haskell-effectful · arybczak · Jul 30, 2025 · Jun 9, 2025
diff --git a/.github/workflows/haskell-ci.yml b/.github/workflows/haskell-ci.yml
@@ -268,11 +268,11 @@ jobs:
       - name: doctest
         run: |
           if [ $((HCNUMVER < 91200)) -ne 0 ] ; then cd ${PKGDIR_effectful_core} || false ; fi
-          if [ $((HCNUMVER < 91200)) -ne 0 ] ; then doctest  -XHaskell2010 -XBangPatterns -XConstraintKinds -XDataKinds -XDeriveFunctor -XDeriveGeneric -XDerivingStrategies -XFlexibleContexts -XFlexibleInstances -XGADTs -XGeneralizedNewtypeDeriving -XImportQualifiedPost -XLambdaCase -XMultiParamTypeClasses -XNoStarIsType -XPolyKinds -XRankNTypes -XRoleAnnotations -XScopedTypeVariables -XStandaloneDeriving -XTupleSections -XTypeApplications -XTypeFamilies -XTypeOperators src ; fi
+          if [ $((HCNUMVER < 91200)) -ne 0 ] ; then doctest  -XHaskell2010 -XBangPatterns -XConstraintKinds -XDataKinds -XDeriveFunctor -XDeriveGeneric -XDerivingStrategies -XFlexibleContexts -XFlexibleInstances -XGADTs -XGeneralizedNewtypeDeriving -XImportQualifiedPost -XLambdaCase -XMultiParamTypeClasses -XNoStarIsType -XPolyKinds -XRankNTypes -XRoleAnnotations -XScopedTypeVariables -XStandaloneDeriving -XTupleSections -XTypeApplications -XTypeFamilies -XTypeOperators -XUndecidableInstances src ; fi
           if [ $((HCNUMVER < 91200)) -ne 0 ] ; then cd ${PKGDIR_effectful_th} || false ; fi
-          if [ $((HCNUMVER < 91200)) -ne 0 ] ; then doctest  -XHaskell2010 -XBangPatterns -XConstraintKinds -XDataKinds -XDeriveFunctor -XDeriveGeneric -XDerivingStrategies -XFlexibleContexts -XFlexibleInstances -XGADTs -XGeneralizedNewtypeDeriving -XImportQualifiedPost -XLambdaCase -XMultiParamTypeClasses -XNoStarIsType -XPolyKinds -XRankNTypes -XRecordWildCards -XRoleAnnotations -XScopedTypeVariables -XStandaloneDeriving -XTupleSections -XTypeApplications -XTypeFamilies -XTypeOperators src ; fi
+          if [ $((HCNUMVER < 91200)) -ne 0 ] ; then doctest  -XHaskell2010 -XBangPatterns -XConstraintKinds -XDataKinds -XDeriveFunctor -XDeriveGeneric -XDerivingStrategies -XFlexibleContexts -XFlexibleInstances -XGADTs -XGeneralizedNewtypeDeriving -XImportQualifiedPost -XLambdaCase -XMultiParamTypeClasses -XNoStarIsType -XPolyKinds -XRankNTypes -XRecordWildCards -XRoleAnnotations -XScopedTypeVariables -XStandaloneDeriving -XTupleSections -XTypeApplications -XTypeFamilies -XTypeOperators -XUndecidableInstances src ; fi
           if [ $((HCNUMVER < 91200)) -ne 0 ] ; then cd ${PKGDIR_effectful} || false ; fi
-          if [ $((HCNUMVER < 91200)) -ne 0 ] ; then doctest  -XHaskell2010 -XBangPatterns -XConstraintKinds -XDataKinds -XDeriveFunctor -XDeriveGeneric -XDerivingStrategies -XFlexibleContexts -XFlexibleInstances -XGADTs -XGeneralizedNewtypeDeriving -XImportQualifiedPost -XLambdaCase -XMultiParamTypeClasses -XNoStarIsType -XPolyKinds -XRankNTypes -XRecordWildCards -XRoleAnnotations -XScopedTypeVariables -XStandaloneDeriving -XTupleSections -XTypeApplications -XTypeFamilies -XTypeOperators src ; fi
+          if [ $((HCNUMVER < 91200)) -ne 0 ] ; then doctest  -XHaskell2010 -XBangPatterns -XConstraintKinds -XDataKinds -XDeriveFunctor -XDeriveGeneric -XDerivingStrategies -XFlexibleContexts -XFlexibleInstances -XGADTs -XGeneralizedNewtypeDeriving -XImportQualifiedPost -XLambdaCase -XMultiParamTypeClasses -XNoStarIsType -XPolyKinds -XRankNTypes -XRecordWildCards -XRoleAnnotations -XScopedTypeVariables -XStandaloneDeriving -XTupleSections -XTypeApplications -XTypeFamilies -XTypeOperators -XUndecidableInstances src ; fi
       - name: cabal check
         run: |
           cd ${PKGDIR_effectful_core} || false

diff --git a/doctest.sh b/doctest.sh
@@ -37,7 +37,8 @@ run_doctest() {
     -XTupleSections \
     -XTypeApplications \
     -XTypeFamilies \
-    -XTypeOperators
+    -XTypeOperators \
+    -XUndecidableInstances
   popd
 }
 

diff --git a/effectful-core/CHANGELOG.md b/effectful-core/CHANGELOG.md
@@ -1,3 +1,7 @@
+# effectful-core-2.6.1.0 (????-??-??)
+* Add `MonadError`, `MonadReader`, `MonadState` and `MonadWriter` instances for
+  `Eff` for compatibility with existing code.
+
 # effectful-core-2.6.0.0 (2025-06-13)
 * Adjust `generalBracket` with `base >= 4.21` to make use of the new exception
   annotation mechanism.

diff --git a/effectful-core/effectful-core.cabal b/effectful-core/effectful-core.cabal
@@ -1,7 +1,7 @@
 cabal-version:      3.0
 build-type:         Simple
 name:               effectful-core
-version:            2.6.0.0
+version:            2.6.1.0
 license:            BSD-3-Clause
 license-file:       LICENSE
 category:           Control
@@ -60,6 +60,7 @@ common language
                         TypeApplications
                         TypeFamilies
                         TypeOperators
+                        UndecidableInstances
 
 library
     import:         language
@@ -70,6 +71,7 @@ library
                     , containers          >= 0.6
                     , deepseq             >= 1.2
                     , exceptions          >= 0.10.4
+                    , mtl                 >= 2.2.1
                     , monad-control       >= 1.0.3
                     , primitive           >= 0.7.3.0
                     , strict-mutable-base >= 1.1.0.0

diff --git a/effectful-core/src/Effectful/Dispatch/Dynamic.hs b/effectful-core/src/Effectful/Dispatch/Dynamic.hs
@@ -1,6 +1,5 @@
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE ImplicitParams #-}
-{-# LANGUAGE UndecidableInstances #-}
 -- | Dynamically dispatched effects.
 module Effectful.Dispatch.Dynamic
   ( -- * Introduction
@@ -349,8 +348,6 @@ import Effectful.Internal.Utils
 -- __orphan__, __canonical__ instance of @MonadRNG@ for 'Eff' that delegates to
 -- the @RNG@ effect:
 --
--- >>> :set -XUndecidableInstances
---
 -- >>> :{
 --   instance RNG :> es => MonadRNG (Eff es) where
 --     randomInt = send RandomInt

diff --git a/effectful-core/src/Effectful/Error/Dynamic.hs b/effectful-core/src/Effectful/Error/Dynamic.hs
@@ -1,6 +1,8 @@
+{-# OPTIONS_GHC -Wno-orphans #-}
 -- | The dynamically dispatched variant of the 'Error' effect.
 --
--- /Note:/ unless you plan to change interpretations at runtime, it's
+-- /Note:/ unless you plan to change interpretations at runtime or you need the
+-- 'MTL.MonadError' instance for compatibility with existing code, it's
 -- recommended to use the statically dispatched variant,
 -- i.e. "Effectful.Error.Static".
 module Effectful.Error.Dynamic
@@ -28,6 +30,7 @@ module Effectful.Error.Dynamic
   , E.prettyCallStack
   ) where
 
+import Control.Monad.Except qualified as MTL
 import GHC.Stack (withFrozenCallStack)
 
 import Effectful
@@ -148,3 +151,15 @@ tryError
   -- ^ The inner computation.
   -> Eff es (Either (E.CallStack, e) a)
 tryError m = (Right <$> m) `catchError` \es e -> pure $ Left (es, e)
+
+----------------------------------------
+-- Orphan instance
+
+-- | Instance included for compatibility with existing code.
+instance
+  ( Show e
+  , Error e :> es
+  , MTL.MonadError e (Eff es)
+  ) => MTL.MonadError e (Eff es) where
+  throwError = send . ThrowErrorWith show
+  catchError action = send . CatchError action . const
diff --git a/effectful-core/src/Effectful/Internal/Effect.hs b/effectful-core/src/Effectful/Internal/Effect.hs
@@ -1,5 +1,4 @@
 {-# LANGUAGE AllowAmbiguousTypes #-}
-{-# LANGUAGE UndecidableInstances #-}
 {-# OPTIONS_HADDOCK not-home #-}
 -- | Type-safe indexing for 'Effectful.Internal.Monad.Env'.
 --

diff --git a/effectful-core/src/Effectful/Internal/Monad.hs b/effectful-core/src/Effectful/Internal/Monad.hs
@@ -1,5 +1,4 @@
 {-# LANGUAGE CPP #-}
-{-# LANGUAGE UndecidableInstances #-}
 {-# OPTIONS_GHC -Wno-orphans #-}
 {-# OPTIONS_HADDOCK not-home #-}
 -- | The 'Eff' monad.

diff --git a/effectful-core/src/Effectful/Reader/Dynamic.hs b/effectful-core/src/Effectful/Reader/Dynamic.hs
@@ -1,6 +1,8 @@
+{-# OPTIONS_GHC -Wno-orphans #-}
 -- | The dynamically dispatched variant of the 'Reader' effect.
 --
--- /Note:/ unless you plan to change interpretations at runtime, it's
+-- /Note:/ unless you plan to change interpretations at runtime or you need the
+-- 'MTL.MonadReader' instance for compatibility with existing code, it's
 -- recommended to use the statically dispatched variant,
 -- i.e. "Effectful.Reader.Static".
 module Effectful.Reader.Dynamic
@@ -17,6 +19,8 @@ module Effectful.Reader.Dynamic
   , local
   ) where
 
+import Control.Monad.Reader qualified as MTL
+
 import Effectful
 import Effectful.Dispatch.Dynamic
 
@@ -81,3 +85,15 @@ local
   -> Eff es a
   -> Eff es a
 local f = send . Local f
+
+----------------------------------------
+-- Orphan instance
+
+-- | Instance included for compatibility with existing code.
+instance
+  ( Reader r :> es
+  , MTL.MonadReader r (Eff es)
+  ) => MTL.MonadReader r (Eff es) where
+  ask = send Ask
+  local f = send . Local f
+  reader f = f <$> send Ask
diff --git a/effectful-core/src/Effectful/State/Dynamic.hs b/effectful-core/src/Effectful/State/Dynamic.hs
@@ -1,6 +1,8 @@
+{-# OPTIONS_GHC -Wno-orphans #-}
 -- | The dynamically dispatched variant of the 'State' effect.
 --
--- /Note:/ unless you plan to change interpretations at runtime, it's
+-- /Note:/ unless you plan to change interpretations at runtime or you need the
+-- 'MTL.MonadState' instance for compatibility with existing code, it's
 -- recommended to use one of the statically dispatched variants,
 -- i.e. "Effectful.State.Static.Local" or "Effectful.State.Static.Shared".
 module Effectful.State.Dynamic
@@ -29,6 +31,8 @@ module Effectful.State.Dynamic
   , modifyM
   ) where
 
+import Control.Monad.State qualified as MTL
+
 import Effectful
 import Effectful.Dispatch.Dynamic
 import Effectful.State.Static.Local qualified as L
@@ -149,3 +153,15 @@ modifyM
   => (s -> Eff es s)
   -> Eff es ()
 modifyM f = stateM (\s -> ((), ) <$> f s)
+
+----------------------------------------
+-- Orphan instance
+
+-- | Instance included for compatibility with existing code.
+instance
+  ( State s :> es
+  , MTL.MonadState s (Eff es)
+  ) => MTL.MonadState s (Eff es) where
+  get = send Get
+  put = send . Put
+  state = send . State
diff --git a/effectful-core/src/Effectful/Writer/Dynamic.hs b/effectful-core/src/Effectful/Writer/Dynamic.hs
@@ -1,6 +1,8 @@
+{-# OPTIONS_GHC -Wno-orphans #-}
 -- | The dynamically dispatched variant of the 'Writer' effect.
 --
--- /Note:/ unless you plan to change interpretations at runtime, it's
+-- /Note:/ unless you plan to change interpretations at runtime or you need the
+-- 'MTL.MonadWriter' instance for compatibility with existing code, it's
 -- recommended to use one of the statically dispatched variants,
 -- i.e. "Effectful.Writer.Static.Local" or "Effectful.Writer.Static.Shared".
 module Effectful.Writer.Dynamic
@@ -23,6 +25,8 @@ module Effectful.Writer.Dynamic
   , listens
   ) where
 
+import Control.Monad.Writer qualified as MTL
+
 import Effectful
 import Effectful.Dispatch.Dynamic
 import Effectful.Writer.Static.Local qualified as L
@@ -102,3 +106,17 @@ listens
 listens f m = do
   (a, w) <- listen m
   pure (a, f w)
+
+----------------------------------------
+-- Orphan instance
+
+-- | Instance included for compatibility with existing code.
+instance
+  ( Monoid w
+  , Writer w :> es
+  , MTL.MonadWriter w (Eff es)
+  ) => MTL.MonadWriter w (Eff es) where
+  writer (a, w) = a <$ send (Tell w)
+  tell = send . Tell
+  listen = send . Listen
+  pass = error "pass is not implemented due to ambiguous semantics in presence of runtime exceptions"
diff --git a/effectful-plugin/effectful-plugin.cabal b/effectful-plugin/effectful-plugin.cabal
@@ -66,6 +66,7 @@ common language
                         TypeApplications
                         TypeFamilies
                         TypeOperators
+                        UndecidableInstances
 
     if impl(ghc >= 9.4)
       default-extensions: NoFieldSelectors

diff --git a/effectful-th/effectful-th.cabal b/effectful-th/effectful-th.cabal
@@ -57,6 +57,7 @@ common language
                         TypeApplications
                         TypeFamilies
                         TypeOperators
+                        UndecidableInstances
 
 library
     import:         language

diff --git a/effectful/CHANGELOG.md b/effectful/CHANGELOG.md
@@ -1,4 +1,6 @@
 # effectful-2.6.1.0 (????-??-??)
+* Add `MonadError`, `MonadReader`, `MonadState` and `MonadWriter` instances for
+  `Eff` for compatibility with existing code.
 * Re-export `writeTMVar` from `stm-2.5.1.0` in `Effectful.Concurrent.STM`.
 * Add `cancelMany` to `Effectful.Concurrent.Async`.
 

diff --git a/effectful/bench/FileSizes.hs b/effectful/bench/FileSizes.hs
@@ -1,6 +1,5 @@
 {-# LANGUAGE BlockArguments #-}
 {-# LANGUAGE CPP #-}
-{-# LANGUAGE UndecidableInstances #-}
 module FileSizes where
 
 import Control.Exception

diff --git a/effectful/effectful.cabal b/effectful/effectful.cabal
@@ -66,6 +66,7 @@ common language
                         TypeApplications
                         TypeFamilies
                         TypeOperators
+                        UndecidableInstances
 
 library
     import:         language
@@ -74,7 +75,7 @@ library
                     , async               >= 2.2.5
                     , bytestring          >= 0.10
                     , directory           >= 1.3.2
-                    , effectful-core      >= 2.6.0.0   && < 2.6.1.0
+                    , effectful-core      >= 2.6.1.0   && < 2.6.2.0
                     , process             >= 1.6.9
                     , strict-mutable-base >= 1.1.0.0
                     , time                >= 1.9.2

diff --git a/effectful/src/Effectful/Concurrent/Async.hs b/effectful/src/Effectful/Concurrent/Async.hs
@@ -1,4 +1,3 @@
-{-# LANGUAGE UndecidableInstances #-}
 -- | Lifted "Control.Concurrent.Async".
 module Effectful.Concurrent.Async
   ( -- * Effect