Add PolyKinds as default extension to clash-prelude

martijnbastiaan · martijnbastiaan · commit a2f6777ee355 · 2025-09-23T17:51:25.000+02:00
`NoPolyKinds` has confused us multiple times because it would needlessly constrain instances to `SomeClass (t :: Type)` instead of the more general `SomeClass (t :: k)`. See the changelog in this commit for more information. This also greatly simplifies the reproducer for #1242, as we couldn't get the behavior we wanted with a local `BitPack` definition -- necessary because instances such as the ones in T1242 are not possible anymore. The current code in T1242 is built to make sure Clash sees an unresolved type family which it *has* to resolve in order to produce HDL. The reproducer was tested by manually reverting the patch for #1242.
diff --git a/changelog/2025-08-25T09_57_02+02_00_enable_polykinds_for_clash_prelude b/changelog/2025-08-25T09_57_02+02_00_enable_polykinds_for_clash_prelude
@@ -0,0 +1 @@
+CHANGED: The `clash-prelude` package now uses `PolyKinds` by default, meaning all its type class instances are now more general than before. As an example, without `PolyKinds`, `BitPack (Proxy a)` would only be defined when `a :: Type`, with the extension it would be defined for any  `a :: k`. After this change, some instances in user code might now overlap.
diff --git a/clash-prelude/LICENSE b/clash-prelude/LICENSE
@@ -1,6 +1,6 @@
 Copyright (c) 2013-2016, University of Twente,
               2016-2019, Myrtle Software Ltd,
-              2017-2019, QBayLogic B.V., Google Inc.
+              2017-2025, QBayLogic B.V., Google Inc.
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
diff --git a/clash-prelude/clash-prelude.cabal b/clash-prelude/clash-prelude.cabal
@@ -131,6 +131,7 @@ common common-options
                       KindSignatures
                       MagicHash
                       NoStarIsType
+                      PolyKinds
                       PostfixOperators
                       ScopedTypeVariables
                       StandaloneDeriving
diff --git a/clash-prelude/src/Clash/Annotations/SynthesisAttributes.hs b/clash-prelude/src/Clash/Annotations/SynthesisAttributes.hs
@@ -1,6 +1,6 @@
 {-|
   Copyright   :  (C) 2018,      Google Inc.,
-                     2021-2023, QBayLogic B.V.
+                     2021-2025, QBayLogic B.V.
   License     :  BSD2 (see the file LICENSE)
   Maintainer  :  QBayLogic B.V. <devops@qbaylogic.com>
 
@@ -13,7 +13,6 @@
 {-# LANGUAGE DeriveAnyClass #-}
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE NoGeneralizedNewtypeDeriving #-}
-{-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE QuasiQuotes #-}
 {-# LANGUAGE TemplateHaskellQuotes #-}
 
diff --git a/clash-prelude/src/Clash/Annotations/TH.hs b/clash-prelude/src/Clash/Annotations/TH.hs
@@ -51,7 +51,6 @@ makeTopEntity 'topEntity
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE PatternSynonyms #-}
-{-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
diff --git a/clash-prelude/src/Clash/Class/BitPack/Internal.hs b/clash-prelude/src/Clash/Class/BitPack/Internal.hs
@@ -58,6 +58,7 @@ import Clash.Sized.Internal.BitVector
 >>> :m -Prelude
 >>> :set -XDataKinds
 >>> import Clash.Prelude
+>>> import Data.Proxy
 -}
 
 -- | Convert data to/from a 'BitVector'. This allows functions to be defined
@@ -92,7 +93,7 @@ import Clash.Sized.Internal.BitVector
 -- places.
 --
 -- Clash provides some generic functions on packable types in the prelude, such
--- as indexing into packable stuctures (see "Clash.Class.BitPack.BitIndex") and
+-- as indexing into packable structures (see "Clash.Class.BitPack.BitIndex") and
 -- bitwise reduction of packable data (see "Clash.Class.BitPack.BitReduction").
 --
 class KnownNat (BitSize a) => BitPack a where
@@ -478,7 +479,16 @@ instance ( BitPack a
          ) => BitPack (Either a b)
 
 instance BitPack a => BitPack (Maybe a)
-instance BitPack (Proxy a) => BitPack (Proxy a)
+
+-- | Any 'Proxy' has a bit size of 0 and hence packs to @0 :: BitVector 0@.
+--
+-- >>> pack (Proxy @())
+-- 0
+-- >>> pack (Proxy @5)
+-- 0
+-- >>> pack (Proxy @Bool)
+-- 0
+instance BitPack (Proxy a)
 
 instance BitPack a => BitPack (Complex a)
 instance BitPack a => BitPack (Down a)
diff --git a/clash-prelude/src/Clash/Class/HasDomain/Common.hs b/clash-prelude/src/Clash/Class/HasDomain/Common.hs
@@ -4,7 +4,6 @@ License    :  BSD2 (see the file LICENSE)
 Maintainer :  Christiaan Baaij <christiaan.baaij@gmail.com>
 -}
 
-{-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 
 module Clash.Class.HasDomain.Common {-# DEPRECATED "Experimental feature multiple hidden has been removed. This module will therefore be removed in Clash 1.12." #-}
diff --git a/clash-prelude/src/Clash/Class/HasDomain/HasSingleDomain.hs b/clash-prelude/src/Clash/Class/HasDomain/HasSingleDomain.hs
@@ -1,6 +1,6 @@
 {-|
 Copyright  :  (C) 2019, Myrtle Software Ltd
-                  2022, QBayLogic B.V.
+                  2022-2025, QBayLogic B.V.
 License    :  BSD2 (see the file LICENSE)
 Maintainer :  QBayLogic B.V. <devops@qbaylogic.com>
 
@@ -10,7 +10,6 @@ Internals for "Clash.Class.HasDomain"
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE FlexibleInstances #-}
-{-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE UndecidableInstances #-}
diff --git a/clash-prelude/src/Clash/Class/HasDomain/HasSpecificDomain.hs b/clash-prelude/src/Clash/Class/HasDomain/HasSpecificDomain.hs
@@ -1,5 +1,6 @@
 {-|
 Copyright  :  (C) 2019, Myrtle Software Ltd
+                  2022-2025, QBayLogic B.V.
 License    :  BSD2 (see the file LICENSE)
 Maintainer :  Christiaan Baaij <christiaan.baaij@gmail.com>
 
@@ -10,7 +11,6 @@ Internals for "Clash.Class.HasDomain"
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
-{-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE UndecidableInstances #-}
diff --git a/clash-prelude/src/Clash/NamedTypes.hs b/clash-prelude/src/Clash/NamedTypes.hs
@@ -34,7 +34,6 @@ fifo @System
 -}
 
 {-# LANGUAGE NoGeneralizedNewtypeDeriving #-}
-{-# LANGUAGE PolyKinds #-}
 
 {-# LANGUAGE Safe #-}
 
diff --git a/clash-prelude/src/Clash/Signal/Delayed/Bundle.hs b/clash-prelude/src/Clash/Signal/Delayed/Bundle.hs
@@ -8,7 +8,6 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE NoImplicitPrelude #-}
-{-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TypeFamilyDependencies #-}
 
 module Clash.Signal.Delayed.Bundle (
diff --git a/clash-prelude/src/Clash/Signal/Internal.hs b/clash-prelude/src/Clash/Signal/Internal.hs
@@ -17,7 +17,6 @@ Maintainer :  QBayLogic B.V. <devops@qbaylogic.com>
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE NamedFieldPuns #-}
 {-# LANGUAGE PatternSynonyms #-}
-{-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE RoleAnnotations #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE TemplateHaskell #-}
diff --git a/clash-prelude/src/Clash/XException.hs-boot b/clash-prelude/src/Clash/XException.hs-boot
@@ -1,13 +1,14 @@
 module Clash.XException where
 
+import Data.Kind (Type)
 import GHC.Stack (HasCallStack)
 
 isX :: a -> Either String a
 
-class ShowX a
+class ShowX (a :: Type)
 showsPrecX :: ShowX a => Int -> a -> ShowS
 
-class NFDataX a
+class NFDataX (a :: Type)
 deepErrorX :: NFDataX a => HasCallStack => String -> a
 hasUndefined :: NFDataX a => a -> Bool
 rnfX :: NFDataX a => a -> ()
diff --git a/clash-prelude/tests/Clash/Tests/Reset.hs b/clash-prelude/tests/Clash/Tests/Reset.hs
@@ -2,7 +2,6 @@
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE DataKinds #-}
-{-# LANGUAGE PolyKinds #-}
 
 {-# OPTIONS_GHC -Wno-orphans #-}
 
diff --git a/tests/shouldwork/Basic/T1242.hs b/tests/shouldwork/Basic/T1242.hs
@@ -1,50 +1,14 @@
-{-# LANGUAGE StandaloneDeriving    #-}
-{-# LANGUAGE UndecidableInstances  #-}
-{-# LANGUAGE MultiParamTypeClasses #-}
 module T1242 where
 
-import           Clash.Prelude
-import           Control.DeepSeq   (NFData)
-import           GHC.Generics      (Generic)
+import Clash.Prelude
 
-data RepKind = AppRep | WireRep
-  deriving (Generic, Show, Eq, NFData, NFDataX)
+class C a where
+  type T a :: Nat
 
-newtype U16 (t :: RepKind) = U16 (Unsigned 16)
+data Wrapper (t :: Bool)
 
-deriving instance Generic (U16 a)
-deriving instance Eq (U16 a)
-deriving instance Show (U16 a)
-deriving instance NFData (U16 a)
-deriving instance NFDataX (U16 a)
-deriving instance BitPack (U16 'AppRep)
+instance C (t 'True) where
+  type T (t 'True) = 16
 
-class WireApp w a where
-    toWire :: a -> w
-    toApp :: w -> a
-
-instance WireApp (U16 'WireRep) (U16 'AppRep) where
-    toWire v = case v of U16 x -> U16 x
-    toApp v = case v of U16 x -> U16 x
-
-instance (BitPack (t 'AppRep), WireApp (t 'WireRep)  (t 'AppRep)) => BitPack (t 'WireRep) where
-  type BitSize (t 'WireRep) = BitSize (t 'AppRep)
-  pack x = bv
-    where
-        bv :: BitVector (BitSize (t 'WireRep))
-        bv = pack app
-        app :: t 'AppRep
-        app = toApp $ x
-  unpack x = toWire app
-    where
-        app :: t 'AppRep
-        app = unpack x
-
-data Record
-  = Record
-  { f1 :: U16 'WireRep
-  , f2 :: U16 'WireRep
-  } deriving (Generic, NFData, Show, Eq, BitPack, NFDataX)
-
-topEntity :: Signal System Bool -> Signal System Bool
-topEntity _ = pure (unpack 0 == Record (U16 0) (U16 0))
+topEntity :: Vec (T (Wrapper 'True)) Int
+topEntity = undefined

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	+CHANGED: The `clash-prelude` package now uses `PolyKinds` by default, meaning all its type class instances are now more general than before. As an example, without `PolyKinds`, `BitPack (Proxy a)` would only be defined when `a :: Type`, with the extension it would be defined for any `a :: k`. After this change, some instances in user code might now overlap.