Vladislav Zavialov pushed to branch wip/int-index/visible-forall-gadts at Glasgow Haskell Compiler / GHC
Commits:
4951a1da by Vladislav Zavialov at 2025-06-21T15:51:14+03:00
Visible forall in GADTs (#25127)
Add support for visible dependent quantification `forall a -> t` in
types of data constructors, e.g.
data KindVal a where
K :: forall k.
forall (a::k) -> -- now allowed!
k ->
KindVal a
For details, see docs/users_guide/exts/required_type_arguments.rst,
which has gained a new subsection.
DataCon in compiler/GHC/Core/DataCon.hs
---------------------------------------
The main change in this patch is that DataCon, the Core representation
of a data constructor, now uses a different type to store user-written
type variable binders:
- dcUserTyVarBinders :: [InvisTVBinder]
+ dcUserTyVarBinders :: [TyVarBinder]
where
type TyVarBinder = VarBndr TyVar ForAllTyFlag
type InvisTVBinder = VarBndr TyVar Specificity
and
data Specificity = InferredSpec | SpecifiedSpec
data ForAllTyFlag = Invisible Specificity | Required
This change necessitates some boring, mechanical changes scattered
throughout the diff:
... is now used in place of ...
-----------------+---------------
TyVarBinder | InvisTVBinder
IfaceForAllBndr | IfaceForAllSpecBndr
Specified | SpecifiedSpec
Inferred | InferredSpec
mkForAllTys | mkInvisForAllTys
additionally,
tyVarSpecToBinders -- added or removed calls
ifaceForAllSpecToBndrs -- removed calls
Visibility casts in mkDataConRep
--------------------------------
Type abstractions in Core (/\a. e) always have type (forall a. t)
because coreTyLamForAllTyFlag = Specified. This is also true of data
constructor workers. So we may be faced with the following:
data con worker: (forall a. blah)
data con wrapper: (forall a -> blah)
In this case the wrapper must use a visibility cast (e |> ForAllCo ...)
with appropriately set fco_vis{L,R}. Relevant functions:
mkDataConRep in compiler/GHC/Types/Id/Make.hs
dataConUserTyVarBindersNeedWrapper in compiler/GHC/Core/DataCon.hs
mkForAllVisCos in compiler/GHC/Core/Coercion.hs
mkCoreTyLams in compiler/GHC/Core/Make.hs
mkWpForAllCast in compiler/GHC/Tc/Types/Evidence.hs
More specifically:
- dataConUserTyVarBindersNeedWrapper has been updated to answer "yes"
if there are visible foralls in the type of the data constructor.
- mkDataConRep now uses mkCoreTyLams to generate the big lambda
abstractions (/\a b c. e) in the data con wrapper.
- mkCoreTyLams is a variant of mkCoreLams that applies visibility casts
as needed. It similar in purpose to the pre-existing mkWpForAllCast,
so the common bits have been factored out into mkForAllVisCos.
ConDecl in compiler/Language/Haskell/Syntax/Decls.hs
----------------------------------------------------
The surface syntax representation of a data constructor declaration is
ConDecl. In accordance with the proposal, only GADT syntax is extended
with support for visible forall, so we are interested in ConDeclGADT.
ConDeclGADT's field con_bndrs has been renamed to con_outer_bndrs
and is now accompanied by con_inner_bndrs:
con_outer_bndrs :: XRec pass (HsOuterSigTyVarBndrs pass)
con_inner_bndrs :: [HsForAllTelescope pass]
Visible foralls always end up in con_inner_bndrs. The outer binders are
stored and processed separately to support implicit quantification and
the forall-or-nothing rule, a design established by HsSigType.
A side effect of this change is that even in absence of visible foralls,
GHC now permits multiple invisible foralls, e.g.
data T a where { MkT :: forall a b. forall c d. ... -> T a }
But of course, this is done in service of making at least some of these
foralls visible. The entire compiler front-end has been updated to deal
with con_inner_bndrs. See the following modified or added functions:
Parser:
mkGadtDecl in compiler/GHC/Parser/PostProcess.hs
splitLHsGadtTy in compiler/GHC/Hs/Type.hs
Pretty-printer:
pprConDecl in compiler/GHC/Hs/Decls.hs
pprHsForAllTelescope in compiler/GHC/Hs/Type.hs
Renamer:
rnConDecl in compiler/GHC/Rename/Module.hs
bindHsForAllTelescopes in compiler/GHC/Rename/HsType.hs
extractHsForAllTelescopes in compiler/GHC/Rename/HsType.hs
Type checker:
tcConDecl in compiler/GHC/Tc/TyCl.hs
tcGadtConTyVarBndrs in compiler/GHC/Tc/Gen/HsType.hs
Template Haskell
----------------
The TH AST is left unchanged for the moment to avoid breakage. An
attempt to quote or reify a data constructor declaration with visible
forall in its type will result an error:
data ThRejectionReason -- in GHC/HsToCore/Errors/Types.hs
= ...
| ThDataConVisibleForall -- new error constructor
However, as noted in the previous section, GHC now permits multiple
invisible foralls, and TH was updated accordingly. Updated code:
repC in compiler/GHC/HsToCore/Quote.hs
reifyDataCon in compiler/GHC/Tc/Gen/Splice.hs
ppr @Con in libraries/ghc-boot-th/GHC/Boot/TH/Ppr.hs
Pattern matching
----------------
Everything described above concerns data constructor declarations, but
what about their use sites? Now it is trickier to type check a pattern
match fn(Con a b c)=... because we can no longer assume that a,b,c are
all value arguments. Indeed, some or all of them may very well turn out
to be required type arguments.
To that end, see the changes to:
tcDataConPat in compiler/GHC/Tc/Gen/Pat.hs
splitConTyArgs in compiler/GHC/Tc/Gen/Pat.hs
and the new helpers split_con_ty_args, zip_pats_bndrs.
This is also the reason the TcRnTooManyTyArgsInConPattern error
constructor has been removed. The new code emits TcRnArityMismatch
or TcRnIllegalInvisibleTypePattern.
Summary
-------
DataCon, ConDecl, as well as all related functions have been updated to
support required type arguments in data constructors.
Test cases:
HieGadtConSigs GadtConSigs_th_dump1 GadtConSigs_th_pprint1
T25127_data T25127_data_inst T25127_infix
T25127_newtype T25127_fail_th_quote T25127_fail_arity
TyAppPat_Tricky
Co-authored-by: mniip