diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f29b71f..5bb99de 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,31 +13,18 @@ jobs:
   build:
     strategy:
       matrix:
-        ghc: ['9.0.2', '9.2.7', '9.4.4', '9.6.1']
-        cabal: ['3.4.1.0', '3.6.2.0', '3.8.1.0', '3.10.1.0']
-        os: ['ubuntu-latest', 'macos-latest']
-        exclude:
-          # Cabal 3.4.1.0 supports GHC version < 9.1
-          - cabal: '3.4.1.0'
-            ghc: '9.2.7'
-          - cabal: '3.4.1.0'
-            ghc: '9.4.4'
-          - cabal: '3.4.1.0'
-            ghc: '9.6.1'
-          # Cabal 3.6.2.0 supports GHC version < 9.4
-          - cabal: '3.6.2.0'
-            ghc: '9.4.4'
-          - cabal: '3.6.2.0'
-            ghc: '9.6.1'
+        ghc: ['9.0.2', '9.2.8', '9.4.8', '9.6.7', '9.8.4', '9.10.3']
+        cabal: ['3.10.2.0', '3.12.1.0', '3.14.2.0', '3.16.1.0']
+        os: ['ubuntu-latest']
     runs-on: ${{ matrix.os }}
     name: platform ${{ matrix.os }} GHC ${{ matrix.ghc }} cabal ${{ matrix.cabal }}
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
 
       - name: Setup Haskell
         id: setup-haskell
-        uses: haskell/actions/setup@v2
+        uses: haskell-actions/setup@v2
         with:
           ghc-version: ${{ matrix.ghc }}
           cabal-version: ${{ matrix.cabal }}
@@ -49,9 +36,6 @@ jobs:
           key: ${{ runner.os }}-${{ matrix.ghc }}-${{ github.sha }}
           restore-keys: ${{ runner.os }}-${{ matrix.ghc }}-
 
-      - name: Update cabal package list
-        run: cabal update
-
       - name: Build dependencies
         run: cabal build --only-dependencies
 
@@ -65,7 +49,6 @@ jobs:
         run: cabal test --index-state HEAD
 
       - name: Test on oldest dependencies
-        if: matrix.cabal == '3.10.1.0'
         run: cabal test --prefer-oldest
 
   format:
@@ -75,4 +58,6 @@ jobs:
         uses: actions/checkout@v3
 
       - name: Check code formatting
-        uses: fourmolu/fourmolu-action@v6
+        uses: haskell-actions/run-fourmolu@v12
+        with:
+          version: "0.19.0.1"
diff --git a/cabal.project b/cabal.project
index 7cce255..5d14d99 100644
--- a/cabal.project
+++ b/cabal.project
@@ -1,4 +1,4 @@
-index-state: 2023-03-14T09:58:19Z
+index-state: 2026-04-09T08:51:20Z
 packages: *.cabal
 write-ghc-environment-files: always
 tests: True
diff --git a/crem.cabal b/crem.cabal
index d7294f5..996c1b4 100644
--- a/crem.cabal
+++ b/crem.cabal
@@ -1,6 +1,6 @@
 cabal-version: 2.0
 
--- This file has been generated from package.yaml by hpack version 0.35.2.
+-- This file has been generated from package.yaml by hpack version 0.38.3.
 --
 -- see: https://github.com/sol/hpack
 
@@ -19,9 +19,11 @@ license-file:   LICENSE
 build-type:     Simple
 tested-with:
     GHC ==9.0.2
-  , GHC ==9.2.7
-  , GHC ==9.4.4
-  , GHC ==9.6.1
+  , GHC ==9.2.8
+  , GHC ==9.4.8
+  , GHC ==9.6.7
+  , GHC ==9.8.4
+  , GHC ==9.10.3
 extra-source-files:
     README.md
     CHANGELOG.md
@@ -29,11 +31,6 @@ extra-source-files:
 flag errors
   description: enable -Werror
   manual: True
-  default: False
-
-flag test-doctest
-  description: run doctests
-  manual: True
   default: True
 
 library
@@ -52,14 +49,15 @@ library
       DerivingStrategies
       LambdaCase
       PackageImports
+      RoleAnnotations
   ghc-options: -Weverything -Wno-safe -Wno-unsafe -Wno-missing-safe-haskell-mode -Wno-implicit-prelude -Wno-missing-export-lists -Wno-missing-home-modules -Wno-missing-import-lists -Wno-all-missed-specialisations -Wno-prepositive-qualified-module
   build-depends:
-      base >=4.15 && <4.19
+      base >=4.15 && <4.21
     , machines >=0.7.3 && <0.8
     , nothunks >=0.1 && <0.4
     , profunctors >=3.2 && <5.7
-    , singletons-base >=3.0 && <3.3
-    , text >=1.2 && <2.1
+    , singletons-base >=3.0 && <3.5
+    , text >=1.2 && <2.2
   default-language: Haskell2010
   if impl(ghc >= 9.2)
     ghc-options: -Wno-missing-kind-signatures
@@ -142,9 +140,10 @@ library crem-examples
       DerivingStrategies
       LambdaCase
       PackageImports
+      RoleAnnotations
   ghc-options: -Weverything -Wno-safe -Wno-unsafe -Wno-missing-safe-haskell-mode -Wno-implicit-prelude -Wno-missing-export-lists -Wno-missing-home-modules -Wno-missing-import-lists -Wno-all-missed-specialisations -Wno-prepositive-qualified-module
   build-depends:
-      base >=4.15 && <4.19
+      base >=4.15 && <4.21
     , crem
     , profunctors
     , singletons-base
@@ -211,9 +210,10 @@ executable hobbit-game
       DerivingStrategies
       LambdaCase
       PackageImports
+      RoleAnnotations
   ghc-options: -Weverything -Wno-safe -Wno-unsafe -Wno-missing-safe-haskell-mode -Wno-implicit-prelude -Wno-missing-export-lists -Wno-missing-home-modules -Wno-missing-import-lists -Wno-all-missed-specialisations -Wno-prepositive-qualified-module
   build-depends:
-      base >=4.15 && <4.19
+      base >=4.15 && <4.21
     , crem
     , crem-examples
   default-language: Haskell2010
@@ -278,9 +278,10 @@ executable hobbit-map
       DerivingStrategies
       LambdaCase
       PackageImports
+      RoleAnnotations
   ghc-options: -Weverything -Wno-safe -Wno-unsafe -Wno-missing-safe-haskell-mode -Wno-implicit-prelude -Wno-missing-export-lists -Wno-missing-home-modules -Wno-missing-import-lists -Wno-all-missed-specialisations -Wno-prepositive-qualified-module
   build-depends:
-      base >=4.15 && <4.19
+      base >=4.15 && <4.21
     , crem
     , crem-examples
     , text
@@ -347,12 +348,15 @@ test-suite crem-doctests
       DerivingStrategies
       LambdaCase
       PackageImports
+      RoleAnnotations
   ghc-options: -Weverything -Wno-safe -Wno-unsafe -Wno-missing-safe-haskell-mode -Wno-implicit-prelude -Wno-missing-export-lists -Wno-missing-home-modules -Wno-missing-import-lists -Wno-all-missed-specialisations -Wno-prepositive-qualified-module -threaded -Wno-unused-packages
   build-depends:
-      base >=4.15 && <4.19
+      Cabal
+    , base >=4.15 && <4.21
     , crem
     , crem-examples
-    , doctest-parallel >=0.2.3 && <0.4
+    , doctest-parallel >=0.2.3 && <0.5
+    , ghc
   default-language: Haskell2010
   if impl(ghc >= 9.2)
     ghc-options: -Wno-missing-kind-signatures
@@ -406,10 +410,6 @@ test-suite crem-doctests
         TypeSynonymInstances
   if flag(errors)
     ghc-options: -Werror
-  if flag(test-doctest)
-    buildable: True
-  else
-    buildable: False
 
 test-suite crem-spec
   type: exitcode-stdio-1.0
@@ -427,11 +427,12 @@ test-suite crem-spec
       DerivingStrategies
       LambdaCase
       PackageImports
+      RoleAnnotations
   ghc-options: -Weverything -Wno-safe -Wno-unsafe -Wno-missing-safe-haskell-mode -Wno-implicit-prelude -Wno-missing-export-lists -Wno-missing-home-modules -Wno-missing-import-lists -Wno-all-missed-specialisations -Wno-prepositive-qualified-module
   build-tool-depends:
       hspec-discover:hspec-discover
   build-depends:
-      base >=4.15 && <4.19
+      base >=4.15 && <4.21
     , crem
     , crem-examples
     , hspec >=2.7 && <2.12
diff --git a/decision-log/2026-04-09-renounce-literate-haskell.yaml b/decision-log/2026-04-09-renounce-literate-haskell.yaml
new file mode 100644
index 0000000..0e80ea8
--- /dev/null
+++ b/decision-log/2026-04-09-renounce-literate-haskell.yaml
@@ -0,0 +1,12 @@
+name: renounce literate Haskell
+date: 2026-04-09
+context: >
+  Currently we are using literate Haskell for one example where the comments take the most part of the file.
+
+  This always created issues with `doctest-parallel` execution, since the library is not able to parse literate Haskell files.
+
+  Moreover, lately we found that, when trying to use CPP conditionals inside a literate Haskell files, then it fails to compile.
+decision: >
+  We decide to renounce literate Haskell, since it is creating more issues that the value it is providing
+consequences: >
+  We `unlit` the literate Haskell file, and we turn it into a normal Haskell module with a lot of comments
diff --git a/decision-log/2026-04-10-remove-macos-from-ci.yaml b/decision-log/2026-04-10-remove-macos-from-ci.yaml
new file mode 100644
index 0000000..08b3acb
--- /dev/null
+++ b/decision-log/2026-04-10-remove-macos-from-ci.yaml
@@ -0,0 +1,19 @@
+name: remove macos from ci
+date: 2026-04-10
+context: >
+  With the support for GHC 9.10 and newer cabal versions, we stumbled into an issue with respect to builds with `macos` machines.
+
+  The issue present itself as follows, during `cabal build`:
+
+  ```
+  <no location info>: error:
+      Warning: Couldn't figure out LLVM version!
+               Make sure you have installed LLVM between [9 and 13)
+  ghc-9.0.2: could not execute: opt
+  ```
+
+  I tried applying the suggestions frm https://discourse.haskell.org/t/cabal-and-llvm-issue/3672 but was not able to solve the issue.
+decision: >
+  For the moment the decision is to remove `macos` machines from CI and hide the issue under the carpet.
+consequences: >
+  I will create an issue about this if someone more knowledgeable than me wants to fix this.
diff --git a/examples/Crem/Example/Cart/Aggregate.hs b/examples/Crem/Example/Cart/Aggregate.hs
index 2a503cb..9a1daa3 100644
--- a/examples/Crem/Example/Cart/Aggregate.hs
+++ b/examples/Crem/Example/Cart/Aggregate.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DerivingVia #-}
 {-# LANGUAGE TemplateHaskell #-}
@@ -5,6 +6,14 @@
 {-# LANGUAGE UndecidableInstances #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wmissing-deriving-strategies
 {-# OPTIONS_GHC -Wno-missing-deriving-strategies #-}
+
+#if __GLASGOW_HASKELL__ >= 908
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-poly-kind-signatures
+{-# OPTIONS_GHC -Wno-missing-poly-kind-signatures #-}
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-role-annotations
+{-# OPTIONS_GHC -Wno-missing-role-annotations #-}
+#endif
+
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunticked-promoted-constructors
 {-# OPTIONS_GHC -Wno-unticked-promoted-constructors #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunused-type-patterns
@@ -20,21 +29,21 @@ import Crem.Topology
 import "singletons-base" Data.Singletons.Base.TH
 
 $( singletons
-    [d|
-      data CartVertex
-        = WaitingForPayment
-        | InitiatingPayment
-        | PaymentComplete
-        deriving stock (Eq, Show, Enum, Bounded)
-
-      cartTopology :: Topology CartVertex
-      cartTopology =
-        Topology
-          [ (WaitingForPayment, [InitiatingPayment])
-          , (InitiatingPayment, [PaymentComplete])
-          , (PaymentComplete, [])
-          ]
-      |]
+     [d|
+       data CartVertex
+         = WaitingForPayment
+         | InitiatingPayment
+         | PaymentComplete
+         deriving stock (Eq, Show, Enum, Bounded)
+
+       cartTopology :: Topology CartVertex
+       cartTopology =
+         Topology
+           [ (WaitingForPayment, [InitiatingPayment])
+           , (InitiatingPayment, [PaymentComplete])
+           , (PaymentComplete, [])
+           ]
+       |]
  )
 
 deriving via AllVertices CartVertex instance RenderableVertices CartVertex
diff --git a/examples/Crem/Example/Cart/Shipping.hs b/examples/Crem/Example/Cart/Shipping.hs
index b508ead..cd5282b 100644
--- a/examples/Crem/Example/Cart/Shipping.hs
+++ b/examples/Crem/Example/Cart/Shipping.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DerivingVia #-}
 {-# LANGUAGE TemplateHaskell #-}
@@ -5,6 +6,14 @@
 {-# LANGUAGE UndecidableInstances #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wmissing-deriving-strategies
 {-# OPTIONS_GHC -Wno-missing-deriving-strategies #-}
+
+#if __GLASGOW_HASKELL__ >= 908
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-poly-kind-signatures
+{-# OPTIONS_GHC -Wno-missing-poly-kind-signatures #-}
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-role-annotations
+{-# OPTIONS_GHC -Wno-missing-role-annotations #-}
+#endif
+
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wredundant-constraints
 {-# OPTIONS_GHC -Wno-redundant-constraints #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunticked-promoted-constructors
@@ -31,13 +40,13 @@ data ShippingCommand
 data ShippingEvent
 
 $( singletons
-    [d|
-      data ShippingVertex = ShippingVertex
-        deriving stock (Eq, Show, Enum, Bounded)
+     [d|
+       data ShippingVertex = ShippingVertex
+         deriving stock (Eq, Show, Enum, Bounded)
 
-      shippingTopology :: Topology ShippingVertex
-      shippingTopology = Topology []
-      |]
+       shippingTopology :: Topology ShippingVertex
+       shippingTopology = Topology []
+       |]
  )
 
 deriving via AllVertices ShippingVertex instance RenderableVertices ShippingVertex
diff --git a/examples/Crem/Example/LockDoor.hs b/examples/Crem/Example/LockDoor.hs
index 8154e4e..8b75be9 100644
--- a/examples/Crem/Example/LockDoor.hs
+++ b/examples/Crem/Example/LockDoor.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DerivingVia #-}
 {-# LANGUAGE TemplateHaskell #-}
@@ -7,6 +8,14 @@
 {-# OPTIONS_GHC -Wno-all-missed-specialisations #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wmissing-deriving-strategies
 {-# OPTIONS_GHC -Wno-missing-deriving-strategies #-}
+
+#if __GLASGOW_HASKELL__ >= 908
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-poly-kind-signatures
+{-# OPTIONS_GHC -Wno-missing-poly-kind-signatures #-}
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-role-annotations
+{-# OPTIONS_GHC -Wno-missing-role-annotations #-}
+#endif
+
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunticked-promoted-constructors
 {-# OPTIONS_GHC -Wno-unticked-promoted-constructors #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunused-type-patterns
@@ -20,21 +29,21 @@ import Crem.Topology
 import "singletons-base" Data.Singletons.Base.TH
 
 $( singletons
-    [d|
-      data LockDoorVertex
-        = IsLockOpen
-        | IsLockClosed
-        | IsLockLocked
-        deriving stock (Eq, Show, Enum, Bounded)
+     [d|
+       data LockDoorVertex
+         = IsLockOpen
+         | IsLockClosed
+         | IsLockLocked
+         deriving stock (Eq, Show, Enum, Bounded)
 
-      lockDoorTopology :: Topology LockDoorVertex
-      lockDoorTopology =
-        Topology
-          [ (IsLockOpen, [IsLockClosed])
-          , (IsLockClosed, [IsLockOpen, IsLockLocked])
-          , (IsLockLocked, [IsLockClosed])
-          ]
-      |]
+       lockDoorTopology :: Topology LockDoorVertex
+       lockDoorTopology =
+         Topology
+           [ (IsLockOpen, [IsLockClosed])
+           , (IsLockClosed, [IsLockOpen, IsLockLocked])
+           , (IsLockLocked, [IsLockClosed])
+           ]
+       |]
  )
 
 deriving via AllVertices LockDoorVertex instance RenderableVertices LockDoorVertex
diff --git a/examples/Crem/Example/RiskManager/Aggregate.hs b/examples/Crem/Example/RiskManager/Aggregate.hs
index 285c7b0..fa66e8a 100644
--- a/examples/Crem/Example/RiskManager/Aggregate.hs
+++ b/examples/Crem/Example/RiskManager/Aggregate.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DerivingVia #-}
 {-# LANGUAGE TemplateHaskell #-}
@@ -5,6 +6,14 @@
 {-# LANGUAGE UndecidableInstances #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wmissing-deriving-strategies
 {-# OPTIONS_GHC -Wno-missing-deriving-strategies #-}
+
+#if __GLASGOW_HASKELL__ >= 908
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-poly-kind-signatures
+{-# OPTIONS_GHC -Wno-missing-poly-kind-signatures #-}
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-role-annotations
+{-# OPTIONS_GHC -Wno-missing-role-annotations #-}
+#endif
+
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunticked-promoted-constructors
 {-# OPTIONS_GHC -Wno-unticked-promoted-constructors #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunused-type-patterns
@@ -19,25 +28,25 @@ import Crem.Topology
 import "singletons-base" Data.Singletons.Base.TH
 
 $( singletons
-    [d|
-      data AggregateVertex
-        = NoDataVertex
-        | CollectedUserDataVertex
-        | CollectedLoanDetailsFirstVertex
-        | ReceivedCreditBureauDataFirstVertex
-        | CollectedAllDataVertex
-        deriving stock (Eq, Show, Enum, Bounded)
+     [d|
+       data AggregateVertex
+         = NoDataVertex
+         | CollectedUserDataVertex
+         | CollectedLoanDetailsFirstVertex
+         | ReceivedCreditBureauDataFirstVertex
+         | CollectedAllDataVertex
+         deriving stock (Eq, Show, Enum, Bounded)
 
-      aggregateTopology :: Topology AggregateVertex
-      aggregateTopology =
-        Topology
-          [ (NoDataVertex, [CollectedUserDataVertex])
-          , (CollectedUserDataVertex, [CollectedLoanDetailsFirstVertex, ReceivedCreditBureauDataFirstVertex])
-          , (CollectedLoanDetailsFirstVertex, [CollectedAllDataVertex])
-          , (ReceivedCreditBureauDataFirstVertex, [CollectedAllDataVertex])
-          , (CollectedAllDataVertex, [])
-          ]
-      |]
+       aggregateTopology :: Topology AggregateVertex
+       aggregateTopology =
+         Topology
+           [ (NoDataVertex, [CollectedUserDataVertex])
+           , (CollectedUserDataVertex, [CollectedLoanDetailsFirstVertex, ReceivedCreditBureauDataFirstVertex])
+           , (CollectedLoanDetailsFirstVertex, [CollectedAllDataVertex])
+           , (ReceivedCreditBureauDataFirstVertex, [CollectedAllDataVertex])
+           , (CollectedAllDataVertex, [])
+           ]
+       |]
  )
 
 deriving via AllVertices AggregateVertex instance RenderableVertices AggregateVertex
@@ -49,6 +58,8 @@ data AggregateState (vertex :: AggregateVertex) where
   ReceivedCreditBureauDataFirst :: UserData -> CreditBureauData -> AggregateState 'ReceivedCreditBureauDataFirstVertex
   CollectedAllData :: UserData -> LoanDetails -> CreditBureauData -> AggregateState 'CollectedAllDataVertex
 
+type role AggregateState nominal
+
 riskAggregate :: BaseMachine AggregateTopology RiskCommand (Maybe RiskEvent)
 riskAggregate =
   BaseMachineT
diff --git a/examples/Crem/Example/RiskManager/Projection.hs b/examples/Crem/Example/RiskManager/Projection.hs
index 769f64c..fc7a2fb 100644
--- a/examples/Crem/Example/RiskManager/Projection.hs
+++ b/examples/Crem/Example/RiskManager/Projection.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DeriveAnyClass #-}
 {-# LANGUAGE DerivingVia #-}
@@ -6,6 +7,14 @@
 {-# LANGUAGE UndecidableInstances #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wmissing-deriving-strategies
 {-# OPTIONS_GHC -Wno-missing-deriving-strategies #-}
+
+#if __GLASGOW_HASKELL__ >= 908
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-poly-kind-signatures
+{-# OPTIONS_GHC -Wno-missing-poly-kind-signatures #-}
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-role-annotations
+{-# OPTIONS_GHC -Wno-missing-role-annotations #-}
+#endif
+
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunticked-promoted-constructors
 {-# OPTIONS_GHC -Wno-unticked-promoted-constructors #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunused-type-patterns
@@ -50,15 +59,15 @@ instance Monoid ReceivedData where
       }
 
 $( singletons
-    [d|
-      data ProjectionVertex
-        = SingleProjectionVertex
-        deriving stock (Eq, Show, Enum, Bounded)
+     [d|
+       data ProjectionVertex
+         = SingleProjectionVertex
+         deriving stock (Eq, Show, Enum, Bounded)
 
-      projectionTopology :: Topology ProjectionVertex
-      projectionTopology =
-        Topology []
-      |]
+       projectionTopology :: Topology ProjectionVertex
+       projectionTopology =
+         Topology []
+       |]
  )
 
 deriving via AllVertices ProjectionVertex instance RenderableVertices ProjectionVertex
@@ -66,6 +75,8 @@ deriving via AllVertices ProjectionVertex instance RenderableVertices Projection
 data ProjectionState (vertex :: ProjectionVertex) where
   SingleProjectionState :: ReceivedData -> ProjectionState 'SingleProjectionVertex
 
+type role ProjectionState nominal
+
 riskProjection :: BaseMachine ProjectionTopology RiskEvent ReceivedData
 riskProjection =
   BaseMachineT
diff --git a/examples/Crem/Example/Switch.hs b/examples/Crem/Example/Switch.hs
index 625bde6..e26fdaf 100644
--- a/examples/Crem/Example/Switch.hs
+++ b/examples/Crem/Example/Switch.hs
@@ -12,16 +12,16 @@ import Crem.Topology
 import "singletons-base" Data.Singletons.Base.TH
 
 $( singletons
-    [d|
-      -- topology with a two vertices and one edge from each vertex to the
-      -- other
-      switchTopology :: Topology Bool
-      switchTopology =
-        Topology
-          [ (True, [False])
-          , (False, [True])
-          ]
-      |]
+     [d|
+       -- topology with a two vertices and one edge from each vertex to the
+       -- other
+       switchTopology :: Topology Bool
+       switchTopology =
+         Topology
+           [ (True, [False])
+           , (False, [True])
+           ]
+       |]
  )
 
 switchMachine :: SBool a -> BaseMachine SwitchTopology () ()
diff --git a/examples/Crem/Example/TheHobbit.hs b/examples/Crem/Example/TheHobbit.hs
index b41349e..40310f0 100644
--- a/examples/Crem/Example/TheHobbit.hs
+++ b/examples/Crem/Example/TheHobbit.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DerivingVia #-}
 {-# LANGUAGE TemplateHaskell #-}
@@ -5,6 +6,14 @@
 {-# LANGUAGE UndecidableInstances #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wmissing-deriving-strategies
 {-# OPTIONS_GHC -Wno-missing-deriving-strategies #-}
+
+#if __GLASGOW_HASKELL__ >= 908
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-poly-kind-signatures
+{-# OPTIONS_GHC -Wno-missing-poly-kind-signatures #-}
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-role-annotations
+{-# OPTIONS_GHC -Wno-missing-role-annotations #-}
+#endif
+
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunticked-promoted-constructors
 {-# OPTIONS_GHC -Wno-unticked-promoted-constructors #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunused-type-patterns
@@ -41,29 +50,29 @@ instance Monoid HobbitMessage where
   mempty = HobbitMessage ""
 
 $( singletons
-    [d|
-      data HobbitVertex
-        = TunnelLikeHall
-        | Lonelands
-        | TrollsClearing
-        | Rivendell
-        | MistyMountain
-        | TrollsPath
-        | TrollsCave
-        deriving stock (Eq, Show, Enum, Bounded)
-
-      hobbitTopology :: Topology HobbitVertex
-      hobbitTopology =
-        Topology
-          [ (TunnelLikeHall, [Lonelands])
-          , (Lonelands, [TunnelLikeHall, TrollsClearing])
-          , (TrollsClearing, [Rivendell, TrollsPath])
-          , (Rivendell, [TrollsClearing, MistyMountain])
-          , (MistyMountain, [Rivendell])
-          , (TrollsPath, [TrollsClearing, TrollsCave])
-          , (TrollsCave, [TrollsPath])
-          ]
-      |]
+     [d|
+       data HobbitVertex
+         = TunnelLikeHall
+         | Lonelands
+         | TrollsClearing
+         | Rivendell
+         | MistyMountain
+         | TrollsPath
+         | TrollsCave
+         deriving stock (Eq, Show, Enum, Bounded)
+
+       hobbitTopology :: Topology HobbitVertex
+       hobbitTopology =
+         Topology
+           [ (TunnelLikeHall, [Lonelands])
+           , (Lonelands, [TunnelLikeHall, TrollsClearing])
+           , (TrollsClearing, [Rivendell, TrollsPath])
+           , (Rivendell, [TrollsClearing, MistyMountain])
+           , (MistyMountain, [Rivendell])
+           , (TrollsPath, [TrollsClearing, TrollsCave])
+           , (TrollsCave, [TrollsPath])
+           ]
+       |]
  )
 
 deriving via AllVertices HobbitVertex instance RenderableVertices HobbitVertex
@@ -84,65 +93,46 @@ data HobbitState (vertex :: HobbitVertex) where
   TrollsPathState :: KeyState -> HobbitState 'TrollsPath
   TrollsCaveState :: HobbitState 'TrollsCave
 
+type role HobbitState nominal
+
 stateMessage :: HobbitState vertex -> HobbitMessage
 stateMessage TunnelLikeHallState =
   HobbitMessage
-    "You are in a tunnel-like hall.\n\
-    \You can only go east to the Lonelands"
+    "You are in a tunnel-like hall.\nYou can only go east to the Lonelands"
 stateMessage LonelandsState =
   HobbitMessage
-    "You are in the lonelands.\n\
-    \You can either go west to a tunnel-like hall\n\
-    \or go east to the Trolls clearing"
+    "You are in the lonelands.\nYou can either go west to a tunnel-like hall\nor go east to the Trolls clearing"
 stateMessage (TrollsClearingState keyState) =
   if keyState == DayDawned
     then
       HobbitMessage
-        "You are in the Trolls clearing.\n\
-        \You could go north to the Trolls path,\n\
-        \you can go east to Rivendell\n\
-        \or you could get the key for the TrollsCave"
+        "You are in the Trolls clearing.\nYou could go north to the Trolls path,\nyou can go east to Rivendell\nor you could get the key for the TrollsCave"
     else
       HobbitMessage
-        "You are in the Trolls clearing.\n\
-        \You could go north to the Trolls path,\n\
-        \you can go east to Rivendell"
+        "You are in the Trolls clearing.\nYou could go north to the Trolls path,\nyou can go east to Rivendell"
 stateMessage (RivendellState _) =
   HobbitMessage
-    "You are in Rivendell.\n\
-    \You could either go west to the Trolls clearing\n\
-    \or go east to the Misty mountains\n"
+    "You are in Rivendell.\nYou could either go west to the Trolls clearing\nor go east to the Misty mountains\n"
 stateMessage (MistyMountainState _) =
   HobbitMessage
-    "You are in the Misty mountains.\n\
-    \You can only go east to Rivendell"
+    "You are in the Misty mountains.\nYou can only go east to Rivendell"
 stateMessage (TrollsPathState keyState) =
   case keyState of
     NoKey ->
       HobbitMessage
-        "You are in the Trolls path.\n\
-        \You can go south to the Trolls clearing\n\
-        \or you can wait a bit"
+        "You are in the Trolls path.\nYou can go south to the Trolls clearing\nor you can wait a bit"
     DayDawned ->
       HobbitMessage
-        "You are in the Trolls path.\n\
-        \You can go south to the Trolls clearing\n\
-        \or you can wait some more"
+        "You are in the Trolls path.\nYou can go south to the Trolls clearing\nor you can wait some more"
     GotKey ->
       HobbitMessage
-        "You are in the Trolls path.\n\
-        \You can go south to the Trolls clearing,\n\
-        \you can unlock the door to the Trolls cave\n\
-        \or you can wait some more"
+        "You are in the Trolls path.\nYou can go south to the Trolls clearing,\nyou can unlock the door to the Trolls cave\nor you can wait some more"
     DoorUnlocked ->
       HobbitMessage
-        "You are in the Trolls path.\n\
-        \You can go south to the Trolls clearing\n\
-        \or you can go north to the Trolls cave"
+        "You are in the Trolls path.\nYou can go south to the Trolls clearing\nor you can go north to the Trolls cave"
 stateMessage TrollsCaveState =
   HobbitMessage
-    "Welcome to the Trolls cave!\n\
-    \Now you can go back south to the Trolls path"
+    "Welcome to the Trolls cave!\nNow you can go back south to the Trolls path"
 
 hobbitResult
   :: (Applicative m, AllowedTransition HobbitTopology initialVertex finalVertex)
diff --git a/examples/Crem/Example/TriangularMachine.hs b/examples/Crem/Example/TriangularMachine.hs
index 26ab66c..a3918a4 100644
--- a/examples/Crem/Example/TriangularMachine.hs
+++ b/examples/Crem/Example/TriangularMachine.hs
@@ -9,6 +9,8 @@ import Crem.StateMachine (StateMachine, unrestrictedMachine)
 data TriangularState (a :: ()) where
   OnlyState :: Int -> TriangularState '()
 
+type role TriangularState nominal
+
 triangular :: StateMachine Int Int
 triangular =
   unrestrictedMachine
diff --git a/examples/Crem/Example/TwoSwitchesGate.hs b/examples/Crem/Example/TwoSwitchesGate.hs
new file mode 100644
index 0000000..c046626
--- /dev/null
+++ b/examples/Crem/Example/TwoSwitchesGate.hs
@@ -0,0 +1,259 @@
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE DerivingVia #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE UndecidableInstances #-}
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wmissing-deriving-strategies
+{-# OPTIONS_GHC -Wno-missing-deriving-strategies #-}
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunticked-promoted-constructors
+{-# OPTIONS_GHC -Wno-unticked-promoted-constructors #-}
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunused-type-patterns
+{-# OPTIONS_GHC -Wno-unused-type-patterns #-}
+
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-poly-kind-signatures
+#if __GLASGOW_HASKELL__ >= 908
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-poly-kind-signatures
+{-# OPTIONS_GHC -Wno-missing-poly-kind-signatures #-}
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-role-annotations
+{-# OPTIONS_GHC -Wno-missing-role-annotations #-}
+#endif
+module Crem.Example.TwoSwitchesGate where
+
+import "base" Data.Functor.Identity
+import "crem" Crem.BaseMachine
+import "crem" Crem.Render.Render
+import "crem" Crem.Render.RenderFlow
+import "crem" Crem.Render.RenderableVertices (AllVertices (..), RenderableVertices)
+import "crem" Crem.StateMachine
+import "crem" Crem.Topology
+import "profunctors" Data.Profunctor
+import "singletons-base" Data.Singletons.Base.TH
+import "text" Data.Text (pack)
+
+-- We would like to implement a gate opening mechanism controlled by two switches. We would like the gate to open only when the two switches are on.
+
+-- We would like to implement this by composing several small state machines: one for every switch, one for making sure that we actually receive the right message from both switches, and one for actually opening the gate.
+
+-- Let's start with the switch.
+
+-- The first thing we need to do is to define the topology of our machine, meaning the allowed transitions in its state space.
+
+-- For a switch, there are only two states. Either the switch is on or it is off.
+
+-- Moreover, we want those switches to be usable only once, and therefore we want to forbid the transition from the `on`` to the `off` position. In other terms, we allow only to go from the `off` position to the `on` position.
+
+$( singletons
+     [d|
+       data SwitchVertex
+         = SwitchIsOn
+         | SwitchIsOff
+         deriving stock (Eq, Show, Bounded, Enum)
+
+       switchTopology :: Topology SwitchVertex
+       switchTopology =
+         Topology
+           [(SwitchIsOff, [SwitchIsOn])]
+       |]
+ )
+
+-- Notice that we need to wrap this in `singletons` because we will soon need to use this data type as a kind, to store information in the type of our state machines.
+
+-- We need also an instance of `RenderableVertices SwitchVertex` to decide which vertices to render for our machine. To obtain that, we use `deriving via` together with the `AllVertices` newtype.
+
+deriving via AllVertices SwitchVertex instance RenderableVertices SwitchVertex
+
+-- Next we need to define which data every vertex of our topology should contain. To express that we use a generalized algebraid data type indexed with `SwitchVertex`
+
+data SwitchState (vertex :: SwitchVertex) where
+  OnState :: SwitchState 'SwitchIsOn
+  OffState :: SwitchState 'SwitchIsOff
+
+type role SwitchState nominal
+
+-- In this case, for every vertex there is just one possible state.
+
+-- At this point we need to define which inputs our machine should handle and which outputs it should emit. In the case there is only one meaningful input, the request of turning on the switch, and one meaningful output, the notification that the switch has been turned on.
+
+data SwitchInput = TurnOn
+
+data SwitchOutput = TurnedOn
+  deriving stock (Show)
+
+instance Semigroup SwitchOutput where
+  TurnedOn <> TurnedOn = TurnedOn
+
+-- At this point we can actually implement our switch as a `BaseMachine`
+
+switch :: () -> BaseMachine SwitchTopology SwitchInput SwitchOutput
+switch _ =
+  BaseMachineT
+    { initialState = InitialState OffState
+    , action = \case
+        OnState -> \_ -> pureResult TurnedOn OnState
+        OffState -> \_ -> pureResult TurnedOn OnState
+    }
+
+-- We start from the `OffState` and every time we receive a request to turn the switch on, we return a message informing the external world that the switch in turned on and we update the state accordingly if needed.
+
+-- Since we need two separate switches, we can create them by invoking the `switch` function twice
+
+switch1 :: BaseMachine SwitchTopology SwitchInput SwitchOutput
+switch1 = switch ()
+
+switch2 :: BaseMachine SwitchTopology SwitchInput SwitchOutput
+switch2 = switch ()
+
+-- This concludes the implementation of our switch machine. Next, we would like to implement a machine which receives as inputs the output of two switches and emits a message whenever both the switches have been turned on.
+
+-- Again, we need to start thinking about the topology of our machine. Since we need to track the state of the two switches, we will have four vertices
+
+$( singletons
+     [d|
+       data BothVertex
+         = NoSwitchOn
+         | OnlyFirstSwitchOn
+         | OnlySecondSwitchOn
+         | BothSwitchesOn
+         deriving (Eq, Show, Enum, Bounded)
+
+       bothTopology :: Topology BothVertex
+       bothTopology =
+         Topology
+           [ (NoSwitchOn, [OnlyFirstSwitchOn, OnlySecondSwitchOn])
+           , (OnlyFirstSwitchOn, [BothSwitchesOn])
+           , (OnlySecondSwitchOn, [BothSwitchesOn])
+           ]
+       |]
+ )
+
+deriving via AllVertices BothVertex instance RenderableVertices BothVertex
+
+-- The topology again constrains the machine with the invariant the we can only turn on switches.
+
+-- Next we need to define the state space, assigning a data type to every vertex in the topology. In this case we don't have the need to attach data to our vertices so we can simply define
+
+data BothState (vertex :: BothVertex) where
+  NoSwitchOnState :: BothState 'NoSwitchOn
+  OnlyFirstSwitchOnState :: BothState 'OnlyFirstSwitchOn
+  OnlySecondSwitchOnState :: BothState 'OnlySecondSwitchOn
+  BothSwitchesOnState :: BothState 'BothSwitchesOn
+
+-- Before defining the logic of the machine, we need to define its inputs and outputs. Since we would like it to monitor the outputs of both switches, its input type could be
+
+type BothInput = Either SwitchOutput SwitchOutput
+
+-- Its output instead will be a potential message to actually open the gate
+
+data OpenGate = OpenGate
+
+type BothOutput = Maybe OpenGate
+
+-- and eventually we can define the logic of our state machine
+
+bothMachine :: BaseMachine BothTopology BothInput BothOutput
+bothMachine =
+  BaseMachineT
+    { initialState = InitialState NoSwitchOnState
+    , action = \case
+        NoSwitchOnState -> \case
+          Left _ -> pureResult Nothing OnlyFirstSwitchOnState
+          Right _ -> pureResult Nothing OnlySecondSwitchOnState
+        OnlyFirstSwitchOnState -> \case
+          Left _ -> pureResult Nothing OnlyFirstSwitchOnState
+          Right _ -> pureResult (Just OpenGate) BothSwitchesOnState
+        OnlySecondSwitchOnState -> \case
+          Left _ -> pureResult (Just OpenGate) BothSwitchesOnState
+          Right _ -> pureResult Nothing OnlySecondSwitchOnState
+        BothSwitchesOnState -> \_ -> pureResult Nothing BothSwitchesOnState
+    }
+
+-- The last machine that we need is one representing the actual gate. Since the logic is exactly the same as the one of the switches, we can actually reuse what we defined above
+
+gate :: BaseMachine SwitchTopology SwitchInput SwitchOutput
+gate = switch ()
+
+-- Now we have all the machines we wanted and we need to connect them appropriately.
+
+-- We have the two switches which produce a `SwitchOutput` and the `bothMachine` which accepts inputs of type `Either SwitchOutput SwitchOutput`.
+
+-- We need to pair up the two switches, first, and then connect them to the `bothMachine`. We need to pair the two switches in a way that allows us to decide whether to run one or the other: this is exactly what the `Alternative` constructor of the `StateMachineT` data type allows us to do.
+
+switches :: StateMachine (Either SwitchInput SwitchInput) (Either SwitchOutput SwitchOutput)
+switches = Basic switch1 `Alternative` Basic switch2
+
+-- Notice that we had to wrap our `switch` machines with `Basic` to turn them into `StateMachine`s, which is the more composable type used by `Alternative`.
+
+-- Now we have the output of `switches` which coincides with the input of `bothMachine`, and therefore we can pass every output we get from `switches` to `bothMachine`. We use the `Sequential` constractor exactly for this
+
+bothSwitches :: StateMachine (Either SwitchInput SwitchInput) BothOutput
+bothSwitches = switches `Sequential` Basic bothMachine
+
+-- Now we have a machine which emits `BothOutput = Maybe OpenGate`. Our `gate` machine on the other hand accepts inputs of type `SwitchInput`. To connect those, we need to do some adjusting.
+
+-- First, we can translate an `OpenGate` into a `SwitchInput`
+
+openGateToSwitchInput :: OpenGate -> SwitchInput
+openGateToSwitchInput OpenGate = TurnOn
+
+-- and we can use this function to adapt our `gate` machine so that it accepts `OpenGate` as input.
+
+gate' :: BaseMachine SwitchTopology OpenGate SwitchOutput
+gate' = lmap openGateToSwitchInput gate
+
+-- Still `bothSwitches` emits values of type `Maybe OpenGate`. We could lift our `gate'` machine to `Maybe OpenGate` inputs using the `maybeM` combinator.
+
+maybeGate :: BaseMachine SwitchTopology (Maybe OpenGate) (Maybe SwitchOutput)
+maybeGate = maybeM gate'
+
+-- At this point we could conclude our composition, joining together `bothMachine` and `maybeGate`
+
+gateMachine :: StateMachine (Either SwitchInput SwitchInput) (Maybe SwitchOutput)
+gateMachine = bothSwitches `Sequential` Basic maybeGate
+
+-- Now we have a single machine which describes out whole flow.
+
+-- Now, there are two things which we could do with `gateMachine`.
+
+-- The first thing is actually executing it. To do it we can use the `runMultiple` function.
+
+-- We can try to to turn on both switches and verify that the gate actually opened
+
+-- |
+-- >>> openedGate
+-- Just TurnedOn
+openedGate :: Maybe SwitchOutput
+openedGate = fst . runIdentity $ runMultiple gateMachine [Left TurnOn, Right TurnOn]
+
+-- Or we can turn just the first switch several times without opening the gate
+
+-- |
+-- >>> closedGate
+-- Nothing
+closedGate :: Maybe SwitchOutput
+closedGate = fst . runIdentity $ runMultiple gateMachine [Left TurnOn, Left TurnOn, Left TurnOn]
+
+-- The other thing we can do is actually rendering a diagram representing how the `gateMachine` works.
+
+-- The best rendering we can get displays the flow of the machine and the state space for every step of the flow
+
+-- |
+-- >>> gateFlow
+-- Right "state switch1 {\nswitch1_SwitchIsOn\nswitch1_SwitchIsOff\nswitch1_SwitchIsOff --> switch1_SwitchIsOn\n}\nstate switch2 {\nswitch2_SwitchIsOn\nswitch2_SwitchIsOff\nswitch2_SwitchIsOff --> switch2_SwitchIsOn\n}\nstate fork_choice_switch1switch2 <<choice>>\nstate join_choice_switch1switch2 <<choice>>\nfork_choice_switch1switch2 --> switch1\nfork_choice_switch1switch2 --> switch2\nswitch1 --> join_choice_switch1switch2\nswitch2 --> join_choice_switch1switch2\nstate both {\nboth_NoSwitchOn\nboth_OnlyFirstSwitchOn\nboth_OnlySecondSwitchOn\nboth_BothSwitchesOn\nboth_NoSwitchOn --> both_OnlyFirstSwitchOn\nboth_NoSwitchOn --> both_OnlySecondSwitchOn\nboth_OnlyFirstSwitchOn --> both_BothSwitchesOn\nboth_OnlySecondSwitchOn --> both_BothSwitchesOn\n}\njoin_choice_switch1switch2 --> both\nstate gate {\ngate_SwitchIsOn\ngate_SwitchIsOff\ngate_SwitchIsOff --> gate_SwitchIsOn\n}\nboth --> gate"
+gateFlow :: Either String Mermaid
+gateFlow =
+  (\(mermaid, _, _) -> mermaid)
+    <$> renderFlow
+      ( BinaryLabel
+          ( BinaryLabel
+              ( BinaryLabel
+                  (LeafLabel . MachineLabel . pack $ "switch1")
+                  (LeafLabel . MachineLabel . pack $ "switch2")
+              )
+              (LeafLabel . MachineLabel . pack $ "both")
+          )
+          (LeafLabel . MachineLabel . pack $ "gate")
+      )
+      (gateMachine @Identity)
+
+-- The result is a diagram which looks like [this](https://mermaid.live/edit#pako:eNqNVN9vgjAQ_lfIPYORCgjE-LBsS_aw-eDbQkI6KMKU1kDd5oz_-0qhcwyr8tAf331333G99gAJSwmEUHPMyX2BVxUurQ8UUQkY9WfBk9w2DhE1xNdt46Wcn-oF1eBZpjUYljU_G-jYF0V9UaQRRTpRpBFF50UzVq3jJGdFQuIuO5XHbNbi87kiv7OC3kC-EPNPEW7kNWfSnUYD6lNQPHSdJ3_mjfFc1bpZxy-srZAqtAQXdLN_LKqaa2xLkjCanjHeiaGFSd0znGRkoldVdHyt8iDYyU-b1DCg3kn0zoVGUG6qzKtm6MrcrAf93AdVM_9HZdyBv0hFHqMyggklqUpcpOJuS9EIeE5KEkEolimu1hEIN8HbbVPh8JAWnFUQZnhTExPwjrPlniYQ8mpHFKl7H35ZW0xfGevtITzAF4RWYAfeaDwOpp7rIdeEPYROgEaub6PA95GY3KMJ39I7GE0dx5-4E9vxJv50LOhEpvPcvk3yiTr-AO8tobo) where you can clearly see the overall structure of the machine we created, and for every step of the flow the state space of the basic state machine governing that step.
diff --git a/examples/Crem/Example/TwoSwitchesGate.lhs b/examples/Crem/Example/TwoSwitchesGate.lhs
deleted file mode 100644
index 0a844e0..0000000
--- a/examples/Crem/Example/TwoSwitchesGate.lhs
+++ /dev/null
@@ -1,243 +0,0 @@
-> {-# LANGUAGE DataKinds #-}
-> {-# LANGUAGE DerivingVia #-}
-> {-# LANGUAGE TemplateHaskell #-}
-> {-# LANGUAGE TypeFamilies #-}
-> {-# LANGUAGE UndecidableInstances #-}
-> -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wmissing-deriving-strategies
-> {-# OPTIONS_GHC -Wno-missing-deriving-strategies #-}
-> -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunticked-promoted-constructors
-> {-# OPTIONS_GHC -Wno-unticked-promoted-constructors #-}
-> -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunused-type-patterns
-> {-# OPTIONS_GHC -Wno-unused-type-patterns #-}
->
-> module Crem.Example.TwoSwitchesGate where
->
-> import "crem" Crem.BaseMachine
-> import "crem" Crem.Render.Render
-> import "crem" Crem.Render.RenderableVertices (AllVertices(..), RenderableVertices)
-> import "crem" Crem.Render.RenderFlow
-> import "crem" Crem.StateMachine
-> import "crem" Crem.Topology
-> import "base" Data.Functor.Identity
-> import "profunctors" Data.Profunctor
-> import "singletons-base" Data.Singletons.Base.TH
-> import "text" Data.Text (pack)
-
-We would like to implement a gate opening mechanism controlled by two switches. We would like the gate to open only when the two switches are on.
-
-We would like to implement this by composing several small state machines: one for every switch, one for making sure that we actually receive the right message from both switches, and one for actually opening the gate.
-
-Let's start with the switch.
-
-The first thing we need to do is to define the topology of our machine, meaning the allowed transitions in its state space.
-
-For a switch, there are only two states. Either the switch is on or it is off.
-
-Moreover, we want those switches to be usable only once, and therefore we want to forbid the transition from the `on`` to the `off` position. In other terms, we allow only to go from the `off` position to the `on` position.
-
-> $( singletons
->     [d|
->       data SwitchVertex
->         = SwitchIsOn
->         | SwitchIsOff
->         deriving stock (Eq, Show, Bounded, Enum)
->
->       switchTopology :: Topology SwitchVertex
->       switchTopology = Topology
->         [(SwitchIsOff, [ SwitchIsOn ])]
->     |]
->  )
-
-Notice that we need to wrap this in `singletons` because we will soon need to use this data type as a kind, to store information in the type of our state machines.
-
-We need also an instance of `RenderableVertices SwitchVertex` to decide which vertices to render for our machine. To obtain that, we use `deriving via` together with the `AllVertices` newtype.
-
-> deriving via AllVertices SwitchVertex instance RenderableVertices SwitchVertex
-
-Next we need to define which data every vertex of our topology should contain. To express that we use a generalized algebraid data type indexed with `SwitchVertex`
-
-> data SwitchState (vertex :: SwitchVertex) where
->   OnState :: SwitchState 'SwitchIsOn
->   OffState :: SwitchState 'SwitchIsOff
-
-In this case, for every vertex there is just one possible state.
-
-At this point we need to define which inputs our machine should handle and which outputs it should emit. In the case there is only one meaningful input, the request of turning on the switch, and one meaningful output, the notification that the switch has been turned on.
-
-> data SwitchInput = TurnOn
->
-> data SwitchOutput = TurnedOn
->   deriving stock Show
->
-> instance Semigroup SwitchOutput where
->   TurnedOn <> TurnedOn = TurnedOn
-
-At this point we can actually implement our switch as a `BaseMachine`
-
-> switch :: () -> BaseMachine SwitchTopology SwitchInput SwitchOutput
-> switch _ =
->   BaseMachineT
->     { initialState = InitialState OffState
->     , action = \case
->         OnState -> \_ -> pureResult TurnedOn OnState
->         OffState -> \_ -> pureResult TurnedOn OnState
->     }
-
-We start from the `OffState` and every time we receive a request to turn the switch on, we return a message informing the external world that the switch in turned on and we update the state accordingly if needed.
-
-Since we need two separate switches, we can create them by invoking the `switch` function twice
-
-> switch1 :: BaseMachine SwitchTopology SwitchInput SwitchOutput
-> switch1 = switch ()
->
-> switch2 :: BaseMachine SwitchTopology SwitchInput SwitchOutput
-> switch2 = switch ()
-
-This concludes the implementation of our switch machine. Next, we would like to implement a machine which receives as inputs the output of two switches and emits a message whenever both the switches have been turned on.
-
-Again, we need to start thinking about the topology of our machine. Since we need to track the state of the two switches, we will have four vertices
-
-> $( singletons
->     [d|
->       data BothVertex
->         = NoSwitchOn
->         | OnlyFirstSwitchOn
->         | OnlySecondSwitchOn
->         | BothSwitchesOn
->         deriving (Eq, Show, Enum, Bounded)
->
->       bothTopology :: Topology BothVertex
->       bothTopology = Topology
->         [ (NoSwitchOn, [OnlyFirstSwitchOn, OnlySecondSwitchOn])
->         , (OnlyFirstSwitchOn, [BothSwitchesOn])
->         , (OnlySecondSwitchOn, [BothSwitchesOn])
->         ]
->     |]
->  )
->
-> deriving via AllVertices BothVertex instance RenderableVertices BothVertex
-
-The topology again constrains the machine with the invariant the we can only turn on switches.
-
-Next we need to define the state space, assigning a data type to every vertex in the topology. In this case we don't have the need to attach data to our vertices so we can simply define
-
-> data BothState (vertex :: BothVertex) where
->   NoSwitchOnState :: BothState 'NoSwitchOn
->   OnlyFirstSwitchOnState :: BothState 'OnlyFirstSwitchOn
->   OnlySecondSwitchOnState :: BothState 'OnlySecondSwitchOn
->   BothSwitchesOnState :: BothState 'BothSwitchesOn
-
-Before defining the logic of the machine, we need to define its inputs and outputs. Since we would like it to monitor the outputs of both switches, its input type could be
-
-> type BothInput = Either SwitchOutput SwitchOutput
-
-Its output instead will be a potential message to actually open the gate
-
-> data OpenGate = OpenGate
->
-> type BothOutput = Maybe OpenGate
-
-and eventually we can define the logic of our state machine
-
-> bothMachine :: BaseMachine BothTopology BothInput BothOutput
-> bothMachine =
->   BaseMachineT
->     { initialState = InitialState NoSwitchOnState
->     , action = \case
->         NoSwitchOnState -> \case
->           Left _ -> pureResult Nothing OnlyFirstSwitchOnState
->           Right _ -> pureResult Nothing OnlySecondSwitchOnState
->         OnlyFirstSwitchOnState -> \case
->           Left _ -> pureResult Nothing OnlyFirstSwitchOnState
->           Right _ -> pureResult (Just OpenGate) BothSwitchesOnState
->         OnlySecondSwitchOnState -> \case
->           Left _ -> pureResult (Just OpenGate) BothSwitchesOnState
->           Right _ -> pureResult Nothing OnlySecondSwitchOnState
->         BothSwitchesOnState -> \_ -> pureResult Nothing BothSwitchesOnState
->     }
-
-The last machine that we need is one representing the actual gate. Since the logic is exactly the same as the one of the switches, we can actually reuse what we defined above
-
-> gate :: BaseMachine SwitchTopology SwitchInput SwitchOutput
-> gate = switch ()
-
-Now we have all the machines we wanted and we need to connect them appropriately.
-
-We have the two switches which produce a `SwitchOutput` and the `bothMachine` which accepts inputs of type `Either SwitchOutput SwitchOutput`.
-
-We need to pair up the two switches, first, and then connect them to the `bothMachine`. We need to pair the two switches in a way that allows us to decide whether to run one or the other: this is exactly what the `Alternative` constructor of the `StateMachineT` data type allows us to do.
-
-> switches :: StateMachine (Either SwitchInput SwitchInput) (Either SwitchOutput SwitchOutput)
-> switches = Basic switch1 `Alternative` Basic switch2
-
-Notice that we had to wrap our `switch` machines with `Basic` to turn them into `StateMachine`s, which is the more composable type used by `Alternative`.
-
-Now we have the output of `switches` which coincides with the input of `bothMachine`, and therefore we can pass every output we get from `switches` to `bothMachine`. We use the `Sequential` constractor exactly for this
-
-> bothSwitches :: StateMachine (Either SwitchInput SwitchInput) BothOutput
-> bothSwitches = switches `Sequential` Basic bothMachine
-
-Now we have a machine which emits `BothOutput = Maybe OpenGate`. Our `gate` machine on the other hand accepts inputs of type `SwitchInput`. To connect those, we need to do some adjusting.
-
-First, we can translate an `OpenGate` into a `SwitchInput`
-
-> openGateToSwitchInput :: OpenGate -> SwitchInput
-> openGateToSwitchInput OpenGate = TurnOn
-
-and we can use this function to adapt our `gate` machine so that it accepts `OpenGate` as input.
-
-> gate' :: BaseMachine SwitchTopology OpenGate SwitchOutput
-> gate' = lmap openGateToSwitchInput gate
-
-Still `bothSwitches` emits values of type `Maybe OpenGate`. We could lift our `gate'` machine to `Maybe OpenGate` inputs using the `maybeM` combinator.
-
-> maybeGate :: BaseMachine SwitchTopology (Maybe OpenGate) (Maybe SwitchOutput)
-> maybeGate = maybeM gate'
-
-At this point we could conclude our composition, joining together `bothMachine` and `maybeGate`
-
-> gateMachine :: StateMachine (Either SwitchInput SwitchInput) (Maybe SwitchOutput)
-> gateMachine = bothSwitches `Sequential` Basic maybeGate
-
-Now we have a single machine which describes out whole flow.
-
-Now, there are two things which we could do with `gateMachine`.
-
-The first thing is actually executing it. To do it we can use the `runMultiple` function.
-
-We can try to to turn on both switches and verify that the gate actually opened
-
-> -- |
-> -- >>> openedGate
-> -- Just TurnedOn
-> openedGate :: Maybe SwitchOutput
-> openedGate = fst . runIdentity $ runMultiple gateMachine [Left TurnOn, Right TurnOn]
-
-Or we can turn just the first switch several times without opening the gate
-
-> -- |
-> -- >>> closedGate
-> -- Nothing
-> closedGate :: Maybe SwitchOutput
-> closedGate = fst . runIdentity $ runMultiple gateMachine [Left TurnOn, Left TurnOn, Left TurnOn]
-
-The other thing we can do is actually rendering a diagram representing how the `gateMachine` works.
-
-The best rendering we can get displays the flow of the machine and the state space for every step of the flow
-
-> -- |
-> -- >>> gateFlow
-> -- Right "state switch1 {\nswitch1_SwitchIsOn\nswitch1_SwitchIsOff\nswitch1_SwitchIsOff --> switch1_SwitchIsOn\n}\nstate switch2 {\nswitch2_SwitchIsOn\nswitch2_SwitchIsOff\nswitch2_SwitchIsOff --> switch2_SwitchIsOn\n}\nstate fork_choice_switch1switch2 <<choice>>\nstate join_choice_switch1switch2 <<choice>>\nfork_choice_switch1switch2 --> switch1\nfork_choice_switch1switch2 --> switch2\nswitch1 --> join_choice_switch1switch2\nswitch2 --> join_choice_switch1switch2\nstate both {\nboth_NoSwitchOn\nboth_OnlyFirstSwitchOn\nboth_OnlySecondSwitchOn\nboth_BothSwitchesOn\nboth_NoSwitchOn --> both_OnlyFirstSwitchOn\nboth_NoSwitchOn --> both_OnlySecondSwitchOn\nboth_OnlyFirstSwitchOn --> both_BothSwitchesOn\nboth_OnlySecondSwitchOn --> both_BothSwitchesOn\n}\njoin_choice_switch1switch2 --> both\nstate gate {\ngate_SwitchIsOn\ngate_SwitchIsOff\ngate_SwitchIsOff --> gate_SwitchIsOn\n}\nboth --> gate"
-> gateFlow :: Either String Mermaid
-> gateFlow = (\(mermaid, _, _) -> mermaid) <$>
->   renderFlow
->     (BinaryLabel
->        (BinaryLabel
->           (BinaryLabel
->             (LeafLabel . MachineLabel . pack $ "switch1")
->             (LeafLabel . MachineLabel . pack $ "switch2"))
->           (LeafLabel . MachineLabel . pack $ "both"))
->        (LeafLabel . MachineLabel . pack $ "gate"))
->     (gateMachine @Identity)
-
-The result is a diagram which looks like [this](https://mermaid.live/edit#pako:eNqNVN9vgjAQ_lfIPYORCgjE-LBsS_aw-eDbQkI6KMKU1kDd5oz_-0qhcwyr8tAf331333G99gAJSwmEUHPMyX2BVxUurQ8UUQkY9WfBk9w2DhE1xNdt46Wcn-oF1eBZpjUYljU_G-jYF0V9UaQRRTpRpBFF50UzVq3jJGdFQuIuO5XHbNbi87kiv7OC3kC-EPNPEW7kNWfSnUYD6lNQPHSdJ3_mjfFc1bpZxy-srZAqtAQXdLN_LKqaa2xLkjCanjHeiaGFSd0znGRkoldVdHyt8iDYyU-b1DCg3kn0zoVGUG6qzKtm6MrcrAf93AdVM_9HZdyBv0hFHqMyggklqUpcpOJuS9EIeE5KEkEolimu1hEIN8HbbVPh8JAWnFUQZnhTExPwjrPlniYQ8mpHFKl7H35ZW0xfGevtITzAF4RWYAfeaDwOpp7rIdeEPYROgEaub6PA95GY3KMJ39I7GE0dx5-4E9vxJv50LOhEpvPcvk3yiTr-AO8tobo) where you can clearly see the overall structure of the machine we created, and for every step of the flow the state space of the basic state machine governing that step.
diff --git a/examples/Crem/Example/Uno.hs b/examples/Crem/Example/Uno.hs
index 923a756..e3ae8e6 100644
--- a/examples/Crem/Example/Uno.hs
+++ b/examples/Crem/Example/Uno.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 {-# HLINT ignore "Redundant id" #-}
 {-# LANGUAGE DeriveAnyClass #-}
@@ -7,6 +8,14 @@
 {-# LANGUAGE UndecidableInstances #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wmissing-deriving-strategies
 {-# OPTIONS_GHC -Wno-missing-deriving-strategies #-}
+
+#if __GLASGOW_HASKELL__ >= 908
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-poly-kind-signatures
+{-# OPTIONS_GHC -Wno-missing-poly-kind-signatures #-}
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-role-annotations
+{-# OPTIONS_GHC -Wno-missing-role-annotations #-}
+#endif
+
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunrecognised-pragmas
 {-# OPTIONS_GHC -Wno-unrecognised-pragmas #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunticked-promoted-constructors
@@ -168,15 +177,15 @@ data Event
 -- * Topology
 
 $( singletons
-    [d|
-      data UnoVertex
-        = Initial
-        | Started
-        deriving stock (Eq, Show, Enum, Bounded)
-
-      unoTopology :: Topology UnoVertex
-      unoTopology = Topology [(Initial, [Started])]
-      |]
+     [d|
+       data UnoVertex
+         = Initial
+         | Started
+         deriving stock (Eq, Show, Enum, Bounded)
+
+       unoTopology :: Topology UnoVertex
+       unoTopology = Topology [(Initial, [Started])]
+       |]
  )
 
 deriving via AllVertices UnoVertex instance RenderableVertices UnoVertex
@@ -192,6 +201,8 @@ data UnoState (vertex :: UnoVertex) where
   UnoInitialState :: UnoState 'Initial
   UnoStartedState :: StateData -> UnoState 'Started
 
+type role UnoState nominal
+
 -- * Errors
 
 data GameError
diff --git a/flake.lock b/flake.lock
index 8624f85..7699d76 100644
--- a/flake.lock
+++ b/flake.lock
@@ -3,11 +3,11 @@
     "flake-compat": {
       "flake": false,
       "locked": {
-        "lastModified": 1673956053,
-        "narHash": "sha256-4gtG9iQuiKITOjNQQeQIpoIB6b16fm+504Ch3sNKLd8=",
+        "lastModified": 1767039857,
+        "narHash": "sha256-vNpUSpF5Nuw8xvDLj2KCwwksIbjua2LZCqhV1LNRDns=",
         "owner": "edolstra",
         "repo": "flake-compat",
-        "rev": "35bb57c0c8d8b62bbfd284272c928ceb64ddbde9",
+        "rev": "5edf11c44bc78a0d334f6334cdaf7d60d732daab",
         "type": "github"
       },
       "original": {
@@ -21,11 +21,11 @@
         "systems": "systems"
       },
       "locked": {
-        "lastModified": 1681202837,
-        "narHash": "sha256-H+Rh19JDwRtpVPAWp64F+rlEtxUWBAQW28eAi3SRSzg=",
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
         "owner": "numtide",
         "repo": "flake-utils",
-        "rev": "cfacdce06f30d2b68473a46042957675eebb3401",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
         "type": "github"
       },
       "original": {
@@ -36,11 +36,11 @@
     },
     "nix-filter": {
       "locked": {
-        "lastModified": 1681154353,
-        "narHash": "sha256-MCJ5FHOlbfQRFwN0brqPbCunLEVw05D/3sRVoNVt2tI=",
+        "lastModified": 1757882181,
+        "narHash": "sha256-+cCxYIh2UNalTz364p+QYmWHs0P+6wDhiWR4jDIKQIU=",
         "owner": "numtide",
         "repo": "nix-filter",
-        "rev": "f529f42792ade8e32c4be274af6b6d60857fbee7",
+        "rev": "59c44d1909c72441144b93cf0f054be7fe764de5",
         "type": "github"
       },
       "original": {
@@ -51,11 +51,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1681303793,
-        "narHash": "sha256-JEdQHsYuCfRL2PICHlOiH/2ue3DwoxUX7DJ6zZxZXFk=",
+        "lastModified": 1773821835,
+        "narHash": "sha256-TJ3lSQtW0E2JrznGVm8hOQGVpXjJyXY2guAxku2O9A4=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "fe2ecaf706a5907b5e54d979fbde4924d84b65fc",
+        "rev": "b40629efe5d6ec48dd1efba650c797ddbd39ace0",
         "type": "github"
       },
       "original": {
@@ -67,16 +67,16 @@
     },
     "nixpkgs-stable": {
       "locked": {
-        "lastModified": 1681269223,
-        "narHash": "sha256-i6OeI2f7qGvmLfD07l1Az5iBL+bFeP0RHixisWtpUGo=",
+        "lastModified": 1773814637,
+        "narHash": "sha256-GNU+ooRmrHLfjlMsKdn0prEKVa0faVanm0jrgu1J/gY=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "87edbd74246ccdfa64503f334ed86fa04010bab9",
+        "rev": "fea3b367d61c1a6592bc47c72f40a9f3e6a53e96",
         "type": "github"
       },
       "original": {
         "owner": "nixos",
-        "ref": "nixos-22.11",
+        "ref": "nixos-25.11",
         "repo": "nixpkgs",
         "type": "github"
       }
diff --git a/flake.nix b/flake.nix
index 582c821..3f75f57 100644
--- a/flake.nix
+++ b/flake.nix
@@ -3,7 +3,7 @@
 
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
-    nixpkgs-stable.url = "github:nixos/nixpkgs/nixos-22.11";
+    nixpkgs-stable.url = "github:nixos/nixpkgs/nixos-25.11";
     flake-utils.url = "github:numtide/flake-utils";
     nix-filter.url = "github:numtide/nix-filter";
     flake-compat = {
@@ -38,13 +38,13 @@
         haskellPackages.override {
           overrides = self: super: {
             hpack = pkgs.hpack;
-            crem = pkgs.haskell.lib.compose.disableCabalFlag "test-doctest" ((self.callCabal2nix "crem" src { }).overrideAttrs (attrs: {
+            crem = (self.callCabal2nix "crem" src { }).overrideAttrs (attrs: {
               # doctest-parallel needs to know where the compiled crem package is
               preCheck = ''
                 export GHC_PACKAGE_PATH="dist/package.conf.inplace:$GHC_PACKAGE_PATH"
               '';
-            }));
-            fourmolu = pkgs.haskell.packages.ghc944.fourmolu;
+            });
+            fourmolu = pkgs.haskell.packages."ghc${ghcVersion}".fourmolu;
           };
         };
 
@@ -62,7 +62,7 @@
           configurations;
 
       # The version of GHC used for default package and development shell.
-      defaultGhcVersion = "ghc90";
+      defaultGhcVersion = "ghc910";
 
       # This is a shell utility that watches source files for changes, and triggers a
       # command when they change.
diff --git a/nix/haskell-configurations.nix b/nix/haskell-configurations.nix
index 0f0e1bd..b062b59 100644
--- a/nix/haskell-configurations.nix
+++ b/nix/haskell-configurations.nix
@@ -12,4 +12,7 @@
   }
   { ghcVersion = "92"; }
   { ghcVersion = "94"; }
+  { ghcVersion = "96"; }
+  { ghcVersion = "98"; }
+  { ghcVersion = "910"; }
 ]
diff --git a/package.yaml b/package.yaml
index 29f4292..55535e9 100644
--- a/package.yaml
+++ b/package.yaml
@@ -16,9 +16,11 @@ description:
   flow and their state space."
 tested-with:
   - GHC ==9.0.2
-  - GHC ==9.2.7
-  - GHC ==9.4.4
-  - GHC ==9.6.1
+  - GHC ==9.2.8
+  - GHC ==9.4.8
+  - GHC ==9.6.7
+  - GHC ==9.8.4
+  - GHC ==9.10.3
 
 extra-source-files:
   - README.md
@@ -27,13 +29,8 @@ extra-source-files:
 flags:
   errors:
     description: enable -Werror
-    default: False
-    manual: True
-
-  test-doctest:
-    description: run doctests
-    manual: True
     default: True
+    manual: True
 
 when:
   - condition: impl(ghc >= 9.2)
@@ -99,6 +96,7 @@ default-extensions:
   - DerivingStrategies # https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/deriving_strategies.html#extension-DerivingStrategies
   - LambdaCase         # https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/lambda_case.html#extension-LambdaCase
   - PackageImports     # https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/package_qualified_imports.html#extension-PackageImports
+  - RoleAnnotations    # https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/roles.html#extension-RoleAnnotations
 
 # Options inspired by: https://medium.com/mercury-bank/enable-all-the-warnings-a0517bc081c3
 ghc-options:
@@ -117,16 +115,16 @@ ghc-options:
   - -Wno-prepositive-qualified-module  # https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wprepositive-qualified-module
 
 dependencies:
-  - base >= 4.15 && < 4.19
+  - base >= 4.15 && < 4.21
 
 library:
   source-dirs:      src
   dependencies:
-    - profunctors >= 3.2 && < 5.7
-    - singletons-base >= 3.0 && < 3.3
-    - text >= 1.2 && < 2.1
-    - nothunks >= 0.1 && < 0.4
-    - machines >=0.7.3 && <0.8
+      - profunctors >= 3.2 && < 5.7
+      - text >= 1.2 && < 2.2
+      - nothunks >= 0.1 && < 0.4
+      - machines >=0.7.3 && <0.8
+      - singletons-base >= 3.0 && < 3.5
   # Disable adding Paths_crem to other-modules, because it does not conform to our style guide.
   # https://github.com/sol/hpack#handling-of-paths_-modules
   when:
@@ -172,13 +170,10 @@ tests:
     dependencies:
       - crem
       - crem-examples
-      - doctest-parallel >= 0.2.3 && < 0.4
+      - doctest-parallel >= 0.2.3 && < 0.5
+      - ghc
+      - Cabal
     when:
-    - condition: flag(test-doctest)
-      then:
-        buildable: True
-      else:
-        buildable: False
     - condition: false
       other-modules: Paths_crem
 
diff --git a/spec/Crem/Render/RenderFlowSpec.hs b/spec/Crem/Render/RenderFlowSpec.hs
index 792f142..67a2d09 100644
--- a/spec/Crem/Render/RenderFlowSpec.hs
+++ b/spec/Crem/Render/RenderFlowSpec.hs
@@ -20,7 +20,7 @@ spec =
           `shouldBe` Right
             ( Mermaid "state lockMachine {"
                 <> ( renderLabelledGraph "lockMachine" . baseMachineAsGraph @_ @_ @_ @_ @Identity $
-                      lockDoorMachine SIsLockClosed
+                       lockDoorMachine SIsLockClosed
                    )
                 <> Mermaid "}"
             , MachineLabel "lockMachine"
diff --git a/spec/Crem/RiskManagerSpec.hs b/spec/Crem/RiskManagerSpec.hs
index b4fd202..f171315 100644
--- a/spec/Crem/RiskManagerSpec.hs
+++ b/spec/Crem/RiskManagerSpec.hs
@@ -136,58 +136,58 @@ spec =
       it "registers one user when a registration event is received" $ do
         run readModel (UserDataRegistered myUserData)
           `shouldOutput` [ ReceivedData
-                            { receivedUserData = Just myUserData
-                            , receivedLoanDetails = Nothing
-                            , receivedCreditBureauData = Nothing
-                            }
+                             { receivedUserData = Just myUserData
+                             , receivedLoanDetails = Nothing
+                             , receivedCreditBureauData = Nothing
+                             }
                          ]
 
       it "registers two users when two registration events are received" $ do
         runMultiple readModel [UserDataRegistered myUserData, UserDataRegistered notMyUserData]
           `shouldOutput` [ ReceivedData
-                            { receivedUserData = Just myUserData
-                            , receivedLoanDetails = Nothing
-                            , receivedCreditBureauData = Nothing
-                            }
+                             { receivedUserData = Just myUserData
+                             , receivedLoanDetails = Nothing
+                             , receivedCreditBureauData = Nothing
+                             }
                          , ReceivedData
-                            { receivedUserData = Just notMyUserData
-                            , receivedLoanDetails = Nothing
-                            , receivedCreditBureauData = Nothing
-                            }
+                             { receivedUserData = Just notMyUserData
+                             , receivedLoanDetails = Nothing
+                             , receivedCreditBureauData = Nothing
+                             }
                          ]
 
     describe "whole" $ do
       it "registers one user when a registration command is received" $ do
         run whole (RegisterUserData myUserData)
           `shouldOutput` [ ReceivedData
-                            { receivedUserData = Just myUserData
-                            , receivedLoanDetails = Nothing
-                            , receivedCreditBureauData = Nothing
-                            }
+                             { receivedUserData = Just myUserData
+                             , receivedLoanDetails = Nothing
+                             , receivedCreditBureauData = Nothing
+                             }
                          , ReceivedData
-                            { receivedUserData = Just myUserData
-                            , receivedLoanDetails = Nothing
-                            , receivedCreditBureauData = Just creditBureauData
-                            }
+                             { receivedUserData = Just myUserData
+                             , receivedLoanDetails = Nothing
+                             , receivedCreditBureauData = Just creditBureauData
+                             }
                          ]
 
       it "registers two users when two registration commands are received" $ do
         runMultiple whole [RegisterUserData myUserData, RegisterUserData notMyUserData]
           `shouldOutput` [ ReceivedData
-                            { receivedUserData = Just myUserData
-                            , receivedLoanDetails = Nothing
-                            , receivedCreditBureauData = Nothing
-                            }
+                             { receivedUserData = Just myUserData
+                             , receivedLoanDetails = Nothing
+                             , receivedCreditBureauData = Nothing
+                             }
                          , ReceivedData
-                            { receivedUserData = Just myUserData
-                            , receivedLoanDetails = Nothing
-                            , receivedCreditBureauData = Just creditBureauData
-                            }
+                             { receivedUserData = Just myUserData
+                             , receivedLoanDetails = Nothing
+                             , receivedCreditBureauData = Just creditBureauData
+                             }
                          , ReceivedData
-                            { receivedUserData = Just notMyUserData
-                            , receivedLoanDetails = Nothing
-                            , receivedCreditBureauData = Just creditBureauData
-                            }
+                             { receivedUserData = Just notMyUserData
+                             , receivedLoanDetails = Nothing
+                             , receivedCreditBureauData = Just creditBureauData
+                             }
                          ]
 
     describe "riskApplication" $ do
diff --git a/src/Crem/BaseMachine.hs b/src/Crem/BaseMachine.hs
index f155262..75e7bbe 100644
--- a/src/Crem/BaseMachine.hs
+++ b/src/Crem/BaseMachine.hs
@@ -25,7 +25,8 @@ data
     m
     (topology :: Topology vertex)
     (input :: Type)
-    (output :: Type) = forall state.
+    (output :: Type)
+  = forall state.
   BaseMachineT
   { initialState :: InitialState state
   , action
@@ -35,6 +36,8 @@ data
       -> ActionResult m topology state initialVertex output
   }
 
+type role BaseMachineT representational nominal representational nominal
+
 -- | A `BaseMachine` is an effectful machine for every possible monad @m@.
 -- Needing to work for every monad, in fact it can not perform any kind of
 -- effect and needs to be pure in nature.
@@ -113,6 +116,8 @@ instance (Applicative m) => Choice (BaseMachineT m topology) where
 data InitialState (state :: vertex -> Type) where
   InitialState :: state vertex -> InitialState state
 
+type role InitialState representational
+
 -- | The result of an action of the state machine.
 -- An @ActionResult m topology state initialVertex output@ contains an @output@
 -- and a @state finalVertex@, where the transition from @initialVertex@ to
@@ -130,6 +135,8 @@ data
     => m (output, state finalVertex)
     -> ActionResult m topology state initialVertex output
 
+type role ActionResult representational nominal nominal nominal nominal
+
 -- | Allows to change the computational context of an `ActionResult` from @m@
 -- to @n@, given we have a [natural transformation](https://stackoverflow.com/a/58364172/2718064)
 -- from @m@ to @n@.
diff --git a/src/Crem/Decider.hs b/src/Crem/Decider.hs
index 1d467ae..5f0809f 100644
--- a/src/Crem/Decider.hs
+++ b/src/Crem/Decider.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 
 -- | The [Decider pattern](https://thinkbeforecoding.com/post/2021/12/17/functional-event-sourcing-decider)
@@ -10,7 +11,11 @@ module Crem.Decider where
 
 import Crem.BaseMachine (ActionResult (..), BaseMachine, BaseMachineT (..), InitialState (..))
 import Crem.Topology (AllowedTransition, Topology)
-import Data.Foldable (foldl')
+
+#if __GLASGOW_HASKELL__ < 910
+import "base" Data.Foldable (foldl')
+#endif
+
 import "base" Data.Kind (Type)
 
 -- | A @Decider topology input output@ is a Decider which receives inputs of
@@ -31,7 +36,8 @@ data
   Decider
     (topology :: Topology vertex)
     input
-    output = forall state.
+    output
+  = forall state.
   Decider
   { deciderInitialState :: InitialState state
   , decide :: forall vertex'. input -> state vertex' -> output
@@ -42,6 +48,8 @@ data
       -> EvolutionResult topology state initialVertex output
   }
 
+type role Decider nominal representational representational
+
 -- | A smart wrapper over the machine state, which allows to enforce that only
 -- transitions allowed by the @topology@ are actually performed.
 data
@@ -49,13 +57,15 @@ data
     (topology :: Topology vertex)
     (state :: vertex -> Type)
     (initialVertex :: vertex)
-    output
+    (output :: k)
   where
   EvolutionResult
     :: (AllowedTransition topology initialVertex finalVertex)
     => state finalVertex
     -> EvolutionResult topology state initialVertex output
 
+type role EvolutionResult nominal representational nominal phantom
+
 -- | translate a `Decider` into a `BaseMachine`
 deciderMachine
   :: Decider topology input output
diff --git a/src/Crem/Graph.hs b/src/Crem/Graph.hs
index 03f14ad..cdec402 100644
--- a/src/Crem/Graph.hs
+++ b/src/Crem/Graph.hs
@@ -12,6 +12,8 @@ newtype Graph a = Graph [(a, a)]
   deriving stock (Eq, Show)
   deriving newtype (NoThunks)
 
+type role Graph representational
+
 -- | The product graph.
 -- It has as vertices the product of the set of vertices of the initial graph.
 -- It has as edge from @(a1, b1)@ to @(a2, b2)@ if and only if there is an edge
diff --git a/src/Crem/Render/RenderFlow.hs b/src/Crem/Render/RenderFlow.hs
index 4b3d118..98d9471 100644
--- a/src/Crem/Render/RenderFlow.hs
+++ b/src/Crem/Render/RenderFlow.hs
@@ -21,7 +21,9 @@ data TreeMetadata a
   | BinaryLabel (TreeMetadata a) (TreeMetadata a)
   deriving stock (Show)
 
-instance NoThunks a => NoThunks (TreeMetadata a) where
+type role TreeMetadata representational
+
+instance (NoThunks a) => NoThunks (TreeMetadata a) where
   showTypeOf _ = "TreeMetadata"
   wNoThunks ctxt tm =
     case tm of
diff --git a/src/Crem/Render/RenderableVertices.hs b/src/Crem/Render/RenderableVertices.hs
index 4a0b023..a424a86 100644
--- a/src/Crem/Render/RenderableVertices.hs
+++ b/src/Crem/Render/RenderableVertices.hs
@@ -23,6 +23,8 @@ class RenderableVertices a where
 -- `RenderableVertices` which lists all the terms of type @a@.
 newtype AllVertices a = AllVertices a
 
+type role AllVertices representational
+
 instance (Enum a, Bounded a) => RenderableVertices (AllVertices a) where
   vertices :: [AllVertices a]
   vertices = AllVertices <$> [minBound .. maxBound]
diff --git a/src/Crem/StateMachine.hs b/src/Crem/StateMachine.hs
index 3baf2b2..0eb8439 100644
--- a/src/Crem/StateMachine.hs
+++ b/src/Crem/StateMachine.hs
@@ -77,6 +77,8 @@ data StateMachineT m input output where
     -> StateMachineT m b (n c)
     -> StateMachineT m a (n c)
 
+type role StateMachineT representational nominal nominal
+
 instance NoThunks (StateMachineT m input output) where
   showTypeOf _ = "StateMachineT"
   wNoThunks ctxt sm =
diff --git a/src/Crem/Topology.hs b/src/Crem/Topology.hs
index 7a238ce..bac62eb 100644
--- a/src/Crem/Topology.hs
+++ b/src/Crem/Topology.hs
@@ -1,7 +1,21 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
+
+#if __GLASGOW_HASKELL__ >= 908
+{-# LANGUAGE TypeAbstractions #-}
+#endif
+
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE UndecidableInstances #-}
+
+#if __GLASGOW_HASKELL__ >= 908
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-poly-kind-signatures
+{-# OPTIONS_GHC -Wno-missing-poly-kind-signatures #-}
+-- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag-Wmissing-role-annotations
+{-# OPTIONS_GHC -Wno-missing-role-annotations #-}
+#endif
+
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wredundant-constraints
 {-# OPTIONS_GHC -Wno-redundant-constraints #-}
 -- https://downloads.haskell.org/ghc/latest/docs/users_guide/using-warnings.html#ghc-flag--Wunticked-promoted-constructors
@@ -37,10 +51,10 @@ import "singletons-base" Prelude.Singletons
 -- Since we are using this information at the type level, and then we want to
 -- bring it down to the value level, we wrap it in `singletons`
 $( singletons
-    [d|
-      newtype Topology vertex = Topology
-        {edges :: [(vertex, [vertex])]}
-      |]
+     [d|
+       newtype Topology vertex = Topology
+         {edges :: [(vertex, [vertex])]}
+       |]
  )
 
 -- ** AllowedTransition
@@ -62,8 +76,8 @@ data AllowTransition (topology :: Topology vertex) (initial :: vertex) (final ::
   -- | If we know that we have an edge from @a@ to @b@ in @map@,
   -- then we also have an edge from @a@ to @b@ if we add another vertex
   AllowAddingVertex
-    :: AllowTransition ('Topology map) a b
-    -> AllowTransition ('Topology (x ': map)) a b
+    :: AllowTransition ('Topology tmap) a b
+    -> AllowTransition ('Topology (x ': tmap)) a b
 
 instance NoThunks (AllowTransition topology initial final) where
   showTypeOf _ = "AllowTransition"
@@ -87,9 +101,9 @@ instance {-# INCOHERENT #-} (AllowedTransition ('Topology ('(a, l1) ': l2)) a b)
   allowsTransition =
     AllowAddingEdge (allowsTransition :: AllowTransition ('Topology ('(a, l1) ': l2)) a b)
 
-instance {-# INCOHERENT #-} (AllowedTransition ('Topology map) a b) => AllowedTransition ('Topology (x ': map)) a b where
+instance {-# INCOHERENT #-} (AllowedTransition ('Topology tmap) a b) => AllowedTransition ('Topology (x ': tmap)) a b where
   allowsTransition =
-    AllowAddingVertex (allowsTransition :: AllowTransition ('Topology map) a b)
+    AllowAddingVertex (allowsTransition :: AllowTransition ('Topology tmap) a b)
 
 instance {-# INCOHERENT #-} AllowedTransition topology a a where
   allowsTransition = AllowIdentityEdge
@@ -100,10 +114,10 @@ instance {-# INCOHERENT #-} AllowedTransition topology a a where
 -- Given a type @a@ for vertices, only trivial transitions, i.e. staying
 -- at the same vertex, are allowed
 $( singletons
-    [d|
-      trivialTopology :: Topology a
-      trivialTopology = Topology []
-      |]
+     [d|
+       trivialTopology :: Topology a
+       trivialTopology = Topology []
+       |]
  )
 
 -- ** Allow all topology
@@ -111,8 +125,8 @@ $( singletons
 -- | Given a type @a@ for vertices, every transition from one vertex to
 -- any other is allowed
 $( singletons
-    [d|
-      allowAllTopology :: (Bounded a, Enum a) => Topology a
-      allowAllTopology = Topology [(a, [minBound .. maxBound]) | a <- [minBound .. maxBound]]
-      |]
+     [d|
+       allowAllTopology :: (Bounded a, Enum a) => Topology a
+       allowAllTopology = Topology [(a, [minBound .. maxBound]) | a <- [minBound .. maxBound]]
+       |]
  )