[Git][ghc/ghc][master] DeriveFunctor: Check for last type variables using dataConUnivTyVars
Marge Bot (@marge-bot)
gitlab at gitlab.haskell.org
Sun Sep 18 12:01:03 UTC 2022
Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC
Commits:
8a666ad2 by Ryan Scott at 2022-09-18T08:00:44-04:00
DeriveFunctor: Check for last type variables using dataConUnivTyVars
Previously, derived instances of `Functor` (as well as the related classes
`Foldable`, `Traversable`, and `Generic1`) would determine which constraints to
infer by checking for fields that contain the last type variable. The problem
was that this last type variable was taken from `tyConTyVars`. For GADTs, the
type variables in each data constructor are _not_ the same type variables as
in `tyConTyVars`, leading to #22167.
This fixes the issue by instead checking for the last type variable using
`dataConUnivTyVars`. (This is very similar in spirit to the fix for #21185,
which also replaced an errant use of `tyConTyVars` with type variables from
each data constructor.)
Fixes #22167.
- - - - -
5 changed files:
- compiler/GHC/Tc/Deriv/Functor.hs
- compiler/GHC/Tc/Deriv/Generics.hs
- compiler/GHC/Tc/Deriv/Infer.hs
- + testsuite/tests/deriving/should_compile/T22167.hs
- testsuite/tests/deriving/should_compile/all.T
Changes:
=====================================
compiler/GHC/Tc/Deriv/Functor.hs
=====================================
@@ -538,8 +538,36 @@ functorLikeTraverse var (FT { ft_triv = caseTrivial, ft_var = caseVar
go _ _ = (caseTrivial,False)
--- Return all syntactic subterms of ty that contain var somewhere
--- These are the things that should appear in instance constraints
+-- | Return all syntactic subterms of a 'Type' that are applied to the 'TyVar'
+-- argument. This determines what constraints should be inferred for derived
+-- 'Functor', 'Foldable', and 'Traversable' instances in "GHC.Tc.Deriv.Infer".
+-- For instance, if we have:
+--
+-- @
+-- data Foo a = MkFoo Int a (Maybe a) (Either Int (Maybe a))
+-- @
+--
+-- Then the following would hold:
+--
+-- * @'deepSubtypesContaining' a Int@ would return @[]@, since @Int@ does not
+-- contain the type variable @a@ at all.
+--
+-- * @'deepSubtypesContaining' a a@ would return @[]@. Although the type @a@
+-- contains the type variable @a@, it is not /applied/ to @a@, which is the
+-- criterion that 'deepSubtypesContaining' checks for.
+--
+-- * @'deepSubtypesContaining' a (Maybe a)@ would return @[Maybe]@, as @Maybe@
+-- is applied to @a at .
+--
+-- * @'deepSubtypesContaining' a (Either Int (Maybe a))@ would return
+-- @[Either Int, Maybe]@. Both of these types are applied to @a@ through
+-- composition.
+--
+-- As used in "GHC.Tc.Deriv.Infer", the 'Type' argument will always come from
+-- 'derivDataConInstArgTys', so it is important that the 'TyVar' comes from
+-- 'dataConUnivTyVars' to match. Make sure /not/ to take the 'TyVar' from
+-- 'tyConTyVars', as these differ from the 'dataConUnivTyVars' when the data
+-- type is a GADT. (See #22167 for what goes wrong if 'tyConTyVars' is used.)
deepSubtypesContaining :: TyVar -> Type -> [TcType]
deepSubtypesContaining tv
= functorLikeTraverse tv
=====================================
compiler/GHC/Tc/Deriv/Generics.hs
=====================================
@@ -93,10 +93,25 @@ gen_Generic_binds gk loc dit = do
************************************************************************
-}
+-- | Called by 'GHC.Tc.Deriv.Infer.inferConstraints'; generates a list of
+-- types, each of which must be a 'Functor' in order for the 'Generic1'
+-- instance to work. For instance, if we have:
+--
+-- @
+-- data Foo a = MkFoo Int a (Maybe a) (Either Int (Maybe a))
+-- @
+--
+-- Then @'get_gen1_constrained_tys' a (f (g a))@ would return @[Either Int]@,
+-- as a derived 'Generic1' instance would need to call 'fmap' at that type.
+-- Invoking @'get_gen1_constrained_tys' a@ on any of the other fields would
+-- return @[]@.
+--
+-- 'get_gen1_constrained_tys' is very similar in spirit to
+-- 'deepSubtypesContaining' in "GHC.Tc.Deriv.Functor". Just like with
+-- 'deepSubtypesContaining', it is important that the 'TyVar' argument come
+-- from 'dataConUnivTyVars'. (See #22167 for what goes wrong if 'tyConTyVars'
+-- is used.)
get_gen1_constrained_tys :: TyVar -> Type -> [Type]
--- called by GHC.Tc.Deriv.Infer.inferConstraints; generates a list of
--- types, each of which must be a Functor in order for the Generic1 instance to
--- work.
get_gen1_constrained_tys argVar
= argTyFold argVar $ ArgTyAlg { ata_rec0 = const []
, ata_par1 = [], ata_rec1 = const []
=====================================
compiler/GHC/Tc/Deriv/Infer.hs
=====================================
@@ -178,9 +178,10 @@ inferConstraintsStock dit@(DerivInstTys { dit_cls_tys = cls_tys
-- Constraints arising from the arguments of each constructor
con_arg_constraints
- :: (CtOrigin -> TypeOrKind
- -> Type
- -> [(ThetaSpec, Maybe Subst)])
+ :: ([TyVar] -> CtOrigin
+ -> TypeOrKind
+ -> Type
+ -> [(ThetaSpec, Maybe Subst)])
-> (ThetaSpec, [TyVar], [TcType], DerivInstTys)
con_arg_constraints get_arg_constraints
= let -- Constraints from the fields of each data constructor.
@@ -195,7 +196,8 @@ inferConstraintsStock dit@(DerivInstTys { dit_cls_tys = cls_tys
, not (isUnliftedType arg_ty)
, let orig = DerivOriginDC data_con arg_n wildcard
, preds_and_mbSubst
- <- get_arg_constraints orig arg_t_or_k arg_ty
+ <- get_arg_constraints (dataConUnivTyVars data_con)
+ orig arg_t_or_k arg_ty
]
-- Stupid constraints from DatatypeContexts. Note that we
-- must gather these constraints from the data constructors,
@@ -237,21 +239,39 @@ inferConstraintsStock dit@(DerivInstTys { dit_cls_tys = cls_tys
is_functor_like = tcTypeKind inst_ty `tcEqKind` typeToTypeKind
|| is_generic1
- get_gen1_constraints :: Class -> CtOrigin -> TypeOrKind -> Type
- -> [(ThetaSpec, Maybe Subst)]
- get_gen1_constraints functor_cls orig t_or_k ty
+ get_gen1_constraints ::
+ Class
+ -> [TyVar] -- The universally quantified type variables for the
+ -- data constructor
+ -> CtOrigin -> TypeOrKind -> Type
+ -> [(ThetaSpec, Maybe Subst)]
+ get_gen1_constraints functor_cls dc_univs orig t_or_k ty
= mk_functor_like_constraints orig t_or_k functor_cls $
- get_gen1_constrained_tys last_tv ty
-
- get_std_constrained_tys :: CtOrigin -> TypeOrKind -> Type
- -> [(ThetaSpec, Maybe Subst)]
- get_std_constrained_tys orig t_or_k ty
+ get_gen1_constrained_tys last_dc_univ ty
+ where
+ -- If we are deriving an instance of 'Generic1' and have made
+ -- it this far, then there should be at least one universal type
+ -- variable, making this use of 'last' safe.
+ last_dc_univ = assert (not (null dc_univs)) $
+ last dc_univs
+
+ get_std_constrained_tys ::
+ [TyVar] -- The universally quantified type variables for the
+ -- data constructor
+ -> CtOrigin -> TypeOrKind -> Type
+ -> [(ThetaSpec, Maybe Subst)]
+ get_std_constrained_tys dc_univs orig t_or_k ty
| is_functor_like
= mk_functor_like_constraints orig t_or_k main_cls $
- deepSubtypesContaining last_tv ty
+ deepSubtypesContaining last_dc_univ ty
| otherwise
= [( [mk_cls_pred orig t_or_k main_cls ty]
, Nothing )]
+ where
+ -- If 'is_functor_like' holds, then there should be at least one
+ -- universal type variable, making this use of 'last' safe.
+ last_dc_univ = assert (not (null dc_univs)) $
+ last dc_univs
mk_functor_like_constraints :: CtOrigin -> TypeOrKind
-> Class -> [Type]
@@ -279,9 +299,6 @@ inferConstraintsStock dit@(DerivInstTys { dit_cls_tys = cls_tys
, tcUnifyTy ki typeToTypeKind
)
- rep_tc_tvs = tyConTyVars rep_tc
- last_tv = last rep_tc_tvs
-
-- Extra Data constraints
-- The Data class (only) requires that for
-- instance (...) => Data (T t1 t2)
@@ -320,7 +337,7 @@ inferConstraintsStock dit@(DerivInstTys { dit_cls_tys = cls_tys
-- Generic1 needs Functor
-- See Note [Getting base classes]
| is_generic1
- -> assert (rep_tc_tvs `lengthExceeds` 0) $
+ -> assert (tyConTyVars rep_tc `lengthExceeds` 0) $
-- Generic1 has a single kind variable
assert (cls_tys `lengthIs` 1) $
do { functorClass <- lift $ tcLookupClass functorClassName
=====================================
testsuite/tests/deriving/should_compile/T22167.hs
=====================================
@@ -0,0 +1,24 @@
+module T22167 where
+
+import GHC.Generics (Generic1)
+
+data T1 f a = MkT1 (f a)
+ deriving (Functor, Foldable, Traversable)
+
+data T2 f a where
+ MkT2 :: f a -> T2 f a
+ deriving (Functor, Foldable, Traversable)
+
+-- A slightly more complicated example from the `syntactic` library
+data (sym1 :+: sym2) sig
+ where
+ InjL :: sym1 a -> (sym1 :+: sym2) a
+ InjR :: sym2 a -> (sym1 :+: sym2) a
+ deriving (Functor, Foldable, Traversable)
+
+-- Test Generic1 instances with inferred Functor constraints
+data G1 f g a = MkG1 (f (g a)) deriving Generic1
+
+data G2 f g a where
+ MkG2 :: f (g a) -> G2 f g a
+ deriving Generic1
=====================================
testsuite/tests/deriving/should_compile/all.T
=====================================
@@ -139,3 +139,4 @@ test('T20387', normal, compile, [''])
test('T20501', normal, compile, [''])
test('T20719', normal, compile, [''])
test('T20994', normal, compile, [''])
+test('T22167', normal, compile, [''])
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/8a666ad2a89a8ad2aa24a6406b88f516afaec671
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/8a666ad2a89a8ad2aa24a6406b88f516afaec671
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