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3c2f4bb4
by sheaf at 2025-11-11T11:47:28-05:00
Preserve user-written kinds in data declarations
This commit ensures that we preserve the user-written kind for data
declarations, e.g. in
type T2T = Type -> Type
type D :: T2T
data D a where { .. }
that we preserve the user-written kind of D as 'T2T', instead of
expanding the type synonym 'T2T' during kind checking.
We do this by storing 'tyConKind' separately from 'tyConResKind'. This
means that 'tyConKind' is not necessarily equal to
'mkTyConKind binders res_kind', as e.g. in the above example the former
is 'T2T' while the latter is 'Type -> Type'.
This is explained in Note [Preserve user-written TyCon kind] in GHC.Core.TyCon.
This is particularly important for Haddock, as the kinds stored in
interface files affect the generated documentation, and we want to
preserve the user-written types as much as possible.
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19859584
by sheaf at 2025-11-11T11:47:28-05:00
Store user-written datacon tvs in interface files
This commit ensures we store the user-written quantified type variables
of data constructors in interface files, e.g. in
data D a where
MkD1 :: forall x. x -> D x
MkD2 :: forall u v. u -> v -> D v
The previous behaviour was to rename the universal variables to match
the universal variables of the data constructor. This was undesirable
because the names that end up in interface files end up mattering for
generated Haddock documentation; it's better to preserve the user-written
type variables.
Moreover, the universal variables may not have been user-written at all,
e.g. in an example such as:
type T2T = Type -> Type
data G :: T2T where
MkG :: forall x. D x
Here GHC will invent the type variable name 'a' for the first binder of
the TyCon G. We really don't want to then rename the user-written 'x'
into the generated 'a'.
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034b2056
by sheaf at 2025-11-11T11:47:28-05:00
DataCon univ_tvs names: pick TyCon over inferred
This commit changes how we compute the names of universal type variables
in GADT data constructors. This augments the existing logic that chose
which type variable name to use, in GHC.Tc.TyCl.mkGADTVars. We continue
to prefer DataCon tv names for user-written binders, but we now prefer
TyCon tv names for inferred (non-user-written) DataCon binders.
This makes a difference in examples such as:
type (:~~:) :: k1 -> k2 -> Type
data a :~~: b where
HRefl :: a :~~: a
Before this patch, we ended up giving HRefl the type:
forall {k2}. forall (a :: k2). a :~~: a
whereas we now give it the type:
forall {k1}. forall (a :: k1). a :~~: a
The important part isn't really 'k1' or 'k2', but more that the inferred
type variable names of the DataCon can be arbitrary/unpredictable (as
they are chosen by GHC and depend on how unification proceeds), so it's
much better to use the more predictable TyCon type variable names.
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95078d00
by sheaf at 2025-11-11T11:47:28-05:00
Backpack Rename: use explicit record construction
This commit updates the Backpack boilerplate in GHC.Iface.Rename to
use explicit record construction rather than record update. This makes
sure that the code stays up to date when the underlying constructors
change (e.g. new fields are added). The rationale is further explained
in Note [Prefer explicit record construction].
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2bf36263
by sheaf at 2025-11-11T11:47:28-05:00
Store # eta binders in TyCon and use for Haddock
This commit stores the number of TyCon binders that were introduced by
eta-expansion (by the function GHC.Tc.Gen.HsType.splitTyConKind).
This is then used to pretty-print the TyCon as the user wrote it, e.g.
for
type Effect :: (Type -> Type) -> Type -> Type
data State s :: Effect where {..} -- arity 3
GHC will eta-expand the data declaration to
data State s a b where {..}
but also store in the 'TyCon' that the number of binders introduced by
this eta expansion is 2. This allows us, in
'Haddock.Convert.synifyTyConKindSig', to recover the original user-written
syntax, preserving the user's intent in Haddock documentation.
See Note [Inline kind signatures with GADTSyntax] in Haddock.Convert.