diff --git a/.github/workflows/ci.yml b/.github/workflows/haskell.yml
similarity index 82%
rename from .github/workflows/ci.yml
rename to .github/workflows/haskell.yml
index b246375..48f97a6 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/haskell.yml
@@ -65,13 +65,22 @@ jobs:
     strategy:
       matrix:
         include:
-          # Check that our upper bounds are correct by building with the latest
-          # version of everything. We use cabal because it uses the latest
-          # versions of our dependencies allowed by our upper bounds.
+          # The purpose of that section is two-fold. First, because we are
+          # using cabal and not stack, cabal will pick the most recent version
+          # of our dependencies which has been released. This way, if a
+          # dependency releases a breaking change, the next monthly CI will
+          # pick that version and notify us if we are affected by the change.
           #
-          # TODO: switch back to "ghc: latest" once we support ghc-9.0
-          - name: newest
-            ghc: "8.10"
+          # Second, since this project is a ghc plugin, we rely on details of
+          # the GHC library which change between versions. It is thus quite
+          # easy to accidentally make a change which works for the latest
+          # version of GHC but not with older versions. CI should thus test all
+          # the supported GHC versions.
+          - name: ghc-8.10
+            ghc: "8.10.7"
+            os: ubuntu-latest
+          - name: ghc-9.0
+            ghc: "9.0.2"
             os: ubuntu-latest
 
     steps:
@@ -118,7 +127,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - ghc: "8.10"
+          - ghc: "9.0"
             os: ubuntu-latest
 
     steps:
diff --git a/package.yaml b/package.yaml
index 9f05ea6..87e2716 100644
--- a/package.yaml
+++ b/package.yaml
@@ -18,6 +18,10 @@ extra-source-files:
 # for release
 ghc-options: -W -Wall
 
+tested-with:
+  - GHC == 9.0.2
+  - GHC == 8.10.7
+
 dependencies:
   - base >= 4.12 && < 5
   - ghc-prim >= 0.5.3
@@ -25,7 +29,7 @@ dependencies:
 library:
   source-dirs:      src
   dependencies:
-    - ghc >= 8.10.2 && < 9.0
+    - ghc >= 8.10.2 && <9.1
     - containers >= 0.6.2.1
     - term-rewriting >= 0.3.0.1
     - transformers >= 0.5.6.2
diff --git a/src/TypeLevel/Rewrite.hs b/src/TypeLevel/Rewrite.hs
index 0738291..27759f6 100644
--- a/src/TypeLevel/Rewrite.hs
+++ b/src/TypeLevel/Rewrite.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE LambdaCase, OverloadedStrings, RecordWildCards, ViewPatterns #-}
+{-# LANGUAGE CPP, LambdaCase, OverloadedStrings, RecordWildCards, ViewPatterns #-}
 module TypeLevel.Rewrite (plugin) where
 
 import Control.Monad
@@ -8,6 +8,17 @@ import Data.Foldable
 import Data.Traversable
 
 -- GHC API
+#if MIN_VERSION_ghc(9,0,0)
+import GHC.Core.Coercion (Role(Representational), mkUnivCo)
+import GHC.Tc.Types.Constraint (CtEvidence(ctev_loc), Ct, ctEvExpr, ctLoc, mkNonCanonical)
+import GHC.Plugins (PredType, SDoc, eqType, fsep, ppr)
+import GHC.Plugins (Plugin(pluginRecompile, tcPlugin), CommandLineOption, defaultPlugin, purePlugin)
+import GHC.Tc.Types.Evidence (EvExpr, EvTerm, evCast)
+import GHC.Tc.Plugin (newWanted)
+import GHC.Core.TyCo.Rep (UnivCoProvenance(PluginProv))
+import GHC.Plugins (synTyConDefn_maybe)
+import GHC.Tc.Types (TcPluginResult(..), TcPluginM, ErrCtxt, pushErrCtxtSameOrigin, TcPlugin(..))
+#else
 import Coercion (Role(Representational), mkUnivCo)
 import Constraint (CtEvidence(ctev_loc), Ct, ctEvExpr, ctLoc, mkNonCanonical)
 import GhcPlugins (PredType, SDoc, eqType, fsep, ppr)
@@ -17,6 +28,7 @@ import TcPluginM (newWanted)
 import TcRnTypes
 import TyCoRep (UnivCoProvenance(PluginProv))
 import TyCon (synTyConDefn_maybe)
+#endif
 
 import TypeLevel.Rewrite.Internal.ApplyRules
 import TypeLevel.Rewrite.Internal.DecomposedConstraint
diff --git a/src/TypeLevel/Rewrite/Internal/ApplyRules.hs b/src/TypeLevel/Rewrite/Internal/ApplyRules.hs
index 3587918..e6e7cfa 100644
--- a/src/TypeLevel/Rewrite/Internal/ApplyRules.hs
+++ b/src/TypeLevel/Rewrite/Internal/ApplyRules.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE LambdaCase, TupleSections, ViewPatterns #-}
+{-# LANGUAGE CPP, LambdaCase, TupleSections, ViewPatterns #-}
 {-# OPTIONS -Wno-name-shadowing #-}
 module TypeLevel.Rewrite.Internal.ApplyRules where
 
@@ -13,7 +13,11 @@ import Data.Traversable
 import qualified Data.Map as Map
 
 -- GHC API
+#if MIN_VERSION_ghc(9,0,0)
+import GHC.Plugins (TyVar)
+#else
 import Type (TyVar)
+#endif
 
 -- term-rewriting API
 import Data.Rewriting.Rule (Rule(..))
diff --git a/src/TypeLevel/Rewrite/Internal/DecomposedConstraint.hs b/src/TypeLevel/Rewrite/Internal/DecomposedConstraint.hs
index 87d47af..e121dbd 100644
--- a/src/TypeLevel/Rewrite/Internal/DecomposedConstraint.hs
+++ b/src/TypeLevel/Rewrite/Internal/DecomposedConstraint.hs
@@ -1,12 +1,17 @@
-{-# LANGUAGE DeriveFunctor, DeriveFoldable, DeriveTraversable, LambdaCase, RecordWildCards, ViewPatterns #-}
+{-# LANGUAGE CPP, DeriveFunctor, DeriveFoldable, DeriveTraversable, LambdaCase, RecordWildCards, ViewPatterns #-}
 module TypeLevel.Rewrite.Internal.DecomposedConstraint where
 
 import Control.Applicative
 
 -- GHC API
 import GHC (Class, Type)
+#if MIN_VERSION_ghc(9,0,0)
+import GHC.Tc.Types.Constraint (Ct, ctEvPred, ctEvidence)
+import GHC.Core.Predicate (EqRel(NomEq), Pred(ClassPred, EqPred), classifyPredType, mkClassPred, mkPrimEqPred)
+#else
 import Constraint (Ct, ctEvPred, ctEvidence)
 import Predicate (EqRel(NomEq), Pred(ClassPred, EqPred), classifyPredType, mkClassPred, mkPrimEqPred)
+#endif
 
 
 data DecomposedConstraint a
diff --git a/src/TypeLevel/Rewrite/Internal/Lookup.hs b/src/TypeLevel/Rewrite/Internal/Lookup.hs
index dbb8323..c390638 100644
--- a/src/TypeLevel/Rewrite/Internal/Lookup.hs
+++ b/src/TypeLevel/Rewrite/Internal/Lookup.hs
@@ -1,21 +1,29 @@
-{-# LANGUAGE LambdaCase, ViewPatterns #-}
+{-# LANGUAGE CPP, LambdaCase, ViewPatterns #-}
 module TypeLevel.Rewrite.Internal.Lookup where
 
 import Control.Arrow ((***), first)
 import Data.Tuple (swap)
 
 -- GHC API
-import Finder (cannotFindModule)
 import GHC (DataCon, TyCon, dataConTyCon)
+#if MIN_VERSION_ghc(9,0,0)
+import GHC.Driver.Finder (cannotFindModule)
+import GHC (Module, ModuleName, mkModuleName)
+import GHC.Plugins (mkDataOcc, mkTcOcc)
+import GHC.Utils.Panic (panicDoc)
+import GHC.Tc.Plugin
+  ( FindResult(Found), TcPluginM, findImportedModule, lookupOrig, tcLookupDataCon, tcLookupTyCon
+  , unsafeTcPluginTcM
+  )
+import GHC.Tc.Solver.Monad (getDynFlags)
+#else
+import Finder (cannotFindModule)
 import Module (Module, ModuleName, mkModuleName)
 import OccName (mkDataOcc, mkTcOcc)
 import Panic (panicDoc)
 import TcPluginM
-  ( FindResult(Found), TcPluginM, findImportedModule, lookupOrig, tcLookupDataCon, tcLookupTyCon
-  , unsafeTcPluginTcM
-  )
 import TcSMonad (getDynFlags)
-
+#endif
 
 lookupModule
   :: String  -- ^ module name
diff --git a/src/TypeLevel/Rewrite/Internal/PrettyPrint.hs b/src/TypeLevel/Rewrite/Internal/PrettyPrint.hs
index afc19ed..efad118 100644
--- a/src/TypeLevel/Rewrite/Internal/PrettyPrint.hs
+++ b/src/TypeLevel/Rewrite/Internal/PrettyPrint.hs
@@ -1,12 +1,18 @@
-{-# LANGUAGE LambdaCase, RecordWildCards #-}
+{-# LANGUAGE CPP, LambdaCase, RecordWildCards #-}
 module TypeLevel.Rewrite.Internal.PrettyPrint where
 
 import Data.List (intercalate)
 
 -- GHC API
+#if MIN_VERSION_ghc(9,0,0)
+import GHC.Utils.Outputable (ppr, showSDocUnsafe)
+import GHC.Plugins (TyCon)
+import GHC.Plugins (TyVar, Type)
+#else
 import Outputable (ppr, showSDocUnsafe)
 import TyCon (TyCon)
 import Type (TyVar, Type)
+#endif
 
 -- term-rewriting API
 import Data.Rewriting.Rule (Rule(..))
diff --git a/src/TypeLevel/Rewrite/Internal/TypeEq.hs b/src/TypeLevel/Rewrite/Internal/TypeEq.hs
index 30272a6..acda114 100644
--- a/src/TypeLevel/Rewrite/Internal/TypeEq.hs
+++ b/src/TypeLevel/Rewrite/Internal/TypeEq.hs
@@ -1,8 +1,13 @@
+{-# LANGUAGE CPP #-}
 module TypeLevel.Rewrite.Internal.TypeEq where
 
 import Data.Function
 
+#if MIN_VERSION_ghc(9,0,0)
+import GHC.Plugins (Type, eqType)
+#else
 import GhcPlugins (Type, eqType)
+#endif
 
 
 -- | A newtype around 'Type' which has an 'Eq' instance.
diff --git a/src/TypeLevel/Rewrite/Internal/TypeNode.hs b/src/TypeLevel/Rewrite/Internal/TypeNode.hs
index 97e14ae..ca320d5 100644
--- a/src/TypeLevel/Rewrite/Internal/TypeNode.hs
+++ b/src/TypeLevel/Rewrite/Internal/TypeNode.hs
@@ -1,9 +1,14 @@
-{-# LANGUAGE ViewPatterns #-}
+{-# LANGUAGE CPP, ViewPatterns #-}
 module TypeLevel.Rewrite.Internal.TypeNode where
 
 -- GHC API
+#if MIN_VERSION_ghc(9,0,0)
+import GHC (TyCon)
+import GHC.Plugins (Type, isNumLitTy, isStrLitTy, mkTyConApp, splitTyConApp_maybe)
+#else
 import TyCon (TyCon)
 import Type (Type, isNumLitTy, isStrLitTy, mkTyConApp, splitTyConApp_maybe)
+#endif
 
 import TypeLevel.Rewrite.Internal.TypeEq
 
diff --git a/src/TypeLevel/Rewrite/Internal/TypeRule.hs b/src/TypeLevel/Rewrite/Internal/TypeRule.hs
index c44b46d..0c1fab3 100644
--- a/src/TypeLevel/Rewrite/Internal/TypeRule.hs
+++ b/src/TypeLevel/Rewrite/Internal/TypeRule.hs
@@ -1,11 +1,17 @@
-{-# LANGUAGE LambdaCase, ViewPatterns #-}
+{-# LANGUAGE CPP, LambdaCase, ViewPatterns #-}
 {-# OPTIONS -Wno-name-shadowing #-}
 module TypeLevel.Rewrite.Internal.TypeRule where
 
 -- GHC API
+#if MIN_VERSION_ghc(9,0,0)
+import GHC.Plugins (getOccString)
+import GHC.Core.Predicate (mkPrimEqPred)
+import GHC.Plugins (TyVar, Type, mkTyVarTy)
+#else
 import Name (getOccString)
 import Predicate (mkPrimEqPred)
 import Type (TyVar, Type, mkTyVarTy)
+#endif
 
 -- term-rewriting API
 import Data.Rewriting.Rule (Rule(..))
diff --git a/src/TypeLevel/Rewrite/Internal/TypeTemplate.hs b/src/TypeLevel/Rewrite/Internal/TypeTemplate.hs
index bd870f2..b2fa608 100644
--- a/src/TypeLevel/Rewrite/Internal/TypeTemplate.hs
+++ b/src/TypeLevel/Rewrite/Internal/TypeTemplate.hs
@@ -1,8 +1,12 @@
-{-# LANGUAGE LambdaCase, ViewPatterns #-}
+{-# LANGUAGE CPP, LambdaCase, ViewPatterns #-}
 module TypeLevel.Rewrite.Internal.TypeTemplate where
 
 -- GHC API
+#if MIN_VERSION_ghc(9,0,0)
+import GHC.Plugins (TyVar, Type, getTyVar_maybe)
+#else
 import Type (TyVar, Type, getTyVar_maybe)
+#endif
 
 -- term-rewriting API
 import Data.Rewriting.Term (Term(Fun, Var))
diff --git a/src/TypeLevel/Rewrite/Internal/TypeTerm.hs b/src/TypeLevel/Rewrite/Internal/TypeTerm.hs
index 28ef65a..5e06acd 100644
--- a/src/TypeLevel/Rewrite/Internal/TypeTerm.hs
+++ b/src/TypeLevel/Rewrite/Internal/TypeTerm.hs
@@ -1,8 +1,12 @@
-{-# LANGUAGE LambdaCase, ViewPatterns #-}
+{-# LANGUAGE CPP, LambdaCase, ViewPatterns #-}
 module TypeLevel.Rewrite.Internal.TypeTerm where
 
 -- GHC API
+#if MIN_VERSION_ghc(9,0,0)
+import GHC.Plugins (Type, mkTyConApp)
+#else
 import Type (Type, mkTyConApp)
+#endif
 
 -- term-rewriting API
 import Data.Rewriting.Term (Term(Fun, Var))
diff --git a/stack.yaml b/stack.yaml
index 7e0065b..1db2e48 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -1,7 +1,7 @@
-resolver: nightly-2020-12-10
+resolver: lts-19.33
 packages:
 - '.'
 
 extra-deps:
 - term-rewriting-0.4.0.2
-- union-find-array-0.1.0.2
+- union-find-array-0.1.0.3
diff --git a/test/error-messages-cases/no-plugin/expected b/test/error-messages-cases/no-plugin/expected
index 430509e..b2ac572 100644
--- a/test/error-messages-cases/no-plugin/expected
+++ b/test/error-messages-cases/no-plugin/expected
@@ -1 +1,2 @@
-    • Couldn't match type ‘as ++ (bs ++ cs)’ with ‘(as ++ bs) ++ cs’
+    • Couldn't match type: as ++ (bs ++ cs)
+                     with: (as ++ bs) ++ cs
diff --git a/test/should-compile/GHC/TypeLits/Test.hs b/test/should-compile/GHC/TypeLits/Test.hs
index a228cce..ffc8fdd 100644
--- a/test/should-compile/GHC/TypeLits/Test.hs
+++ b/test/should-compile/GHC/TypeLits/Test.hs
@@ -17,6 +17,7 @@ ex2 :: ( ((s1 `AppendSymbol` "foo") `AppendSymbol` s2)
        ~ (s1 `AppendSymbol` ("foo" `AppendSymbol` s2))
       => r
        )
-    -> proxy s
+    -> proxy s1
+    -> proxy s2
     -> r
-ex2 r _ = r
+ex2 r _ _ = r
diff --git a/typelevel-rewrite-rules.cabal b/typelevel-rewrite-rules.cabal
index c33115e..26a9688 100644
--- a/typelevel-rewrite-rules.cabal
+++ b/typelevel-rewrite-rules.cabal
@@ -1,10 +1,10 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.33.0.
+-- This file has been generated from package.yaml by hpack version 0.35.2.
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 2673197ba6a9e92128e851017519e221b95a23ca70b1f7b84158feae74eba11b
+-- hash: cac2a8aec9cc51f367be3e13c24e6efa7d62ba4ec28e8368137d8292de814188
 
 name:           typelevel-rewrite-rules
 version:        1.0
@@ -17,6 +17,9 @@ author:         Samuel Gélineau
 maintainer:     gelisam+github@gmail.com
 license:        PublicDomain
 build-type:     Simple
+tested-with:
+    GHC == 9.0.2
+  , GHC == 8.10.7
 extra-source-files:
     README.md
     CHANGELOG.md
@@ -48,7 +51,7 @@ library
   build-depends:
       base >=4.12 && <5
     , containers >=0.6.2.1
-    , ghc >=8.10.2 && <9.0
+    , ghc >=8.10.2 && <9.1
     , ghc-prim >=0.5.3
     , term-rewriting >=0.3.0.1
     , transformers >=0.5.6.2