Commit a5fdd185 authored by Vladislav Zavialov's avatar Vladislav Zavialov Committed by Ben Gamari

Guard CUSKs behind a language pragma

GHC Proposal #36 describes a transition plan away from CUSKs and to
top-level kind signatures:

1. Introduce a new extension, -XCUSKs, on by default, that detects CUSKs
   as they currently exist.
2. We turn off the -XCUSKs extension in a few releases and remove it
   sometime thereafter.

This patch implements phase 1 of this plan, introducing a new language
extension to control whether CUSKs are enabled. When top-level kind
signatures are implemented, we can transition to phase 2.
parent c72c369b
......@@ -679,11 +679,15 @@ countTyClDecls decls
-- | Does this declaration have a complete, user-supplied kind signature?
-- See Note [CUSKs: complete user-supplied kind signatures]
hsDeclHasCusk :: TyClDecl GhcRn -> Bool
hsDeclHasCusk (FamDecl { tcdFam = fam_decl })
= famDeclHasCusk False fam_decl
hsDeclHasCusk
:: Bool -- True <=> the -XCUSKs extension is enabled
-> TyClDecl GhcRn
-> Bool
hsDeclHasCusk _cusks_enabled@False _ = False
hsDeclHasCusk cusks_enabled (FamDecl { tcdFam = fam_decl })
= famDeclHasCusk cusks_enabled False fam_decl
-- False: this is not: an associated type of a class with no cusk
hsDeclHasCusk (SynDecl { tcdTyVars = tyvars, tcdRhs = rhs })
hsDeclHasCusk _cusks_enabled@True (SynDecl { tcdTyVars = tyvars, tcdRhs = rhs })
-- NB: Keep this synchronized with 'getInitialKind'
= hsTvbAllKinded tyvars && rhs_annotated rhs
where
......@@ -691,9 +695,9 @@ hsDeclHasCusk (SynDecl { tcdTyVars = tyvars, tcdRhs = rhs })
HsParTy _ lty -> rhs_annotated lty
HsKindSig {} -> True
_ -> False
hsDeclHasCusk (DataDecl { tcdDExt = DataDeclRn { tcdDataCusk = cusk }}) = cusk
hsDeclHasCusk (ClassDecl { tcdTyVars = tyvars }) = hsTvbAllKinded tyvars
hsDeclHasCusk (XTyClDecl _) = panic "hsDeclHasCusk"
hsDeclHasCusk _cusks_enabled@True (DataDecl { tcdDExt = DataDeclRn { tcdDataCusk = cusk }}) = cusk
hsDeclHasCusk _cusks_enabled@True (ClassDecl { tcdTyVars = tyvars }) = hsTvbAllKinded tyvars
hsDeclHasCusk _ (XTyClDecl _) = panic "hsDeclHasCusk"
-- Pretty-printing TyClDecl
-- ~~~~~~~~~~~~~~~~~~~~~~~~
......@@ -787,6 +791,10 @@ declaration before checking all of the others, supporting polymorphic recursion.
See https://gitlab.haskell.org/ghc/ghc/wikis/ghc-kinds/kind-inference#proposed-new-strategy
and #9200 for lots of discussion of how we got here.
The detection of CUSKs is enabled by the -XCUSKs extension, switched on by default.
Under -XNoCUSKs, all declarations are treated as if they have no CUSK.
See https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0036-kind-signatures.rst
PRINCIPLE:
a type declaration has a CUSK iff we could produce a separate kind signature
for it, just like a type signature for a function,
......@@ -1080,11 +1088,13 @@ data FamilyInfo pass
-- | Does this family declaration have a complete, user-supplied kind signature?
-- See Note [CUSKs: complete user-supplied kind signatures]
famDeclHasCusk :: Bool -- ^ True <=> this is an associated type family,
famDeclHasCusk :: Bool -- ^ True <=> the -XCUSKs extension is enabled
-> Bool -- ^ True <=> this is an associated type family,
-- and the parent class has /no/ CUSK
-> FamilyDecl pass
-> Bool
famDeclHasCusk assoc_with_no_cusk
famDeclHasCusk _cusks_enabled@False _ _ = False
famDeclHasCusk _cusks_enabled@True assoc_with_no_cusk
(FamilyDecl { fdInfo = fam_info
, fdTyVars = tyvars
, fdResultSig = L _ resultSig })
......@@ -1095,7 +1105,7 @@ famDeclHasCusk assoc_with_no_cusk
-- Un-associated open type/data families have CUSKs
-- Associated type families have CUSKs iff the parent class does
famDeclHasCusk _ (XFamilyDecl {}) = panic "famDeclHasCusk"
famDeclHasCusk _ _ (XFamilyDecl {}) = panic "famDeclHasCusk"
-- | Does this family declaration have user-supplied return kind signature?
hasReturnKindSignature :: FamilyResultSig a -> Bool
......
......@@ -2279,6 +2279,7 @@ languageExtensions (Just Haskell98)
= [LangExt.ImplicitPrelude,
-- See Note [When is StarIsType enabled]
LangExt.StarIsType,
LangExt.CUSKs,
LangExt.MonomorphismRestriction,
LangExt.NPlusKPatterns,
LangExt.DatatypeContexts,
......@@ -2295,6 +2296,7 @@ languageExtensions (Just Haskell2010)
= [LangExt.ImplicitPrelude,
-- See Note [When is StarIsType enabled]
LangExt.StarIsType,
LangExt.CUSKs,
LangExt.MonomorphismRestriction,
LangExt.DatatypeContexts,
LangExt.TraditionalRecordSyntax,
......@@ -4377,6 +4379,7 @@ xFlagsDeps = [
flagSpec "BinaryLiterals" LangExt.BinaryLiterals,
flagSpec "CApiFFI" LangExt.CApiFFI,
flagSpec "CPP" LangExt.Cpp,
flagSpec "CUSKs" LangExt.CUSKs,
flagSpec "ConstrainedClassMethods" LangExt.ConstrainedClassMethods,
flagSpec "ConstraintKinds" LangExt.ConstraintKinds,
flagSpec "DataKinds" LangExt.DataKinds,
......
......@@ -1552,7 +1552,8 @@ rnTyClDecl (DataDecl { tcdLName = tycon, tcdTyVars = tyvars,
; bindHsQTyVars doc Nothing Nothing kvs tyvars $ \ tyvars' no_rhs_kvs ->
do { (defn', fvs) <- rnDataDefn doc defn
-- See Note [Complete user-supplied kind signatures] in HsDecls
; let cusk = hsTvbAllKinded tyvars' && no_rhs_kvs
; cusks_enabled <- xoptM LangExt.CUSKs
; let cusk = cusks_enabled && hsTvbAllKinded tyvars' && no_rhs_kvs
rn_info = DataDeclRn { tcdDataCusk = cusk
, tcdFVs = fvs }
; traceRn "rndata" (ppr tycon <+> ppr cusk <+> ppr no_rhs_kvs)
......
......@@ -510,8 +510,9 @@ kcTyClGroup decls
-- 3. Generalise the inferred kinds
-- See Note [Kind checking for type and class decls]
; cusks_enabled <- xoptM LangExt.CUSKs
; let (cusk_decls, no_cusk_decls)
= partition (hsDeclHasCusk . unLoc) decls
= partition (hsDeclHasCusk cusks_enabled . unLoc) decls
; poly_cusk_tcs <- getInitialKinds True cusk_decls
......@@ -1040,17 +1041,25 @@ getInitialKind cusk (FamDecl { tcdFam = decl })
getInitialKind cusk (SynDecl { tcdLName = dL->L _ name
, tcdTyVars = ktvs
, tcdRhs = rhs })
= do { tycon <- kcLHsQTyVars name TypeSynonymFlavour cusk ktvs $
case kind_annotation rhs of
= do { cusks_enabled <- xoptM LangExt.CUSKs
; tycon <- kcLHsQTyVars name TypeSynonymFlavour cusk ktvs $
case kind_annotation cusks_enabled rhs of
Just ksig -> tcLHsKindSig (TySynKindCtxt name) ksig
Nothing -> newMetaKindVar
Nothing -> newMetaKindVar
; return [tycon] }
where
-- Keep this synchronized with 'hsDeclHasCusk'.
kind_annotation (dL->L _ ty) = case ty of
HsParTy _ lty -> kind_annotation lty
HsKindSig _ _ k -> Just k
_ -> Nothing
kind_annotation
:: Bool -- cusks_enabled?
-> LHsType GhcRn -- rhs
-> Maybe (LHsKind GhcRn)
kind_annotation False = const Nothing
kind_annotation True = go
where
go (dL->L _ ty) = case ty of
HsParTy _ lty -> go lty
HsKindSig _ _ k -> Just k
_ -> Nothing
getInitialKind _ (DataDecl _ _ _ _ (XHsDataDefn _)) = panic "getInitialKind"
getInitialKind _ (XTyClDecl _) = panic "getInitialKind"
......@@ -1074,18 +1083,20 @@ getFamDeclInitialKind parent_cusk mb_parent_tycon
, fdTyVars = ktvs
, fdResultSig = (dL->L _ resultSig)
, fdInfo = info })
= kcLHsQTyVars name flav fam_cusk ktvs $
case resultSig of
KindSig _ ki -> tcLHsKindSig ctxt ki
TyVarSig _ (dL->L _ (KindedTyVar _ _ ki)) -> tcLHsKindSig ctxt ki
_ -- open type families have * return kind by default
| tcFlavourIsOpen flav -> return liftedTypeKind
-- closed type families have their return kind inferred
-- by default
| otherwise -> newMetaKindVar
= do { cusks_enabled <- xoptM LangExt.CUSKs
; kcLHsQTyVars name flav (fam_cusk cusks_enabled) ktvs $
case resultSig of
KindSig _ ki -> tcLHsKindSig ctxt ki
TyVarSig _ (dL->L _ (KindedTyVar _ _ ki)) -> tcLHsKindSig ctxt ki
_ -- open type families have * return kind by default
| tcFlavourIsOpen flav -> return liftedTypeKind
-- closed type families have their return kind inferred
-- by default
| otherwise -> newMetaKindVar
}
where
assoc_with_no_cusk = isJust mb_parent_tycon && not parent_cusk
fam_cusk = famDeclHasCusk assoc_with_no_cusk decl
fam_cusk cusks_enabled = famDeclHasCusk cusks_enabled assoc_with_no_cusk decl
flav = case info of
DataFamily -> DataFamilyFlavour mb_parent_tycon
OpenTypeFamily -> OpenTypeFamilyFlavour mb_parent_tycon
......
......@@ -9012,6 +9012,11 @@ do so.
Complete user-supplied kind signatures and polymorphic recursion
----------------------------------------------------------------
.. extension:: CUSKs
:shortdesc: Enable detection of complete user-supplied kind signatures.
:since: 8.10.1
Just as in type inference, kind inference for recursive types can only
use *monomorphic* recursion. Consider this (contrived) example: ::
......@@ -9110,6 +9115,13 @@ example, consider ::
According to the rules above ``X`` has a CUSK. Yet, the kind of ``k`` is undetermined.
It is thus quantified over, giving ``X`` the kind ``forall k1 (k :: k1). Proxy k -> Type``.
The detection of CUSKs is enabled by the :extension:`CUSKs` flag, which is
switched on by default. When :extension:`CUSKs` is switched off, there is
currently no way to enable polymorphic recursion in types. In the future, the
notion of a CUSK will be replaced by top-level kind signatures
(`GHC Proposal #36 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0036-kind-signatures.rst>`__),
then, after a transition period, this extension will be turned off by default, and eventually removed.
Kind inference in closed type families
--------------------------------------
......
......@@ -140,4 +140,5 @@ data Extension
| QuantifiedConstraints
| StarIsType
| ImportQualifiedPost
| CUSKs
deriving (Eq, Enum, Show, Generic, Bounded)
......@@ -41,6 +41,7 @@ expectedGhcOnlyExtensions = ["RelaxedLayout",
"AlternativeLayoutRuleTransitional",
"EmptyDataDeriving",
"GeneralisedNewtypeDeriving",
"CUSKs",
"ImportQualifiedPost"]
expectedCabalOnlyExtensions :: [String]
......
......@@ -241,6 +241,7 @@ test('tcfail217', normal, compile_fail, [''])
test('tcfail218', normal, compile_fail, [''])
test('tcfail223', normal, compile_fail, [''])
test('tcfail224', normal, compile_fail, [''])
test('tcfail225', normal, compile_fail, [''])
test('SilentParametersOverlapping', normal, compile, [''])
test('FailDueToGivenOverlapping', normal, compile_fail, [''])
......
{-# LANGUAGE PolyKinds, GADTs #-}
{-# LANGUAGE NoCUSKs #-}
module TcFail225 where
import Data.Kind (Type)
data T (m :: k -> Type) :: k -> Type where
MkT :: m a -> T Maybe (m a) -> T m a
tcfail225.hs:9:19: error:
• Expected kind ‘k -> *’, but ‘Maybe’ has kind ‘* -> *’
• In the first argument of ‘T’, namely ‘Maybe’
In the type ‘T Maybe (m a)’
In the definition of data constructor ‘MkT’
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