNWith,
     reinterpretBy,
     reinterpretNBy,
+    reinterpretWith,
+    reinterpretNWith,
     reinterpretRecWith,
     reinterpretRecNWith,
 
@@ -647,6 +648,8 @@ module Control.Monad.Hefty (
     liftIO,
     module Data.Effect.OpenUnion,
     module Data.Effect,
+    module Data.Effect.Tag,
+    module Data.Effect.Key,
     module Data.Effect.TH,
     module Data.Effect.HFunctor.TH,
     module Data.Effect.Key.TH,
@@ -690,6 +693,7 @@ import Control.Monad.Hefty.Interpret (
     reinterpretRecNHWith,
     reinterpretRecNWith,
     reinterpretRecWith,
+    reinterpretWith,
     runEff,
     runPure,
     stateless,
@@ -794,8 +798,10 @@ import Control.Monad.IO.Class (liftIO)
 import Data.Effect
 import Data.Effect.HFunctor (HFunctor)
 import Data.Effect.HFunctor.TH
+import Data.Effect.Key
 import Data.Effect.Key.TH
 import Data.Effect.OpenUnion
 import Data.Effect.OpenUnion.Sum (type (:+:))
 import Data.Effect.TH
+import Data.Effect.Tag
 import Data.Kind (Type)
diff --git a/heftia/src/Control/Monad/Hefty/Interpret.hs b/heftia/src/Control/Monad/Hefty/Interpret.hs
index 97df2eb..666ff43 100644
--- a/heftia/src/Control/Monad/Hefty/Interpret.hs
+++ b/heftia/src/Control/Monad/Hefty/Interpret.hs
@@ -188,40 +188,40 @@ reinterpretN f = reinterpretRecNWith @n (stateless f)
 {-# INLINE reinterpretN #-}
 
 reinterpretWith
-    :: forall e ef' ef a
+    :: forall e ef' ef eh a
      . (ef `IsSuffixOf` ef')
-    => Interpreter e (Eff '[] ef') a
+    => Interpreter e (Eff eh ef') a
     -> Eff '[] (e ': ef) a
-    -> Eff '[] ef' a
+    -> Eff eh ef' a
 reinterpretWith = reinterpretBy pure
 {-# INLINE reinterpretWith #-}
 
 reinterpretNWith
-    :: forall n e ef' ef a
+    :: forall n e ef' ef eh a
      . (WeakenN n ef ef')
-    => Interpreter e (Eff '[] ef') a
+    => Interpreter e (Eff eh ef') a
     -> Eff '[] (e ': ef) a
-    -> Eff '[] ef' a
+    -> Eff eh ef' a
 reinterpretNWith = reinterpretNBy @n pure
 {-# INLINE reinterpretNWith #-}
 
 reinterpretBy
-    :: forall e ef' ef ans a
+    :: forall e ef' ef eh ans a
      . (ef `IsSuffixOf` ef')
-    => (a -> Eff '[] ef' ans)
-    -> Interpreter e (Eff '[] ef') ans
+    => (a -> Eff eh ef' ans)
+    -> Interpreter e (Eff eh ef') ans
     -> Eff '[] (e ': ef) a
-    -> Eff '[] ef' ans
+    -> Eff eh ef' ans
 reinterpretBy ret hdl = iterAllEffHFBy ret nilH (hdl !+ flip sendUnionBy . weakens)
 {-# INLINE reinterpretBy #-}
 
 reinterpretNBy
-    :: forall n e ef' ef ans a
+    :: forall n e ef' ef eh ans a
      . (WeakenN n ef ef')
-    => (a -> Eff '[] ef' ans)
-    -> Interpreter e (Eff '[] ef') ans
+    => (a -> Eff eh ef' ans)
+    -> Interpreter e (Eff eh ef') ans
     -> Eff '[] (e ': ef) a
-    -> Eff '[] ef' ans
+    -> Eff eh ef' ans
 reinterpretNBy ret hdl = iterAllEffHFBy ret nilH (hdl !+ flip sendUnionBy . weakenN @n)
 {-# INLINE reinterpretNBy #-}
 
@@ -373,14 +373,14 @@ interposeWith = interposeBy pure
 If multiple instances of @e@ exist in the list, the one closest to the head (with the smallest index) will be targeted.
 -}
 interposeBy
-    :: forall e ef ans a
+    :: forall e ef eh ans a
      . (e <| ef)
-    => (a -> Eff '[] ef ans)
+    => (a -> Eff eh ef ans)
     -- ^ Value handler
-    -> Interpreter e (Eff '[] ef) ans
+    -> Interpreter e (Eff eh ef) ans
     -- ^ Effect handler
     -> Eff '[] ef a
-    -> Eff '[] ef ans
+    -> Eff eh ef ans
 interposeBy ret f = iterAllEffHFBy ret nilH \u -> maybe (`sendUnionBy` u) f (prj @e u)
 {-# INLINE interposeBy #-}
 
diff --git a/heftia/src/Data/Effect/OpenUnion/Internal.hs b/heftia/src/Data/Effect/OpenUnion/Internal.hs
index 22dfb83..eb332b2 100644
--- a/heftia/src/Data/Effect/OpenUnion/Internal.hs
+++ b/heftia/src/Data/Effect/OpenUnion/Internal.hs
@@ -15,6 +15,7 @@ Description :  Open unions (type-indexed co-products) for extensible effects.
 -}
 module Data.Effect.OpenUnion.Internal where
 
+import Data.Effect.Key (type (##>), type (#>))
 import Data.Proxy (Proxy (Proxy))
 import Data.Type.Equality (type (==))
 import GHC.TypeError (ErrorMessage (ShowType, Text, (:$$:), (:<>:)), TypeError)
@@ -68,13 +69,23 @@ lexi-lambda/freer-simple#3, which describes the motivation in more detail.
 -}
 instance {-# INCOHERENT #-} IfNotFound e r w
 
-type LookupError (key :: kk) (w :: [ke]) =
-    TypeError
+class IfKeyNotFound (key :: k) (r :: [e]) (w :: [e])
+
+instance
+    ( TypeError
         ( 'Text "The key ‘"
             ':<>: 'ShowType key
             ':<>: 'Text "’ does not exist in the type-level list"
             ':$$: 'Text "  ‘" ':<>: 'ShowType w ':<>: 'Text "’"
         )
+    )
+    => IfKeyNotFound key '[] w
+
+instance IfKeyNotFound key (key #> e ': r) w
+instance IfKeyNotFound key (key ##> e ': r) w
+instance {-# OVERLAPPABLE #-} (IfKeyNotFound key r w) => IfKeyNotFound key (e ': r) w
+
+instance {-# INCOHERENT #-} IfKeyNotFound e r w
 
 infixr 5 ++
 type family xs ++ ys where
diff --git a/heftia/src/Data/Effect/OpenUnion/Internal/FO.hs b/heftia/src/Data/Effect/OpenUnion/Internal/FO.hs
index e1fad28..618005a 100644
--- a/heftia/src/Data/Effect/OpenUnion/Internal/FO.hs
+++ b/heftia/src/Data/Effect/OpenUnion/Internal/FO.hs
@@ -31,11 +31,11 @@ import Data.Effect.OpenUnion.Internal (
     ElemAt,
     ElemIndex,
     FindElem,
+    IfKeyNotFound,
     IfNotFound,
     IsSuffixOf,
     KnownLength,
     Length,
-    LookupError,
     P (unP),
     PrefixLength,
     Reverse,
@@ -153,14 +153,11 @@ instance (FindElem e es, IfNotFound e es es) => Member e es where
 infix 3 <|
 type (<|) = Member
 
-type MemberBy key e es = (key #> e <| es, Lookup key es ~ key #> e)
+type MemberBy key e es = (key #> e <| es, Lookup key es ~ key #> e, IfKeyNotFound key es es)
 
-type Lookup key es = Lookup_ key es es
-
-type family Lookup_ (key :: k) r w :: EffectF where
-    Lookup_ key (key #> e ': _) w = key #> e
-    Lookup_ key (_ ': r) w = Lookup_ key r w
-    Lookup_ key '[] w = LookupError key w
+type family Lookup (key :: k) r :: EffectF where
+    Lookup key (key #> e ': _) = key #> e
+    Lookup key (_ ': r) = Lookup key r
 
 {- | Orthogonal decomposition of a @'Union' (e ': es) :: 'EffectF'@. 'Right' value
 is returned if the @'Union' (e ': es) :: 'EffectF'@ contains @e :: 'EffectF'@, and
diff --git a/heftia/src/Data/Effect/OpenUnion/Internal/HO.hs b/heftia/src/Data/Effect/OpenUnion/Internal/HO.hs
index 274f8b5..2d5a7f2 100644
--- a/heftia/src/Data/Effect/OpenUnion/Internal/HO.hs
+++ b/heftia/src/Data/Effect/OpenUnion/Internal/HO.hs
@@ -36,11 +36,11 @@ import Data.Effect.OpenUnion.Internal (
     ElemAt,
     ElemIndex,
     FindElem,
+    IfKeyNotFound,
     IfNotFound,
     IsSuffixOf,
     KnownLength,
     Length,
-    LookupError,
     P (unP),
     PrefixLength,
     Reverse,
@@ -110,14 +110,11 @@ instance (FindElem e es, IfNotFound e es es) => MemberH e es where
 infix 3 <<|
 type (<<|) = MemberH
 
-type MemberHBy key e es = (key ##> e <<| es, LookupH key es ~ key ##> e)
+type MemberHBy key e es = (key ##> e <<| es, LookupH key es ~ key ##> e, IfKeyNotFound key es es)
 
-type LookupH key es = LookupH_ key es es
-
-type family LookupH_ (key :: k) r w :: EffectH where
-    LookupH_ key (key ##> e ': _) w = key ##> e
-    LookupH_ key (_ ': r) w = LookupH_ key r w
-    LookupH_ key '[] w = LookupError key w
+type family LookupH (key :: k) r :: EffectH where
+    LookupH key (key ##> e ': _) = key ##> e
+    LookupH key (_ ': r) = LookupH key r
 
 decompH :: (HFunctor e) => UnionH (e ': es) f a -> Either (UnionH es f a) (e f a)
 decompH (UnionH 0 a koi) = Right $ hfmap koi $ unsafeCoerce a