[Git][ghc/ghc][wip/T18191] Make GADT constructors adhere to the forall-or-nothing rule properly
Ryan Scott
gitlab at gitlab.haskell.org
Sun May 31 15:47:56 UTC 2020
Ryan Scott pushed to branch wip/T18191 at Glasgow Haskell Compiler / GHC
Commits:
7b89ce1c by Ryan Scott at 2020-05-31T11:47:37-04:00
Make GADT constructors adhere to the forall-or-nothing rule properly
Issue #18191 revealed that the types of GADT constructors don't quite
adhere to the `forall`-or-nothing rule. This patch serves to clean up
this sad state of affairs somewhat. The main change is not in the
code itself, but in the documentation, as this patch introduces two
sections to the GHC User's Guide:
* A "Formal syntax for GADTs" section that presents a BNF-style
grammar for what is and isn't allowed in GADT constructor types.
This mostly exists to codify GHC's existing behavior, but it also
imposes a new restriction that addresses #18191: the outermost
`forall` and/or context in a GADT constructor is not allowed to be
surrounded by parentheses. Doing so would make these
`forall`s/contexts nested, and GADTs do not support nested
`forall`s/contexts at present.
* A "`forall`-or-nothing rule" section that describes exactly what
the `forall`-or-nothing rule is all about. Surprisingly, there was
no mention of this anywhere in the User's Guide up until now!
To adhere the new specification in the "Formal syntax for GADTs"
section of the User's Guide, the following code changes were made:
* A new function, `GHC.Hs.Type.splitLHsGADTPrefixTy`, was introduced.
This is very much like `splitLHsSigmaTy`, except that it avoids
splitting apart any parentheses, which can be syntactically
significant for GADT types. See
`Note [No nested foralls or contexts in GADT constructors]` in
`GHC.Hs.Type`.
* `ConDeclGADTPrefixPs`, an extension constructor for `XConDecl`, was
introduced so that `GHC.Parser.PostProcess.mkGadtDecl` can return
it when given a prefix GADT constructor. Unlike `ConDeclGADT`,
`ConDeclGADTPrefixPs` does not split the GADT type into its argument
and result types, as this cannot be done until after the type is
renamed (see `Note [GADT abstract syntax]` in `GHC.Hs.Decls` for why
this is the case).
* `GHC.Renamer.Module.rnConDecl` now has an additional case for
`ConDeclGADTPrefixPs` that (1) splits apart the full `LHsType` into
its `forall`s, context, argument types, and result type, and
(2) checks for nested `forall`s/contexts. Step (2) used to be
performed the typechecker (in `GHC.Tc.TyCl.badDataConTyCon`) rather
than the renamer, but now the relevant code from the typechecker
can simply be deleted.
One nice side effect of this change is that we are able to give a
more accurate error message for GADT constructors that use visible
dependent quantification (e.g., `MkFoo :: forall a -> a -> Foo a`),
which improves the stderr in the `T16326_Fail6` test case.
Fixes #18191. Bumps the Haddock submodule.
- - - - -
27 changed files:
- compiler/GHC/Hs/Decls.hs
- compiler/GHC/Hs/Instances.hs
- compiler/GHC/Hs/Type.hs
- compiler/GHC/Hs/Utils.hs
- compiler/GHC/Parser.y
- compiler/GHC/Parser/PostProcess.hs
- compiler/GHC/Parser/PostProcess/Haddock.hs
- compiler/GHC/Rename/Module.hs
- compiler/GHC/Tc/TyCl.hs
- compiler/GHC/ThToHs.hs
- docs/users_guide/8.12.1-notes.rst
- docs/users_guide/exts/explicit_forall.rst
- docs/users_guide/exts/gadt_syntax.rst
- testsuite/tests/dependent/should_fail/T16326_Fail6.stderr
- testsuite/tests/gadt/T12087.stderr
- testsuite/tests/gadt/T14320.hs
- + testsuite/tests/gadt/T14320.stderr
- testsuite/tests/gadt/T16427.stderr
- + testsuite/tests/gadt/T18191.hs
- + testsuite/tests/gadt/T18191.stderr
- testsuite/tests/gadt/all.T
- testsuite/tests/ghc-api/annotations/T10399.stdout
- testsuite/tests/ghc-api/annotations/Test10399.hs
- testsuite/tests/parser/should_compile/T15323.hs
- testsuite/tests/parser/should_compile/T15323.stderr
- testsuite/tests/rename/should_compile/T5331.stderr
- utils/haddock
Changes:
=====================================
compiler/GHC/Hs/Decls.hs
=====================================
@@ -5,6 +5,7 @@
{-# LANGUAGE DeriveDataTypeable, DeriveFunctor, DeriveFoldable,
DeriveTraversable #-}
+{-# LANGUAGE CPP #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
@@ -70,7 +71,7 @@ module GHC.Hs.Decls (
ForeignDecl(..), LForeignDecl, ForeignImport(..), ForeignExport(..),
CImportSpec(..),
-- ** Data-constructor declarations
- ConDecl(..), LConDecl,
+ ConDecl(..), LConDecl, ConDeclGADTPrefixPs(..),
HsConDeclDetails, hsConDeclArgTys, hsConDeclTheta,
getConNames, getConArgs,
-- ** Document comments
@@ -109,6 +110,7 @@ import GHC.Core.Coercion
import GHC.Types.ForeignCall
import GHC.Hs.Extension
import GHC.Types.Name
+import GHC.Types.Name.Reader
import GHC.Types.Name.Set
-- others:
@@ -1422,54 +1424,92 @@ type instance XConDeclGADT GhcRn = [Name] -- Implicitly bound type variables
type instance XConDeclGADT GhcTc = NoExtField
type instance XConDeclH98 (GhcPass _) = NoExtField
-type instance XXConDecl (GhcPass _) = NoExtCon
+
+type instance XXConDecl GhcPs = ConDeclGADTPrefixPs
+type instance XXConDecl GhcRn = NoExtCon
+type instance XXConDecl GhcTc = NoExtCon
+
+-- | Stores the types of prefix GADT constructors in the parser. This is used
+-- in lieu of ConDeclGADT, which requires knowing the specific argument and
+-- result types, as this is difficult to determine in general in the parser.
+-- See @Note [GADT abstract syntax]@.
+data ConDeclGADTPrefixPs = ConDeclGADTPrefixPs
+ { con_gp_names :: [Located RdrName]
+ -- ^ The GADT constructor declaration's names.
+ , con_gp_ty :: LHsSigType GhcPs
+ -- ^ The type after the @::@.
+ , con_gp_doc :: Maybe LHsDocString
+ -- ^ A possible Haddock comment.
+ }
{- Note [GADT abstract syntax]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-There's a wrinkle in ConDeclGADT
-
-* For record syntax, it's all uniform. Given:
- data T a where
- K :: forall a. Ord a => { x :: [a], ... } -> T a
- we make the a ConDeclGADT for K with
- con_qvars = {a}
- con_mb_cxt = Just [Ord a]
- con_args = RecCon <the record fields>
- con_res_ty = T a
-
- We need the RecCon before the reanmer, so we can find the record field
- binders in GHC.Hs.Utils.hsConDeclsBinders.
-
-* However for a GADT constr declaration which is not a record, it can
- be hard parse until we know operator fixities. Consider for example
- C :: a :*: b -> a :*: b -> a :+: b
- Initially this type will parse as
- a :*: (b -> (a :*: (b -> (a :+: b))))
- so it's hard to split up the arguments until we've done the precedence
- resolution (in the renamer).
-
- So: - In the parser (GHC.Parser.PostProcess.mkGadtDecl), we put the whole constr
- type into the res_ty for a ConDeclGADT for now, and use
- PrefixCon []
- con_args = PrefixCon []
- con_res_ty = a :*: (b -> (a :*: (b -> (a :+: b))))
-
- - In the renamer (GHC.Rename.Module.rnConDecl), we unravel it after
- operator fixities are sorted. So we generate. So we end
- up with
- con_args = PrefixCon [ a :*: b, a :*: b ]
- con_res_ty = a :+: b
+There are two broad ways to classify GADT constructors:
+
+* Record-syntax constructors. For example:
+
+ data T a where
+ K :: forall a. Ord a => { x :: [a], ... } -> T a
+
+* Prefix constructors, which do not use record syntax. For example:
+
+ data T a where
+ K :: forall a. Ord a => [a] -> ... -> T a
+
+Initially, both forms of GADT constructors are initially parsed as a single
+LHsType. However, GADTs have a certain structure, requiring distinct argument
+and result types, as well as imposing restrictions on where `forall`s and
+contexts can be (see Note [No nested foralls or contexts in GADT constructors]
+in GHC.Hs.Type). As a result, it is convenient to split up the LHsType into
+its individual components, which are stored in the ConDeclGADT constructor of
+ConDecl.
+
+Where should this splitting occur? For GADT constructors with record syntax,
+we split in the parser (in GHC.Parser.PostProcess.mkGadtDecl). We must do this
+splitting before the renamer, as we need the record field names for use in
+GHC.Hs.Utils.hsConDeclsBinders.
+
+For prefix GADT constructors, however, the situation is more complicated. It
+can be difficult to split a prefix GADT type until we know type operator
+fixities. Consider this, for example:
+
+ C :: a :*: b -> a :*: b -> a :+: b
+
+Initially, the type of C will parse as:
+
+ a :*: (b -> (a :*: (b -> (a :+: b))))
+
+So it's hard to split up the arguments until we've done the precedence
+resolution (in the renamer). (Unlike prefix GADT types, record GADT types
+do not have this problem because of their uniform syntax.)
+
+As a result, we deliberately avoid splitting prefix GADT types in the parser.
+Instead, we store the entire LHsType in ConDeclGADTPrefixPs, a GHC-specific
+extension constructor to ConDecl. Later, in the renamer
+(in GHC.Rename.Module.rnConDecl), we resolve the fixities of all type operators
+in the LHsType, which facilitates splitting it into argument and result types
+accurately. We finish renaming a ConDeclGADTPrefixPs by putting the split
+components into a ConDeclGADT. This is why ConDeclGADTPrefixPs has the suffix
+-Ps, as it is only used by the parser.
+
+Note that the existence of ConDeclGADTPrefixPs does not imply that ConDeclGADT
+goes completely unused by the parser. Other consumers of GHC's abstract syntax
+are still free to use ConDeclGADT. Indeed, both Haddock and Template Haskell
+construct values of type `ConDecl GhcPs` by way of ConDeclGADT, as neither of
+them have the same difficulties with operator precedence that GHC's parser
+does. As an example, see GHC.ThToHs.cvtConstr, which converts Template Haskell
+syntax into GHC syntax.
-}
-- | Haskell data Constructor Declaration Details
type HsConDeclDetails pass
= HsConDetails (LBangType pass) (Located [LConDeclField pass])
-getConNames :: ConDecl (GhcPass p) -> [Located (IdP (GhcPass p))]
+getConNames :: ConDecl GhcRn -> [Located Name]
getConNames ConDeclH98 {con_name = name} = [name]
getConNames ConDeclGADT {con_names = names} = names
-getConArgs :: ConDecl pass -> HsConDeclDetails pass
+getConArgs :: ConDecl GhcRn -> HsConDeclDetails GhcRn
getConArgs d = con_args d
hsConDeclArgTys :: HsConDeclDetails pass -> [LBangType pass]
@@ -1518,16 +1558,30 @@ instance Outputable NewOrData where
ppr NewType = text "newtype"
ppr DataType = text "data"
-pp_condecls :: (OutputableBndrId p) => [LConDecl (GhcPass p)] -> SDoc
-pp_condecls cs@(L _ ConDeclGADT{} : _) -- In GADT syntax
+pp_condecls :: forall p. OutputableBndrId p => [LConDecl (GhcPass p)] -> SDoc
+pp_condecls cs
+ | gadt_syntax -- In GADT syntax
= hang (text "where") 2 (vcat (map ppr cs))
-pp_condecls cs -- In H98 syntax
+ | otherwise -- In H98 syntax
= equals <+> sep (punctuate (text " |") (map ppr cs))
+ where
+ gadt_syntax = case cs of
+ [] -> False
+ (L _ ConDeclH98{} : _) -> False
+ (L _ ConDeclGADT{} : _) -> True
+ (L _ (XConDecl x) : _) ->
+ case ghcPass @p of
+ GhcPs | ConDeclGADTPrefixPs{} <- x
+ -> True
+#if __GLASGOW_HASKELL__ < 811
+ GhcRn -> noExtCon x
+ GhcTc -> noExtCon x
+#endif
instance (OutputableBndrId p) => Outputable (ConDecl (GhcPass p)) where
ppr = pprConDecl
-pprConDecl :: (OutputableBndrId p) => ConDecl (GhcPass p) -> SDoc
+pprConDecl :: forall p. OutputableBndrId p => ConDecl (GhcPass p) -> SDoc
pprConDecl (ConDeclH98 { con_name = L _ con
, con_ex_tvs = ex_tvs
, con_mb_cxt = mcxt
@@ -1558,6 +1612,16 @@ pprConDecl (ConDeclGADT { con_names = cons, con_qvars = qvars
ppr_arrow_chain (a:as) = sep (a : map (arrow <+>) as)
ppr_arrow_chain [] = empty
+pprConDecl (XConDecl x) =
+ case ghcPass @p of
+ GhcPs | ConDeclGADTPrefixPs { con_gp_names = cons, con_gp_ty = ty
+ , con_gp_doc = doc } <- x
+ -> ppr_mbDoc doc <+> ppr_con_names cons <+> dcolon <+> ppr ty
+#if __GLASGOW_HASKELL__ < 811
+ GhcRn -> noExtCon x
+ GhcTc -> noExtCon x
+#endif
+
ppr_con_names :: (OutputableBndr a) => [Located a] -> SDoc
ppr_con_names = pprWithCommas (pprPrefixOcc . unLoc)
=====================================
compiler/GHC/Hs/Instances.hs
=====================================
@@ -159,6 +159,8 @@ deriving instance Data (ConDecl GhcPs)
deriving instance Data (ConDecl GhcRn)
deriving instance Data (ConDecl GhcTc)
+deriving instance Data ConDeclGADTPrefixPs
+
-- deriving instance DataIdLR p p => Data (TyFamInstDecl p)
deriving instance Data (TyFamInstDecl GhcPs)
deriving instance Data (TyFamInstDecl GhcRn)
=====================================
compiler/GHC/Hs/Type.hs
=====================================
@@ -58,7 +58,8 @@ module GHC.Hs.Type (
hsLTyVarName, hsLTyVarNames, hsLTyVarLocName, hsExplicitLTyVarNames,
splitLHsInstDeclTy, getLHsInstDeclHead, getLHsInstDeclClass_maybe,
splitLHsPatSynTy,
- splitLHsForAllTyInvis, splitLHsQualTy, splitLHsSigmaTyInvis,
+ splitLHsForAllTyInvis, splitLHsQualTy,
+ splitLHsSigmaTyInvis, splitLHsGADTPrefixTy,
splitHsFunType, hsTyGetAppHead_maybe,
mkHsOpTy, mkHsAppTy, mkHsAppTys, mkHsAppKindTy,
ignoreParens, hsSigType, hsSigWcType, hsPatSigType,
@@ -1347,6 +1348,85 @@ splitLHsSigmaTyInvis ty
, (ctxt, ty2) <- splitLHsQualTy ty1
= (tvs, ctxt, ty2)
+-- | Decompose a prefix GADT type into its constituent parts.
+-- Returns @(mb_tvbs, mb_ctxt, body)@, where:
+--
+-- * @mb_tvbs@ are @Just@ the leading @forall at s, if they are provided.
+-- Otherwise, they are @Nothing at .
+--
+-- * @mb_ctxt@ is @Just@ the context, if it is provided.
+-- Otherwise, it is @Nothing at .
+--
+-- * @body@ is the body of the type after the optional @forall at s and context.
+--
+-- This function is careful not to look through parentheses.
+-- See @Note [No nested foralls or contexts in GADT constructors]@ for why this
+-- is important.
+splitLHsGADTPrefixTy ::
+ LHsType pass
+ -> (Maybe [LHsTyVarBndr Specificity pass], Maybe (LHsContext pass), LHsType pass)
+splitLHsGADTPrefixTy ty
+ | (mb_tvbs, rho) <- split_forall ty
+ , (mb_ctxt, tau) <- split_ctxt rho
+ = (mb_tvbs, mb_ctxt, tau)
+ where
+ -- NB: We do not use splitLHsForAllTyInvis below, since that looks through
+ -- parentheses...
+ split_forall (L _ (HsForAllTy { hst_fvf = ForallInvis, hst_bndrs = bndrs
+ , hst_body = rho }))
+ = (Just bndrs, rho)
+ split_forall sigma
+ = (Nothing, sigma)
+
+ -- ...similarly, we do not use splitLHsQualTy below, since that also looks
+ -- through parentheses.
+ split_ctxt (L _ (HsQualTy { hst_ctxt = cxt, hst_body = tau }))
+ = (Just cxt, tau)
+ split_ctxt tau
+ = (Nothing, tau)
+
+{-
+Note [No nested foralls or contexts in GADT constructors]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+GADT constructors provide some freedom to change the order of foralls in their
+types (see Note [DataCon user type variable binders] in GHC.Core.DataCon), but
+this freedom is still limited. GADTs still require that all quantification
+occurs "prenex". That is, any explicitly quantified type variables must occur
+at the front of the GADT type, followed by any contexts, followed by the body of
+the GADT type, in precisely that order. For instance:
+
+ data T where
+ MkT1 :: forall a b. (Eq a, Eq b) => a -> b -> T
+ -- OK
+ MkT2 :: forall a. Eq a => forall b. a -> b -> T
+ -- Rejected, `forall b` is nested
+ MkT3 :: forall a b. Eq a => Eq b => a -> b -> T
+ -- Rejected, `Eq b` is nested
+ MkT4 :: Int -> forall a. a -> T
+ -- Rejected, `forall a` is nested
+ MkT5 :: forall a. Int -> Eq a => a -> T
+ -- Rejected, `Eq a` is nested
+ MkT6 :: (forall a. a -> T)
+ -- Rejected, `forall a` is nested due to the surrounding parentheses
+ MkT7 :: (Eq a => a -> t)
+ -- Rejected, `Eq a` is nested due to the surrounding parentheses
+
+For the full details, see the "Formal syntax for GADTs" section of the GHC
+User's Guide. GHC enforces that GADT constructors do not have nested `forall`s
+or contexts in two parts:
+
+1. The splitLHsGADTPrefixTy function is careful not to remove parentheses
+ in a prefix GADT constructor's type, as these parentheses can be
+ syntactically significant. If the third result returned by
+ splitLHsGADTPrefixTy contains any `forall`s or contexts, then they are
+ nested.
+2. The renamer (in the ConDeclGADTPrefixPs case of GHC.Rename.Module.rnConDecl)
+ will check for nested `forall`s/contexts in the body of a prefix GADT type,
+ after it has determined what all of the argument types are.
+ (See Note [GADT abstract syntax] in GHC.Hs.Decls for why this check must wait
+ until the renamer.)
+-}
+
-- | Decompose a type of the form @forall <tvs>. body@ into its constituent
-- parts. Only splits type variable binders that
-- were quantified invisibly (e.g., @forall a.@, with a dot).
=====================================
compiler/GHC/Hs/Utils.hs
=====================================
@@ -1242,7 +1242,8 @@ hsTyClForeignBinders tycl_decls foreign_decls
getSelectorNames (ns, fs) = map unLoc ns ++ map (extFieldOcc . unLoc) fs
-------------------
-hsLTyClDeclBinders :: Located (TyClDecl (GhcPass p))
+hsLTyClDeclBinders :: IsPass p
+ => Located (TyClDecl (GhcPass p))
-> ([Located (IdP (GhcPass p))], [LFieldOcc (GhcPass p)])
-- ^ Returns all the /binding/ names of the decl. The first one is
-- guaranteed to be the name of the decl. The first component
@@ -1304,7 +1305,8 @@ getPatSynBinds binds
, L _ (PatSynBind _ psb) <- bagToList lbinds ]
-------------------
-hsLInstDeclBinders :: LInstDecl (GhcPass p)
+hsLInstDeclBinders :: IsPass p
+ => LInstDecl (GhcPass p)
-> ([Located (IdP (GhcPass p))], [LFieldOcc (GhcPass p)])
hsLInstDeclBinders (L _ (ClsInstD
{ cid_inst = ClsInstDecl
@@ -1316,7 +1318,8 @@ hsLInstDeclBinders (L _ (TyFamInstD {})) = mempty
-------------------
-- | the 'SrcLoc' returned are for the whole declarations, not just the names
-hsDataFamInstBinders :: DataFamInstDecl (GhcPass p)
+hsDataFamInstBinders :: IsPass p
+ => DataFamInstDecl (GhcPass p)
-> ([Located (IdP (GhcPass p))], [LFieldOcc (GhcPass p)])
hsDataFamInstBinders (DataFamInstDecl { dfid_eqn = HsIB { hsib_body =
FamEqn { feqn_rhs = defn }}})
@@ -1325,7 +1328,8 @@ hsDataFamInstBinders (DataFamInstDecl { dfid_eqn = HsIB { hsib_body =
-------------------
-- | the 'SrcLoc' returned are for the whole declarations, not just the names
-hsDataDefnBinders :: HsDataDefn (GhcPass p)
+hsDataDefnBinders :: IsPass p
+ => HsDataDefn (GhcPass p)
-> ([Located (IdP (GhcPass p))], [LFieldOcc (GhcPass p)])
hsDataDefnBinders (HsDataDefn { dd_cons = cons })
= hsConDeclsBinders cons
@@ -1335,7 +1339,8 @@ hsDataDefnBinders (HsDataDefn { dd_cons = cons })
type Seen p = [LFieldOcc (GhcPass p)] -> [LFieldOcc (GhcPass p)]
-- Filters out ones that have already been seen
-hsConDeclsBinders :: [LConDecl (GhcPass p)]
+hsConDeclsBinders :: forall p. IsPass p
+ => [LConDecl (GhcPass p)]
-> ([Located (IdP (GhcPass p))], [LFieldOcc (GhcPass p)])
-- See hsLTyClDeclBinders for what this does
-- The function is boringly complicated because of the records
@@ -1365,6 +1370,16 @@ hsConDeclsBinders cons
(remSeen', flds) = get_flds remSeen args
(ns, fs) = go remSeen' rs
+ XConDecl x -> case ghcPass @p of
+ GhcPs | ConDeclGADTPrefixPs { con_gp_names = names } <- x
+ -> (map (L loc . unLoc) names ++ ns, fs)
+#if __GLASGOW_HASKELL__ < 811
+ GhcRn -> noExtCon x
+ GhcTc -> noExtCon x
+#endif
+ where
+ (ns, fs) = go remSeen rs
+
get_flds :: Seen p -> HsConDeclDetails (GhcPass p)
-> (Seen p, [LFieldOcc (GhcPass p)])
get_flds remSeen (RecCon flds)
=====================================
compiler/GHC/Parser.y
=====================================
@@ -2250,9 +2250,8 @@ gadt_constr :: { LConDecl GhcPs }
-- see Note [Difference in parsing GADT and data constructors]
-- Returns a list because of: C,D :: ty
: con_list '::' sigtypedoc
- {% let (gadt,anns) = mkGadtDecl (unLoc $1) $3
- in ams (sLL $1 $> gadt)
- (mu AnnDcolon $2:anns) }
+ {% ams (sLL $1 $> (mkGadtDecl (unLoc $1) $3))
+ [mu AnnDcolon $2] }
{- Note [Difference in parsing GADT and data constructors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
=====================================
compiler/GHC/Parser/PostProcess.hs
=====================================
@@ -685,43 +685,35 @@ mkConDeclH98 name mb_forall mb_cxt args
, con_args = args
, con_doc = Nothing }
+-- | Construct a GADT-style data constructor from the constructor names and
+-- their type. This will return different AST forms for record syntax
+-- constructors and prefix constructors, as the latter must be handled
+-- specially in the renamer. See @Note [GADT abstract syntax]@ in
+-- "GHC.Hs.Decls" for the full story.
mkGadtDecl :: [Located RdrName]
- -> LHsType GhcPs -- Always a HsForAllTy
- -> (ConDecl GhcPs, [AddAnn])
+ -> LHsType GhcPs
+ -> ConDecl GhcPs
mkGadtDecl names ty
- = (ConDeclGADT { con_g_ext = noExtField
- , con_names = names
- , con_forall = L l $ isLHsForAllTy ty'
- , con_qvars = tvs
- , con_mb_cxt = mcxt
- , con_args = args
- , con_res_ty = res_ty
- , con_doc = Nothing }
- , anns1 ++ anns2)
+ | Just (mtvs, mcxt, args, res_ty) <- mb_record_gadt ty
+ = ConDeclGADT { con_g_ext = noExtField
+ , con_names = names
+ , con_forall = L (getLoc ty) $ isJust mtvs
+ , con_qvars = fromMaybe [] mtvs
+ , con_mb_cxt = mcxt
+ , con_args = args
+ , con_res_ty = res_ty
+ , con_doc = Nothing }
+ | otherwise
+ = XConDecl $ ConDeclGADTPrefixPs { con_gp_names = names
+ , con_gp_ty = mkLHsSigType ty
+ , con_gp_doc = Nothing }
where
- (ty'@(L l _),anns1) = peel_parens ty []
- (tvs, rho) = splitLHsForAllTyInvis ty'
- (mcxt, tau, anns2) = split_rho rho []
-
- split_rho (L _ (HsQualTy { hst_ctxt = cxt, hst_body = tau })) ann
- = (Just cxt, tau, ann)
- split_rho (L l (HsParTy _ ty)) ann
- = split_rho ty (ann++mkParensApiAnn l)
- split_rho tau ann
- = (Nothing, tau, ann)
-
- (args, res_ty) = split_tau tau
-
- -- See Note [GADT abstract syntax] in GHC.Hs.Decls
- split_tau (L _ (HsFunTy _ (L loc (HsRecTy _ rf)) res_ty))
- = (RecCon (L loc rf), res_ty)
- split_tau tau
- = (PrefixCon [], tau)
-
- peel_parens (L l (HsParTy _ ty)) ann = peel_parens ty
- (ann++mkParensApiAnn l)
- peel_parens ty ann = (ty, ann)
-
+ mb_record_gadt ty
+ | (mtvs, mcxt, body_ty) <- splitLHsGADTPrefixTy ty
+ , L _ (HsFunTy _ (L loc (HsRecTy _ rf)) res_ty) <- body_ty
+ = Just (mtvs, mcxt, RecCon (L loc rf), res_ty)
+ | otherwise
+ = Nothing
setRdrNameSpace :: RdrName -> NameSpace -> RdrName
-- ^ This rather gruesome function is used mainly by the parser.
=====================================
compiler/GHC/Parser/PostProcess/Haddock.hs
=====================================
@@ -12,24 +12,28 @@ import Control.Monad
-- -----------------------------------------------------------------------------
-- Adding documentation to record fields (used in parsing).
-addFieldDoc :: LConDeclField a -> Maybe LHsDocString -> LConDeclField a
+addFieldDoc :: LConDeclField GhcPs -> Maybe LHsDocString -> LConDeclField GhcPs
addFieldDoc (L l fld) doc
= L l (fld { cd_fld_doc = cd_fld_doc fld `mplus` doc })
-addFieldDocs :: [LConDeclField a] -> Maybe LHsDocString -> [LConDeclField a]
+addFieldDocs :: [LConDeclField GhcPs] -> Maybe LHsDocString -> [LConDeclField GhcPs]
addFieldDocs [] _ = []
addFieldDocs (x:xs) doc = addFieldDoc x doc : xs
-addConDoc :: LConDecl a -> Maybe LHsDocString -> LConDecl a
+addConDoc :: LConDecl GhcPs -> Maybe LHsDocString -> LConDecl GhcPs
addConDoc decl Nothing = decl
-addConDoc (L p c) doc = L p ( c { con_doc = con_doc c `mplus` doc } )
+addConDoc (L p c) doc = L p $ case c of
+ ConDeclH98 { con_doc = old_doc } -> c { con_doc = old_doc `mplus` doc }
+ ConDeclGADT { con_doc = old_doc } -> c { con_doc = old_doc `mplus` doc }
+ XConDecl x@(ConDeclGADTPrefixPs { con_gp_doc = old_doc }) ->
+ XConDecl (x { con_gp_doc = old_doc `mplus` doc })
-addConDocs :: [LConDecl a] -> Maybe LHsDocString -> [LConDecl a]
+addConDocs :: [LConDecl GhcPs] -> Maybe LHsDocString -> [LConDecl GhcPs]
addConDocs [] _ = []
addConDocs [x] doc = [addConDoc x doc]
addConDocs (x:xs) doc = x : addConDocs xs doc
-addConDocFirst :: [LConDecl a] -> Maybe LHsDocString -> [LConDecl a]
+addConDocFirst :: [LConDecl GhcPs] -> Maybe LHsDocString -> [LConDecl GhcPs]
addConDocFirst [] _ = []
addConDocFirst (x:xs) doc = addConDoc x doc : xs
=====================================
compiler/GHC/Rename/Module.hs
=====================================
@@ -1747,8 +1747,9 @@ rnDataDefn doc (HsDataDefn { dd_ND = new_or_data, dd_cType = cType
}
where
h98_style = case condecls of -- Note [Stupid theta]
- (L _ (ConDeclGADT {})) : _ -> False
- _ -> True
+ (L _ (ConDeclGADT {})) : _ -> False
+ (L _ (XConDecl (ConDeclGADTPrefixPs {}))) : _ -> False
+ _ -> True
rn_derivs (L loc ds)
= do { deriv_strats_ok <- xoptM LangExt.DerivingStrategies
@@ -2085,7 +2086,7 @@ rnConDecl decl@(ConDeclH98 { con_name = name, con_ex_tvs = ex_tvs
do { (new_context, fvs1) <- rnMbContext ctxt mcxt
; (new_args, fvs2) <- rnConDeclDetails (unLoc new_name) ctxt args
; let all_fvs = fvs1 `plusFV` fvs2
- ; traceRn "rnConDecl" (ppr name <+> vcat
+ ; traceRn "rnConDecl (ConDeclH98)" (ppr name <+> vcat
[ text "ex_tvs:" <+> ppr ex_tvs
, text "new_ex_dqtvs':" <+> ppr new_ex_tvs ])
@@ -2128,22 +2129,66 @@ rnConDecl decl@(ConDeclGADT { con_names = names
; (new_res_ty, fvs3) <- rnLHsType ctxt res_ty
; let all_fvs = fvs1 `plusFV` fvs2 `plusFV` fvs3
- (args', res_ty')
- = case args of
- InfixCon {} -> pprPanic "rnConDecl" (ppr names)
- RecCon {} -> (new_args, new_res_ty)
- PrefixCon as | (arg_tys, final_res_ty) <- splitHsFunType new_res_ty
- -> ASSERT( null as )
- -- See Note [GADT abstract syntax] in GHC.Hs.Decls
- (PrefixCon arg_tys, final_res_ty)
-
- ; traceRn "rnConDecl2" (ppr names $$ ppr implicit_tkvs $$ ppr explicit_tkvs)
+
+ ; traceRn "rnConDecl (ConDeclGADT)"
+ (ppr names $$ ppr implicit_tkvs $$ ppr explicit_tkvs)
; return (decl { con_g_ext = implicit_tkvs, con_names = new_names
, con_qvars = explicit_tkvs, con_mb_cxt = new_cxt
- , con_args = args', con_res_ty = res_ty'
+ , con_args = new_args, con_res_ty = new_res_ty
, con_doc = mb_doc' },
all_fvs) } }
+-- This case is only used for prefix GADT constructors generated by GHC's
+-- parser, where we do not know the argument types until type operator
+-- precedence has been resolved. See Note [GADT abstract syntax] in
+-- GHC.Hs.Decls for the full story.
+rnConDecl (XConDecl (ConDeclGADTPrefixPs { con_gp_names = names, con_gp_ty = ty
+ , con_gp_doc = mb_doc }))
+ = do { mapM_ (addLocM checkConName) names
+ ; new_names <- mapM lookupLocatedTopBndrRn names
+ ; mb_doc' <- rnMbLHsDoc mb_doc
+
+ ; let ctxt = ConDeclCtx new_names
+ ; (ty', fvs) <- rnHsSigType ctxt TypeLevel Nothing ty
+
+ -- Now that operator precedence has been resolved, we can split the
+ -- GADT type into its individual components below.
+ ; let HsIB { hsib_ext = implicit_tkvs, hsib_body = body } = ty'
+ (mb_explicit_tkvs, mb_cxt, tau) = splitLHsGADTPrefixTy body
+ lhas_forall = L (getLoc body) $ isJust mb_explicit_tkvs
+ explicit_tkvs = fromMaybe [] mb_explicit_tkvs
+ (arg_tys, res_ty) = splitHsFunType tau
+ arg_details = PrefixCon arg_tys
+
+ -- Ensure that there are no nested `forall`s or contexts, per
+ -- Note [No nested foralls or contexts in GADT constructors] in
+ -- GHC.Hs.Type.
+ ; case res_ty of
+ L l (HsForAllTy { hst_fvf = fvf })
+ | ForallVis <- fvf
+ -> setSrcSpan l $ addErr $ withHsDocContext ctxt $ vcat
+ [ text "Illegal visible, dependent quantification" <+>
+ text "in the type of a term"
+ , text "(GHC does not yet support this)" ]
+ | ForallInvis <- fvf
+ -> nested_foralls_contexts_err l ctxt
+ L l (HsQualTy {})
+ -> nested_foralls_contexts_err l ctxt
+ _ -> pure ()
+
+ ; traceRn "rnConDecl (ConDeclGADTPrefixPs)"
+ (ppr names $$ ppr implicit_tkvs $$ ppr explicit_tkvs)
+ ; pure (ConDeclGADT { con_g_ext = implicit_tkvs, con_names = new_names
+ , con_forall = lhas_forall, con_qvars = explicit_tkvs
+ , con_mb_cxt = mb_cxt, con_args = arg_details
+ , con_res_ty = res_ty, con_doc = mb_doc' },
+ fvs) }
+ where
+ nested_foralls_contexts_err :: SrcSpan -> HsDocContext -> RnM ()
+ nested_foralls_contexts_err l ctxt =
+ setSrcSpan l $ addErr $ withHsDocContext ctxt $
+ text "GADT constructor type signature cannot contain nested"
+ <+> quotes forAllLit <> text "s or contexts"
rnMbContext :: HsDocContext -> Maybe (LHsContext GhcPs)
-> RnM (Maybe (LHsContext GhcRn), FreeVars)
=====================================
compiler/GHC/Tc/TyCl.hs
=====================================
@@ -3073,7 +3073,7 @@ dataDeclChecks tc_name new_or_data (L _ stupid_theta) cons
-----------------------------------
-consUseGadtSyntax :: [LConDecl a] -> Bool
+consUseGadtSyntax :: [LConDecl GhcRn] -> Bool
consUseGadtSyntax (L _ (ConDeclGADT {}) : _) = True
consUseGadtSyntax _ = False
-- All constructors have same shape
@@ -4720,50 +4720,12 @@ noClassTyVarErr clas fam_tc
badDataConTyCon :: DataCon -> Type -> SDoc
badDataConTyCon data_con res_ty_tmpl
- | ASSERT( all isTyVar tvs )
- tcIsForAllTy actual_res_ty
- = nested_foralls_contexts_suggestion
- | isJust (tcSplitPredFunTy_maybe actual_res_ty)
- = nested_foralls_contexts_suggestion
- | otherwise
= hang (text "Data constructor" <+> quotes (ppr data_con) <+>
text "returns type" <+> quotes (ppr actual_res_ty))
2 (text "instead of an instance of its parent type" <+> quotes (ppr res_ty_tmpl))
where
actual_res_ty = dataConOrigResTy data_con
- -- This suggestion is useful for suggesting how to correct code like what
- -- was reported in #12087:
- --
- -- data F a where
- -- MkF :: Ord a => Eq a => a -> F a
- --
- -- Although nested foralls or contexts are allowed in function type
- -- signatures, it is much more difficult to engineer GADT constructor type
- -- signatures to allow something similar, so we error in the latter case.
- -- Nevertheless, we can at least suggest how a user might reshuffle their
- -- exotic GADT constructor type signature so that GHC will accept.
- nested_foralls_contexts_suggestion =
- text "GADT constructor type signature cannot contain nested"
- <+> quotes forAllLit <> text "s or contexts"
- $+$ hang (text "Suggestion: instead use this type signature:")
- 2 (ppr (dataConName data_con) <+> dcolon <+> ppr suggested_ty)
-
- -- To construct a type that GHC would accept (suggested_ty), we:
- --
- -- 1) Find the existentially quantified type variables and the class
- -- predicates from the datacon. (NB: We don't need the universally
- -- quantified type variables, since rejigConRes won't substitute them in
- -- the result type if it fails, as in this scenario.)
- -- 2) Split apart the return type (which is headed by a forall or a
- -- context) using tcSplitNestedSigmaTys, collecting the type variables
- -- and class predicates we find, as well as the rho type lurking
- -- underneath the nested foralls and contexts.
- -- 3) Smash together the type variables and class predicates from 1) and
- -- 2), and prepend them to the rho type from 2).
- (tvs, theta, rho) = tcSplitNestedSigmaTys (dataConUserType data_con)
- suggested_ty = mkSpecSigmaTy tvs theta rho
-
badGadtDecl :: Name -> SDoc
badGadtDecl tc_name
= vcat [ text "Illegal generalised algebraic data declaration for" <+> quotes (ppr tc_name)
=====================================
compiler/GHC/ThToHs.hs
=====================================
@@ -50,7 +50,6 @@ import GHC.Utils.Lexeme
import GHC.Utils.Misc
import GHC.Data.FastString
import GHC.Utils.Outputable as Outputable
-import GHC.Utils.Monad ( foldrM )
import qualified Data.ByteString as BS
import Control.Monad( unless, ap )
@@ -595,6 +594,8 @@ cvtConstr (ForallC tvs ctxt con)
add_cxt (L loc cxt1) (Just (L _ cxt2))
= Just (L loc (cxt1 ++ cxt2))
+ add_forall :: [LHsTyVarBndr Hs.Specificity GhcPs] -> LHsContext GhcPs
+ -> ConDecl GhcPs -> ConDecl GhcPs
add_forall tvs' cxt' con@(ConDeclGADT { con_qvars = qvars, con_mb_cxt = cxt })
= con { con_forall = noLoc $ not (null all_tvs)
, con_qvars = all_tvs
@@ -609,7 +610,13 @@ cvtConstr (ForallC tvs ctxt con)
where
all_tvs = tvs' ++ ex_tvs
- add_forall _ _ (XConDecl nec) = noExtCon nec
+ -- The GadtC and RecGadtC cases of cvtConstr will always return a
+ -- ConDeclGADT, not a ConDeclGADTPrefixPs, so this case is unreachable.
+ -- See Note [GADT abstract syntax] in GHC.Hs.Decls for more on the
+ -- distinction between ConDeclGADT and ConDeclGADTPrefixPs.
+ add_forall _ _ con@(XConDecl (ConDeclGADTPrefixPs {})) =
+ pprPanic "cvtConstr.add_forall: Unexpected ConDeclGADTPrefixPs"
+ (Outputable.ppr con)
cvtConstr (GadtC [] _strtys _ty)
= failWith (text "GadtC must have at least one constructor name")
@@ -617,9 +624,8 @@ cvtConstr (GadtC [] _strtys _ty)
cvtConstr (GadtC c strtys ty)
= do { c' <- mapM cNameL c
; args <- mapM cvt_arg strtys
- ; L _ ty' <- cvtType ty
- ; c_ty <- mk_arr_apps args ty'
- ; returnL $ fst $ mkGadtDecl c' c_ty}
+ ; ty' <- cvtType ty
+ ; returnL $ mk_gadt_decl c' (PrefixCon args) ty'}
cvtConstr (RecGadtC [] _varstrtys _ty)
= failWith (text "RecGadtC must have at least one constructor name")
@@ -628,9 +634,19 @@ cvtConstr (RecGadtC c varstrtys ty)
= do { c' <- mapM cNameL c
; ty' <- cvtType ty
; rec_flds <- mapM cvt_id_arg varstrtys
- ; let rec_ty = noLoc (HsFunTy noExtField
- (noLoc $ HsRecTy noExtField rec_flds) ty')
- ; returnL $ fst $ mkGadtDecl c' rec_ty }
+ ; returnL $ mk_gadt_decl c' (RecCon $ noLoc rec_flds) ty' }
+
+mk_gadt_decl :: [Located RdrName] -> HsConDeclDetails GhcPs -> LHsType GhcPs
+ -> ConDecl GhcPs
+mk_gadt_decl names args res_ty
+ = ConDeclGADT { con_g_ext = noExtField
+ , con_names = names
+ , con_forall = noLoc False
+ , con_qvars = []
+ , con_mb_cxt = Nothing
+ , con_args = args
+ , con_res_ty = res_ty
+ , con_doc = Nothing }
cvtSrcUnpackedness :: TH.SourceUnpackedness -> SrcUnpackedness
cvtSrcUnpackedness NoSourceUnpackedness = NoSrcUnpack
@@ -1647,13 +1663,6 @@ See (among other closed issued) https://gitlab.haskell.org/ghc/ghc/issues/14289
-}
-- ---------------------------------------------------------------------
--- | Constructs an arrow type with a specified return type
-mk_arr_apps :: [LHsType GhcPs] -> HsType GhcPs -> CvtM (LHsType GhcPs)
-mk_arr_apps tys return_ty = foldrM go return_ty tys >>= returnL
- where go :: LHsType GhcPs -> HsType GhcPs -> CvtM (HsType GhcPs)
- go arg ret_ty = do { ret_ty_l <- returnL ret_ty
- ; return (HsFunTy noExtField arg ret_ty_l) }
-
split_ty_app :: TH.Type -> CvtM (TH.Type, [LHsTypeArg GhcPs])
split_ty_app ty = go ty []
where
=====================================
docs/users_guide/8.12.1-notes.rst
=====================================
@@ -96,6 +96,27 @@ Language
instantiated through visible type application. More information can be found
here: :ref:`Manually-defining-inferred-variables`.
+* GADT constructor types now properly adhere to :ref:`forall-or-nothing`. As
+ a result, GHC will now reject some GADT constructors that previous versions
+ of GHC would accept, such as the following: ::
+
+ data T where
+ MkT1 :: (forall a. a -> b -> T)
+ MkT2 :: (forall a. a -> T)
+
+ ``MkT1`` and ``MkT2`` are rejected because the lack of an outermost
+ ``forall`` triggers implicit quantification, making the explicit ``forall``s
+ nested. Furthermore, GADT constructors do not permit the use of nested
+ ``forall``s, as explained in :ref:`formal-gadt-syntax`.
+
+ In addition to rejecting nested ``forall``s, GHC is now more stringent about
+ rejecting uses of nested *contexts* in GADT constructors. For example, the
+ following example, which previous versions of GHC would accept, is now
+ rejected:
+
+ data U a where
+ MkU :: (Show a => U a)
+
Compiler
~~~~~~~~
=====================================
docs/users_guide/exts/explicit_forall.rst
=====================================
@@ -45,4 +45,81 @@ Notes:
would warn about the unused type variable `a`.
+.. _forall-or-nothing:
+
+The ``forall``-or-nothing rule
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In certain forms of types, type variables obey what is known as the
+"``forall``-or-nothing" rule: if a type has an outermost, explicit
+``forall``, then all of the type variables in the type must be explicitly
+quantified. These two examples illustrate how the rule works: ::
+
+ f :: forall a b. a -> b -> b -- OK, `a` and `b` are explicitly bound
+ g :: forall a. a -> forall b. b -> b -- OK, `a` and `b` are explicitly bound
+ h :: forall a. a -> b -> b -- Rejected, `b` is not in scope
+
+The type signatures for ``f``, ``g``, and ``h`` all begin with an outermost
+``forall``, so every type variable in these signatures must be explicitly
+bound by a ``forall``. Both ``f`` and ``g`` obey the ``forall``-or-nothing
+rule, since they explicitly quantify ``a`` and ``b``. On the other hand,
+``h`` does not explicitly quantify ``b``, so GHC will reject its type
+signature for being improperly scoped.
+
+In places where the ``forall``-or-nothing rule takes effect, if a type does
+*not* have an outermost ``forall``, then any type variables that are not
+explicitly bound by a ``forall`` become implicitly quantified. For example: ::
+
+ i :: a -> b -> b -- `a` and `b` are implicitly quantified
+ j :: a -> forall b. b -> b -- `a` is implicitly quantified
+ k :: (forall a. a -> b -> b) -- `b` is implicitly quantified
+
+GHC will accept ``i``, ``j``, and ``k``'s type signatures. Note that:
+
+- ``j``'s signature is accepted despite its mixture of implicit and explicit
+ quantification. As long as a ``forall`` is not an outermost one, it is fine
+ to use it among implicitly bound type variables.
+- ``k``'s signature is accepted because the outermost parentheses imply that
+ the ``forall`` is not an outermost ``forall``. The ``forall``-or-nothing
+ rule is one of the few places in GHC where the presence or absence of
+ parentheses can be semantically significant!
+
+The ``forall``-or-nothing rule takes effect in the following places:
+
+- Type signature declarations for functions, values, and class methods
+- Expression type annotations
+- Instance declarations
+- :ref:`class-default-signatures`
+- Type signatures in a :ref:`specialize-pragma` or
+ :ref:`specialize-instance-pragma`
+- :ref:`standalone-kind-signatures`
+- Type signatures for :ref:`gadt` constructors
+- Type signatures for :ref:`pattern-synonyms`
+- :ref:`data-instance-declarations`, :ref:`type-instance-declarations`,
+ :ref:`closed-type-families`, and :ref:`assoc-inst`
+- :ref:`rewrite-rules` in which the type variables are explicitly quantified
+Notes:
+
+- :ref:`pattern-type-sigs` are a notable example of a place where
+ types do *not* obey the ``forall``-or-nothing rule. For example, GHC will
+ accept the following: ::
+
+ f (g :: forall a. a -> b) x = g x :: b
+
+ Furthermore, :ref:`rewrite-rules` do not obey the ``forall``-or-nothing rule
+ when their type variables are not explicitly quantified: ::
+
+ {-# RULES "f" forall (g :: forall a. a -> b) x. f g x = g x :: b #-}
+
+- GADT constructors are extra particular about their ``forall``s. In addition
+ to adhering to the ``forall``-or-nothing rule, GADT constructors also forbid
+ nested ``forall``s. For example, GHC would reject the following GADT: ::
+
+ data T where
+ MkT :: (forall a. a -> b -> T)
+
+ Because of the lack of an outermost ``forall`` in the type of ``MkT``, the
+ ``b`` would be implicitly quantified. In effect, it would be as if one had
+ written ``MkT :: forall b. (forall a. a -> b -> T)``, which contains nested
+ ``forall``s. See :ref:`formal-gadt-syntax`.
=====================================
docs/users_guide/exts/gadt_syntax.rst
=====================================
@@ -103,6 +103,123 @@ implements this behaviour, odd though it is. But for GADT-style
declarations, GHC's behaviour is much more useful, as well as much more
intuitive.
+.. _formal-gadt-syntax:
+
+Formal syntax for GADTs
+~~~~~~~~~~~~~~~~~~~~~~~
+
+To make more precise what is and what is not permitted inside of a GADT-style
+constructor, we provide a BNF-style grammar for GADT below. Note that this
+grammar is subject to change in the future. ::
+
+ gadt_con ::= conids '::' opt_forall opt_ctxt gadt_body
+
+ conids ::= conid
+ | conid ',' conids
+
+ opt_forall ::= <empty>
+ | 'forall' tv_bndrs '.'
+
+ tv_bndrs ::= <empty>
+ | tv_bndr tv_bndrs
+
+ tv_bndr ::= tyvar
+ | '(' tyvar '::' ctype ')'
+
+ opt_ctxt ::= <empty>
+ | btype '=>'
+ | '(' ctxt ')' '=>'
+
+ ctxt ::= ctype
+ | ctype ',' ctxt
+
+ gadt_body ::= prefix_gadt_body
+ | record_gadt_body
+
+ prefix_gadt_body ::= '(' prefix_gadt_body ')'
+ | return_type
+ | opt_unpack btype '->' prefix_gadt_body
+
+ record_gadt_body ::= '{' fieldtypes '}' '->' return_type
+
+ fieldtypes ::= <empty>
+ | fieldnames '::' opt_unpack ctype
+ | fieldnames '::' opt_unpack ctype ',' fieldtypes
+
+ fieldnames ::= fieldname
+ | fieldname ',' fieldnames
+
+ opt_unpack ::= opt_bang
+ : {-# UNPACK #-} opt_bang
+ | {-# NOUNPACK #-} opt_bang
+
+ opt_bang ::= <empty>
+ | '!'
+ | '~'
+
+Where:
+
+- ``btype`` is a type that is not allowed to have an outermost
+ ``forall``/``=>`` unless it is surrounded by parentheses. For example,
+ ``forall a. a`` and ``Eq a => a`` are not legal ``btype``s, but
+ ``(forall a. a)`` and ``(Eq a => a)`` are legal.
+- ``ctype`` is a ``btype`` that has no restrictions on an outermost
+ ``forall``/``=>``, so ``forall a. a`` and ``Eq a => a`` are legal ``ctype``s.
+- ``return_type`` is a type that is not allowed to have ``forall``s, ``=>``s,
+ or ``->``s.
+
+This is a simplified grammar that does not fully delve into all of the
+implementation details of GHC's parser (such as the placement of Haddock
+comments), but it is sufficient to attain an understanding of what is
+syntactically allowed. Some further various observations about this grammar:
+
+- GADT constructor types are currently not permitted to have nested ``forall``s
+ or ``=>``s. (e.g., something like ``MkT :: Int -> forall a. a -> T`` would be
+ rejected.) As a result, ``gadt_sig`` puts all of its quantification and
+ constraints up front with ``opt_forall`` and ``opt_context``. Note that
+ higher-rank ``forall``s and ``=>``s are only permitted if they do not appear
+ directly to the right of a function arrow in a `prefix_gadt_body`. (e.g.,
+ something like ``MkS :: Int -> (forall a. a) -> S`` is allowed, since
+ parentheses separate the ``forall`` from the ``->``.)
+- Furthermore, GADT constructors do not permit outermost parentheses that
+ surround the ``opt_forall`` or ``opt_ctxt``, if at least one of them are
+ used. For example, ``MkU :: (forall a. a -> U)`` would be rejected, since
+ it would treat the ``forall`` as being nested.
+
+ Note that it is acceptable to use parentheses in a ``prefix_gadt_body``.
+ For instance, ``MkV1 :: forall a. (a) -> (V1)`` is acceptable, as is
+ ``MkV2 :: forall a. (a -> V2)``.
+- The function arrows in a ``prefix_gadt_body``, as well as the function
+ arrow in a ``record_gadt_body``, are required to be used infix. For
+ example, ``MkA :: (->) Int A`` would be rejected.
+- GHC uses the function arrows in a ``prefix_gadt_body`` and
+ ``prefix_gadt_body`` to syntactically demarcate the function and result
+ types. Note that GHC does not attempt to be clever about looking through
+ type synonyms here. If you attempt to do this, for instance: ::
+
+ type C = Int -> B
+
+ data B where
+ MkB :: C
+
+ Then GHC will interpret the return type of ``MkB`` to be ``C``, and since
+ GHC requires that the return type must be headed by ``B``, this will be
+ rejected. On the other hand, it is acceptable to use type synonyms within
+ the argument and result types themselves, so the following is permitted: ::
+
+ type B1 = Int
+ type B2 = B
+
+ data B where
+ MkB :: B1 -> B2
+- GHC will accept any combination of ``!``/``~`` and
+ ``{-# UNPACK #-}``/``{-# NOUNPACK #-}``, although GHC will ignore some
+ combinations. For example, GHC will produce a warning if you write
+ ``{-# UNPACK #-} ~Int`` and proceed as if you had written ``Int``.
+
+GADT syntax odds and ends
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
The rest of this section gives further details about GADT-style data
type declarations.
=====================================
testsuite/tests/dependent/should_fail/T16326_Fail6.stderr
=====================================
@@ -1,7 +1,5 @@
-T16326_Fail6.hs:9:3: error:
- • GADT constructor type signature cannot contain nested ‘forall’s or contexts
- Suggestion: instead use this type signature:
- MkFoo :: forall a. a -> Foo a
- • In the definition of data constructor ‘MkFoo’
- In the data type declaration for ‘Foo’
+T16326_Fail6.hs:9:12: error:
+ Illegal visible, dependent quantification in the type of a term
+ (GHC does not yet support this)
+ In the definition of data constructor ‘MkFoo’
=====================================
testsuite/tests/gadt/T12087.stderr
=====================================
@@ -1,35 +1,20 @@
-T12087.hs:6:3: error:
- • GADT constructor type signature cannot contain nested ‘forall’s or contexts
- Suggestion: instead use this type signature:
- MkF1 :: forall a. (Ord a, Eq a) => a -> F1 a
- • In the definition of data constructor ‘MkF1’
- In the data type declaration for ‘F1’
+T12087.hs:6:20: error:
+ GADT constructor type signature cannot contain nested ‘forall’s or contexts
+ In the definition of data constructor ‘MkF1’
-T12087.hs:9:3: error:
- • GADT constructor type signature cannot contain nested ‘forall’s or contexts
- Suggestion: instead use this type signature:
- MkF2 :: forall a. (Ord a, Eq a) => a -> F2 a
- • In the definition of data constructor ‘MkF2’
- In the data type declaration for ‘F2’
+T12087.hs:9:25: error:
+ GADT constructor type signature cannot contain nested ‘forall’s or contexts
+ In the definition of data constructor ‘MkF2’
-T12087.hs:12:3: error:
- • GADT constructor type signature cannot contain nested ‘forall’s or contexts
- Suggestion: instead use this type signature:
- MkF3 :: forall a b. (Eq a, Eq b) => a -> b -> F3 a
- • In the definition of data constructor ‘MkF3’
- In the data type declaration for ‘F3’
+T12087.hs:12:34: error:
+ GADT constructor type signature cannot contain nested ‘forall’s or contexts
+ In the definition of data constructor ‘MkF3’
-T12087.hs:15:3: error:
- • GADT constructor type signature cannot contain nested ‘forall’s or contexts
- Suggestion: instead use this type signature:
- MkF4 :: forall a b. (Eq a, Eq b) => a -> b -> F4 a
- • In the definition of data constructor ‘MkF4’
- In the data type declaration for ‘F4’
+T12087.hs:15:36: error:
+ GADT constructor type signature cannot contain nested ‘forall’s or contexts
+ In the definition of data constructor ‘MkF4’
-T12087.hs:18:3: error:
- • GADT constructor type signature cannot contain nested ‘forall’s or contexts
- Suggestion: instead use this type signature:
- MkF5 :: forall a b. Int -> Int -> a -> Int -> Int -> b -> F5 a
- • In the definition of data constructor ‘MkF5’
- In the data type declaration for ‘F5’
+T12087.hs:18:25: error:
+ GADT constructor type signature cannot contain nested ‘forall’s or contexts
+ In the definition of data constructor ‘MkF5’
=====================================
testsuite/tests/gadt/T14320.hs
=====================================
@@ -10,8 +10,8 @@ data Exp :: Type where
newtype TypedExp :: Type -> Type where
TEGood :: forall a . (Exp -> (TypedExp a))
--- The only difference here is that the type is wrapped in parentheses,
--- but GHC 8.0.1 rejects this program
+-- The presence of outer parentheses makes the `forall` nested, and
+-- GADTs do not permit nested `forall`s.
--
newtype TypedExpToo :: Type -> Type where
TEBad :: (forall a . (Exp -> (TypedExpToo a)))
=====================================
testsuite/tests/gadt/T14320.stderr
=====================================
@@ -0,0 +1,4 @@
+
+T14320.hs:17:14: error:
+ GADT constructor type signature cannot contain nested ‘forall’s or contexts
+ In the definition of data constructor ‘TEBad’
=====================================
testsuite/tests/gadt/T16427.stderr
=====================================
@@ -1,7 +1,4 @@
-T16427.hs:5:14: error:
- • GADT constructor type signature cannot contain nested ‘forall’s or contexts
- Suggestion: instead use this type signature:
- C :: forall b. Int -> b -> D
- • In the definition of data constructor ‘C’
- In the data type declaration for ‘D’
+T16427.hs:5:26: error:
+ GADT constructor type signature cannot contain nested ‘forall’s or contexts
+ In the definition of data constructor ‘C’
=====================================
testsuite/tests/gadt/T18191.hs
=====================================
@@ -0,0 +1,12 @@
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE RankNTypes #-}
+module T18191 where
+
+data T where
+ MkT :: (forall a. a -> b -> T)
+
+data S a where
+ MkS :: (forall a. S a)
+
+data U a where
+ MkU :: (Show a => U a)
=====================================
testsuite/tests/gadt/T18191.stderr
=====================================
@@ -0,0 +1,12 @@
+
+T18191.hs:6:11: error:
+ GADT constructor type signature cannot contain nested ‘forall’s or contexts
+ In the definition of data constructor ‘MkT’
+
+T18191.hs:9:11: error:
+ GADT constructor type signature cannot contain nested ‘forall’s or contexts
+ In the definition of data constructor ‘MkS’
+
+T18191.hs:12:11: error:
+ GADT constructor type signature cannot contain nested ‘forall’s or contexts
+ In the definition of data constructor ‘MkU’
=====================================
testsuite/tests/gadt/all.T
=====================================
@@ -113,10 +113,11 @@ test('T7558', normal, compile_fail, [''])
test('T9380', normal, compile_and_run, [''])
test('T12087', normal, compile_fail, [''])
test('T12468', normal, compile_fail, [''])
-test('T14320', normal, compile, [''])
+test('T14320', normal, compile_fail, [''])
test('T14719', normal, compile_fail, ['-fdiagnostics-show-caret'])
test('T14808', normal, compile, [''])
test('T15009', normal, compile, [''])
test('T15558', normal, compile, [''])
test('T16427', normal, compile_fail, [''])
test('T17423', expect_broken(17423), compile_and_run, [''])
+test('T18191', normal, compile_fail, [''])
=====================================
testsuite/tests/ghc-api/annotations/T10399.stdout
=====================================
@@ -34,9 +34,9 @@
((Test10399.hs:12:30,AnnComma), [Test10399.hs:12:30]),
((Test10399.hs:12:31-32,AnnCloseP), [Test10399.hs:12:32]),
((Test10399.hs:12:31-32,AnnOpenP), [Test10399.hs:12:31]),
-((Test10399.hs:(14,1)-(18,55),AnnData), [Test10399.hs:14:1-4]),
-((Test10399.hs:(14,1)-(18,55),AnnSemi), [Test10399.hs:20:1]),
-((Test10399.hs:(14,1)-(18,55),AnnWhere), [Test10399.hs:14:21-25]),
+((Test10399.hs:(14,1)-(18,53),AnnData), [Test10399.hs:14:1-4]),
+((Test10399.hs:(14,1)-(18,53),AnnSemi), [Test10399.hs:20:1]),
+((Test10399.hs:(14,1)-(18,53),AnnWhere), [Test10399.hs:14:21-25]),
((Test10399.hs:15:5-64,AnnDcolon), [Test10399.hs:15:11-12]),
((Test10399.hs:15:5-64,AnnSemi), [Test10399.hs:16:5]),
((Test10399.hs:15:14-64,AnnDot), [Test10399.hs:15:23]),
@@ -48,37 +48,29 @@
((Test10399.hs:15:45-46,AnnBang), [Test10399.hs:15:45]),
((Test10399.hs:15:45-46,AnnRarrow), [Test10399.hs:15:48-49]),
((Test10399.hs:15:45-64,AnnRarrow), [Test10399.hs:15:48-49]),
-((Test10399.hs:(16,5)-(17,69),AnnCloseP), [Test10399.hs:17:69]),
-((Test10399.hs:(16,5)-(17,69),AnnDcolon), [Test10399.hs:16:12-13]),
-((Test10399.hs:(16,5)-(17,69),AnnOpenP), [Test10399.hs:16:27]),
-((Test10399.hs:(16,5)-(17,69),AnnSemi), [Test10399.hs:18:5]),
-((Test10399.hs:(16,15)-(17,69),AnnDot), [Test10399.hs:16:25]),
-((Test10399.hs:(16,15)-(17,69),AnnForall), [Test10399.hs:16:15-20]),
-((Test10399.hs:(16,27)-(17,69),AnnCloseP), [Test10399.hs:17:69]),
-((Test10399.hs:(16,27)-(17,69),AnnOpenP), [Test10399.hs:16:27]),
-((Test10399.hs:16:28-43,AnnCloseP), [Test10399.hs:16:43, Test10399.hs:16:43]),
-((Test10399.hs:16:28-43,AnnDarrow), [Test10399.hs:16:45-46]),
-((Test10399.hs:16:28-43,AnnOpenP), [Test10399.hs:16:28, Test10399.hs:16:28]),
-((Test10399.hs:16:30-33,AnnComma), [Test10399.hs:16:34]),
-((Test10399.hs:16:48,AnnRarrow), [Test10399.hs:16:50-51]),
-((Test10399.hs:(16,48)-(17,68),AnnRarrow), [Test10399.hs:16:50-51]),
-((Test10399.hs:16:53-66,AnnRarrow), [Test10399.hs:17:45-46]),
-((Test10399.hs:(16,53)-(17,68),AnnRarrow), [Test10399.hs:17:45-46]),
-((Test10399.hs:17:48,AnnRarrow), [Test10399.hs:17:50-51]),
-((Test10399.hs:17:48-68,AnnRarrow), [Test10399.hs:17:50-51]),
-((Test10399.hs:17:66-68,AnnCloseS), [Test10399.hs:17:68]),
-((Test10399.hs:17:66-68,AnnOpenS), [Test10399.hs:17:66]),
-((Test10399.hs:18:5-55,AnnCloseP), [Test10399.hs:18:55]),
-((Test10399.hs:18:5-55,AnnDcolon), [Test10399.hs:18:16-17]),
-((Test10399.hs:18:5-55,AnnOpenP), [Test10399.hs:18:19]),
-((Test10399.hs:18:19-55,AnnCloseP), [Test10399.hs:18:55]),
-((Test10399.hs:18:19-55,AnnOpenP), [Test10399.hs:18:19]),
-((Test10399.hs:18:20-54,AnnDot), [Test10399.hs:18:29]),
-((Test10399.hs:18:20-54,AnnForall), [Test10399.hs:18:20-25]),
-((Test10399.hs:18:31-36,AnnCloseP), [Test10399.hs:18:36]),
-((Test10399.hs:18:31-36,AnnOpenP), [Test10399.hs:18:31]),
-((Test10399.hs:18:31-36,AnnRarrow), [Test10399.hs:18:38-39]),
-((Test10399.hs:18:31-54,AnnRarrow), [Test10399.hs:18:38-39]),
+((Test10399.hs:(16,5)-(17,67),AnnDcolon), [Test10399.hs:16:12-13]),
+((Test10399.hs:(16,5)-(17,67),AnnSemi), [Test10399.hs:18:5]),
+((Test10399.hs:(16,15)-(17,67),AnnDot), [Test10399.hs:16:25]),
+((Test10399.hs:(16,15)-(17,67),AnnForall), [Test10399.hs:16:15-20]),
+((Test10399.hs:16:27-42,AnnCloseP), [Test10399.hs:16:42, Test10399.hs:16:42]),
+((Test10399.hs:16:27-42,AnnDarrow), [Test10399.hs:16:44-45]),
+((Test10399.hs:16:27-42,AnnOpenP), [Test10399.hs:16:27, Test10399.hs:16:27]),
+((Test10399.hs:16:29-32,AnnComma), [Test10399.hs:16:33]),
+((Test10399.hs:16:47,AnnRarrow), [Test10399.hs:16:49-50]),
+((Test10399.hs:(16,47)-(17,67),AnnRarrow), [Test10399.hs:16:49-50]),
+((Test10399.hs:16:52-65,AnnRarrow), [Test10399.hs:17:44-45]),
+((Test10399.hs:(16,52)-(17,67),AnnRarrow), [Test10399.hs:17:44-45]),
+((Test10399.hs:17:47,AnnRarrow), [Test10399.hs:17:49-50]),
+((Test10399.hs:17:47-67,AnnRarrow), [Test10399.hs:17:49-50]),
+((Test10399.hs:17:65-67,AnnCloseS), [Test10399.hs:17:67]),
+((Test10399.hs:17:65-67,AnnOpenS), [Test10399.hs:17:65]),
+((Test10399.hs:18:5-53,AnnDcolon), [Test10399.hs:18:16-17]),
+((Test10399.hs:18:19-53,AnnDot), [Test10399.hs:18:28]),
+((Test10399.hs:18:19-53,AnnForall), [Test10399.hs:18:19-24]),
+((Test10399.hs:18:30-35,AnnCloseP), [Test10399.hs:18:35]),
+((Test10399.hs:18:30-35,AnnOpenP), [Test10399.hs:18:30]),
+((Test10399.hs:18:30-35,AnnRarrow), [Test10399.hs:18:37-38]),
+((Test10399.hs:18:30-53,AnnRarrow), [Test10399.hs:18:37-38]),
((Test10399.hs:20:1-25,AnnCloseQ), [Test10399.hs:20:24-25]),
((Test10399.hs:20:1-25,AnnOpen), [Test10399.hs:20:1-3]),
((Test10399.hs:20:1-25,AnnSemi), [Test10399.hs:22:1]),
=====================================
testsuite/tests/ghc-api/annotations/Test10399.hs
=====================================
@@ -13,9 +13,9 @@ mkPoli = mkBila . map ((,,(),,()) <$> P.base <*> P.pos <*> P.form)
data MaybeDefault v where
SetTo :: forall v . ( Eq v, Show v ) => !v -> MaybeDefault v
- SetTo4 :: forall v a. (( Eq v, Show v ) => v -> MaybeDefault v
- -> a -> MaybeDefault [a])
- TestParens :: (forall v . (Eq v) -> MaybeDefault v)
+ SetTo4 :: forall v a. ( Eq v, Show v ) => v -> MaybeDefault v
+ -> a -> MaybeDefault [a]
+ TestParens :: forall v . (Eq v) -> MaybeDefault v
[t| Map.Map T.Text $tc |]
=====================================
testsuite/tests/parser/should_compile/T15323.hs
=====================================
@@ -3,4 +3,4 @@
module T15323 where
data MaybeDefault v where
- TestParens :: (forall v . (Eq v) => MaybeDefault v)
+ TestParens :: forall v . (Eq v) => MaybeDefault v
=====================================
testsuite/tests/parser/should_compile/T15323.stderr
=====================================
@@ -8,7 +8,7 @@
{ModuleName: T15323}))
(Nothing)
[]
- [({ T15323.hs:(5,1)-(6,56) }
+ [({ T15323.hs:(5,1)-(6,54) }
(TyClD
(NoExtField)
(DataDecl
@@ -33,63 +33,67 @@
[])
(Nothing)
(Nothing)
- [({ T15323.hs:6:5-56 }
- (ConDeclGADT
- (NoExtField)
- [({ T15323.hs:6:5-14 }
- (Unqual
- {OccName: TestParens}))]
- ({ T15323.hs:6:21-55 }
- (True))
- [({ T15323.hs:6:28 }
- (UserTyVar
- (NoExtField)
- (SpecifiedSpec)
- ({ T15323.hs:6:28 }
- (Unqual
- {OccName: v}))))]
- (Just
- ({ T15323.hs:6:32-37 }
- [({ T15323.hs:6:32-37 }
- (HsParTy
- (NoExtField)
- ({ T15323.hs:6:33-36 }
- (HsAppTy
- (NoExtField)
- ({ T15323.hs:6:33-34 }
- (HsTyVar
- (NoExtField)
- (NotPromoted)
- ({ T15323.hs:6:33-34 }
- (Unqual
- {OccName: Eq}))))
- ({ T15323.hs:6:36 }
- (HsTyVar
- (NoExtField)
- (NotPromoted)
- ({ T15323.hs:6:36 }
- (Unqual
- {OccName: v}))))))))]))
- (PrefixCon
- [])
- ({ T15323.hs:6:42-55 }
- (HsAppTy
+ [({ T15323.hs:6:5-54 }
+ (XConDecl
+ (ConDeclGADTPrefixPs
+ [({ T15323.hs:6:5-14 }
+ (Unqual
+ {OccName: TestParens}))]
+ (HsIB
(NoExtField)
- ({ T15323.hs:6:42-53 }
- (HsTyVar
+ ({ T15323.hs:6:20-54 }
+ (HsForAllTy
(NoExtField)
- (NotPromoted)
- ({ T15323.hs:6:42-53 }
- (Unqual
- {OccName: MaybeDefault}))))
- ({ T15323.hs:6:55 }
- (HsTyVar
- (NoExtField)
- (NotPromoted)
- ({ T15323.hs:6:55 }
- (Unqual
- {OccName: v}))))))
- (Nothing)))]
+ (ForallInvis)
+ [({ T15323.hs:6:27 }
+ (UserTyVar
+ (NoExtField)
+ (SpecifiedSpec)
+ ({ T15323.hs:6:27 }
+ (Unqual
+ {OccName: v}))))]
+ ({ T15323.hs:6:31-54 }
+ (HsQualTy
+ (NoExtField)
+ ({ T15323.hs:6:31-36 }
+ [({ T15323.hs:6:31-36 }
+ (HsParTy
+ (NoExtField)
+ ({ T15323.hs:6:32-35 }
+ (HsAppTy
+ (NoExtField)
+ ({ T15323.hs:6:32-33 }
+ (HsTyVar
+ (NoExtField)
+ (NotPromoted)
+ ({ T15323.hs:6:32-33 }
+ (Unqual
+ {OccName: Eq}))))
+ ({ T15323.hs:6:35 }
+ (HsTyVar
+ (NoExtField)
+ (NotPromoted)
+ ({ T15323.hs:6:35 }
+ (Unqual
+ {OccName: v}))))))))])
+ ({ T15323.hs:6:41-54 }
+ (HsAppTy
+ (NoExtField)
+ ({ T15323.hs:6:41-52 }
+ (HsTyVar
+ (NoExtField)
+ (NotPromoted)
+ ({ T15323.hs:6:41-52 }
+ (Unqual
+ {OccName: MaybeDefault}))))
+ ({ T15323.hs:6:54 }
+ (HsTyVar
+ (NoExtField)
+ (NotPromoted)
+ ({ T15323.hs:6:54 }
+ (Unqual
+ {OccName: v})))))))))))
+ (Nothing))))]
({ <no location info> }
[])))))]
(Nothing)
=====================================
testsuite/tests/rename/should_compile/T5331.stderr
=====================================
@@ -5,7 +5,7 @@ T5331.hs:8:17: warning: [-Wunused-foralls (in -Wextra)]
T5331.hs:11:16: warning: [-Wunused-foralls (in -Wextra)]
Unused quantified type variable ‘a’
- In the definition of data constructor ‘W1’
+ In the type ‘forall a. W’
T5331.hs:13:13: warning: [-Wunused-foralls (in -Wextra)]
Unused quantified type variable ‘a’
=====================================
utils/haddock
=====================================
@@ -1 +1 @@
-Subproject commit 8134a3be2c01ab5f1b88fed86c4ad7cc2f417f0a
+Subproject commit ab0ab0a6254f0a8e302a7b13485084cbd2ed1247
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/7b89ce1c3cda917dcf628d24a84163d30d84098a
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/7b89ce1c3cda917dcf628d24a84163d30d84098a
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