diff options
Diffstat (limited to 'compiler/GHC/Tc/Deriv/Functor.hs')
| -rw-r--r-- | compiler/GHC/Tc/Deriv/Functor.hs | 32 |
1 files changed, 30 insertions, 2 deletions
diff --git a/compiler/GHC/Tc/Deriv/Functor.hs b/compiler/GHC/Tc/Deriv/Functor.hs index b3e9fb775c..4b111f7a41 100644 --- a/compiler/GHC/Tc/Deriv/Functor.hs +++ b/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@. +-- +-- * @'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 |