diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 6889945..d5eec3e 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -23,8 +23,8 @@ jobs:
           ~/.cabal/packages
           ~/.cabal/store
           dist-newstyle
-        key: cache-${{ runner.os }}-${{ hashFiles('nixpkgs.nix') }}-v${{ env.cache-invalidation-key }}-${{ hashFiles('linear-base.cabal') }}-${{ github.sha }}
-        restore-keys: cache-${{ runner.os }}-${{ hashFiles('nixpkgs.nix') }}-v${{ env.cache-invalidation-key }}-${{ hashFiles('linear-base.cabal') }}-
+        key: cache-${{ runner.os }}-${{ hashFiles('nixpkgs.nix') }}-v${{ env.cache-invalidation-key }}-${{ hashFiles('capability.cabal') }}-${{ github.sha }}
+        restore-keys: cache-${{ runner.os }}-${{ hashFiles('nixpkgs.nix') }}-v${{ env.cache-invalidation-key }}-${{ hashFiles('capability.cabal') }}-
     - name: Update Cabal's database
       run: $NIXSHELL --run "cabal update"
     - name: Build Cabal's dependencies
@@ -35,3 +35,18 @@ jobs:
       run: $NIXSHELL --run "cabal --flag=dev test"
     - name: Haddock
       run: $NIXSHELL --run "cabal haddock"
+
+  stack-ghc9:
+    name: Build on GHC 9.0
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2.3.4
+    - name: Cache dependencies
+      uses: actions/cache@v2.1.6
+      with:
+        path: ~/.stack
+        key: stack-ghc9-${{ runner.os }}-v${{ env.cache-invalidation-key }}-${{ hashFiles('stack-ghc9.yaml.lock', 'capability.cabal') }}
+    - name: Upgrade stack
+      run: stack upgrade
+    - name: Build
+      run: stack --stack-yaml=stack-ghc9.yaml build --pedantic --test --bench --no-run-tests --no-run-benchmarks
diff --git a/capability.cabal b/capability.cabal
index 8726bd8..d788501 100644
--- a/capability.cabal
+++ b/capability.cabal
@@ -58,7 +58,6 @@ library
     Capability.Stream
     Capability.TypeOf
     Capability.Writer
-    Capability.Writer.Discouraged
   build-depends:
       base >= 4.14 && < 5.0
     , constraints >= 0.1 && < 0.14
@@ -67,7 +66,6 @@ library
     , generic-lens >= 2.0 && < 2.2
     , lens >= 4.16 && < 5.1
     , monad-control >= 1.0 && < 1.1
-    , monad-unlift >= 0.2 && < 0.3
     , mtl >= 2.0 && < 3.0
     , mutable-containers >= 0.3 && < 0.4
     , primitive >= 0.6 && < 0.8
diff --git a/src/Capability.hs b/src/Capability.hs
index 4462c00..613f862 100644
--- a/src/Capability.hs
+++ b/src/Capability.hs
@@ -103,11 +103,6 @@
 -- The use of tags allows one to have independent effects that share a superclass.
 -- E.g. @HasState "foo" Int@ and @HasWriter "bar" String@.
 --
--- Some of the capability modules have a “discouraged” companion (such as
--- "Capability.Writer.Discouraged"). These modules contain deriving-via
--- combinators which you can use if you absolutely must: they are correct, but
--- inefficient, so we recommend that you do not.
---
 -- Finally there are
 --
 -- * "Capability.Derive"
diff --git a/src/Capability/Error.hs b/src/Capability/Error.hs
index fb66776..3d2dac7 100644
--- a/src/Capability/Error.hs
+++ b/src/Capability/Error.hs
@@ -174,8 +174,8 @@ wrapError :: forall innertag t (cs :: [Capability]) inner m a.
   , HasCatch innertag inner (t m)
   , All cs m)
   => (forall m'. All (HasCatch innertag inner ': cs) m' => m' a) -> m a
-wrapError =
-  derive @t @'[HasCatch innertag inner] @cs
+wrapError action =
+  derive @t @'[HasCatch innertag inner] @cs action
 {-# INLINE wrapError #-}
 
 -- XXX: Does it make sense to add a HasMask capability similar to @MonadMask@?
diff --git a/src/Capability/Reader/Internal/Class.hs b/src/Capability/Reader/Internal/Class.hs
index 4d691ee..5266504 100644
--- a/src/Capability/Reader/Internal/Class.hs
+++ b/src/Capability/Reader/Internal/Class.hs
@@ -107,8 +107,8 @@ magnify :: forall innertag t (cs :: [Capability]) inner m a.
   , HasReader innertag inner (t m)
   , All cs m)
   => (forall m'. All (HasReader innertag inner ': cs) m' => m' a) -> m a
-magnify =
-  derive @t @'[HasReader innertag inner] @cs
+magnify action =
+  derive @t @'[HasReader innertag inner] @cs action
 {-# INLINE magnify #-}
 
 --------------------------------------------------------------------------------
diff --git a/src/Capability/Reflection.hs b/src/Capability/Reflection.hs
index ad5a4a7..1235752 100644
--- a/src/Capability/Reflection.hs
+++ b/src/Capability/Reflection.hs
@@ -73,7 +73,7 @@ interpret_ ::
   Reified tag c m ->
   (forall m'. c m' => m' a) ->
   m a
-interpret_ = interpret @tag @'[] @c
+interpret_ dict action = interpret @tag @'[] @c dict action
 {-# INLINE interpret_ #-}
 
 -- | @interpret \@tag \@ambient dict action@
diff --git a/src/Capability/State/Internal/Class.hs b/src/Capability/State/Internal/Class.hs
index c2a4410..e91657f 100644
--- a/src/Capability/State/Internal/Class.hs
+++ b/src/Capability/State/Internal/Class.hs
@@ -132,8 +132,8 @@ zoom :: forall innertag t (cs :: [Capability]) inner m a.
   , HasState innertag inner (t m)
   , All cs m )
   => (forall m'. All (HasState innertag inner ': cs) m' => m' a) -> m a
-zoom =
-  derive @t @'[HasState innertag inner] @cs
+zoom action =
+  derive @t @'[HasState innertag inner] @cs action
 {-# INLINE zoom #-}
 
 --------------------------------------------------------------------------------
diff --git a/src/Capability/Writer/Discouraged.hs b/src/Capability/Writer/Discouraged.hs
deleted file mode 100644
index f923fc6..0000000
--- a/src/Capability/Writer/Discouraged.hs
+++ /dev/null
@@ -1,55 +0,0 @@
-{-# LANGUAGE FlexibleInstances #-}
-{-# LANGUAGE InstanceSigs #-}
-{-# LANGUAGE MagicHash #-}
-{-# LANGUAGE MultiParamTypeClasses #-}
-{-# LANGUAGE RankNTypes #-}
-{-# LANGUAGE ScopedTypeVariables #-}
-{-# LANGUAGE TypeApplications #-}
-{-# LANGUAGE UndecidableInstances #-}
-
-{-# OPTIONS_GHC -Wno-orphans #-}
-{-# OPTIONS_GHC -Wno-simplifiable-class-constraints #-}
-
--- | Defines discouraged instances of writer monad capabilities.
-
-module Capability.Writer.Discouraged
-  ( ) where
-
-import Capability.Accessors
-import Capability.Writer
-import Control.Monad.Trans.Class (lift)
-import Control.Monad.Trans.Unlift (MonadTransUnlift, Unlift(..), askUnlift)
-import Data.Coerce (coerce)
-import GHC.Exts (Proxy#)
-
--- | Lift one layer in a monad transformer stack.
---
--- Note, that if the 'HasWriter' instance is based on 'HasState', then it is
--- more efficient to apply 'Lift' to the underlying state capability. E.g.
--- you should favour
---
--- > deriving (HasWriter tag w) via
--- >   WriterLog (Lift (SomeTrans (MonadState SomeStateMonad)))
---
--- over
---
--- > deriving (HasWriter tag w) via
--- >   Lift (SomeTrans (WriterLog (MonadState SomeStateMonad)))
-instance
-  -- MonadTransUnlift constraint requires -Wno-simplifiable-class-constraints
-  (HasWriter tag w m, MonadTransUnlift t, Monad (t m))
-  => HasWriter tag w (Lift (t m))
-  where
-    writer_ :: forall a. Proxy# tag -> (a, w) -> Lift (t m) a
-    writer_ _ = coerce @((a, w) -> t m a) $ lift . writer @tag
-    {-# INLINE writer_ #-}
-    listen_ :: forall a. Proxy# tag -> Lift (t m) a -> Lift (t m) (a, w)
-    listen_ _ = coerce @(t m a -> t m (a, w)) $ \m -> do
-      u <- askUnlift
-      lift $ listen @tag $ unlift u m
-    {-# INLINE listen_ #-}
-    pass_ :: forall a. Proxy# tag -> Lift (t m) (a, w -> w) -> Lift (t m) a
-    pass_ _ = coerce @(t m (a, w -> w) -> t m a) $ \m -> do
-      u <- askUnlift
-      lift $ pass @tag $ unlift u m
-    {-# INLINE pass_ #-}
diff --git a/stack-ghc9.yaml b/stack-ghc9.yaml
new file mode 100644
index 0000000..55ea160
--- /dev/null
+++ b/stack-ghc9.yaml
@@ -0,0 +1,12 @@
+resolver: nightly-2021-07-04
+
+packages:
+- .
+
+extra-deps:
+# Generic-lens is not yet released in a 9.0-compatible version
+- git: https://github.com/kcsongor/generic-lens.git
+  commit: 8e1fc7dcf444332c474fca17110d4bc554db08c8
+  subdirs:
+  - generic-lens-core
+  - generic-lens
diff --git a/stack-ghc9.yaml.lock b/stack-ghc9.yaml.lock
new file mode 100644
index 0000000..863096a
--- /dev/null
+++ b/stack-ghc9.yaml.lock
@@ -0,0 +1,38 @@
+# This file was autogenerated by Stack.
+# You should not edit this file by hand.
+# For more information, please see the documentation at:
+#   https://docs.haskellstack.org/en/stable/lock_files
+
+packages:
+- completed:
+    subdir: generic-lens-core
+    name: generic-lens-core
+    version: 2.1.0.0
+    git: https://github.com/kcsongor/generic-lens.git
+    pantry-tree:
+      size: 2283
+      sha256: 5a26b16c38ad8d8ebc81182e1d09858858f16dc117a0c7b084d8446d3a17d749
+    commit: 8e1fc7dcf444332c474fca17110d4bc554db08c8
+  original:
+    subdir: generic-lens-core
+    git: https://github.com/kcsongor/generic-lens.git
+    commit: 8e1fc7dcf444332c474fca17110d4bc554db08c8
+- completed:
+    subdir: generic-lens
+    name: generic-lens
+    version: 2.1.0.0
+    git: https://github.com/kcsongor/generic-lens.git
+    pantry-tree:
+      size: 2630
+      sha256: aba81557550537d4cfda2c1b07accff8589da44a1e418a3cde5639eb438d1c86
+    commit: 8e1fc7dcf444332c474fca17110d4bc554db08c8
+  original:
+    subdir: generic-lens
+    git: https://github.com/kcsongor/generic-lens.git
+    commit: 8e1fc7dcf444332c474fca17110d4bc554db08c8
+snapshots:
+- completed:
+    size: 540164
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/nightly/2021/7/4.yaml
+    sha256: 195e3e9394de03e724525e210f82e30c4488f7c8a09fc70850d75d8b79332993
+  original: nightly-2021-07-04