TH-reified type family and data family instances have a paucity of kinds

Consider this data family (and instances):

{-# LANGUAGE TypeFamilies #-}
module Foo where

data family Foo a
data instance Foo ((f :: *        -> *) (a ::       *))
data instance Foo ((f :: (* -> *) -> *) (a :: (* -> *)))

These are two data family instances that GHC distinguishes by the kinds of their type parameters. However, Template Haskell does not give me the same insight that GHC has, because if I call Language.Haskell.TH.reify ''Foo, I get this:

FamilyI
  (DataFamilyD
     Foo.Foo [ KindedTV a_6989586621679025989 StarT ] (Just StarT))
  [ DataInstD
      []
      Foo.Foo
      [ AppT (VarT f_6989586621679026001) (VarT a_6989586621679026000) ]
      Nothing
      []
      []
  , DataInstD
      []
      Foo.Foo
      [ AppT (VarT f_6989586621679026007) (VarT a_6989586621679026006) ]
      Nothing
      []
      []
  ]

Note that neither f nor a have a kind signature in either instance! This makes it completely impossible to tell which is which (aside from the order, which is brittle). It would make my life a lot easier if TH were to include kind signatures for each type variable in a data family instance. I can see two ways to accomplish this:

1. Include a [TyVarBndr] field in DataInstD and NewtypeInstD where each TyVarBndr is a KindedTV.
2. Walk over the Types in a DataInstD/NewtypeInstD and ensure that every occurrence of a VarT is surrounded with SigT to indicate its kind.

While (1) is arguably the cleaner solution, since it makes the kinds easy to discover, it is a breaking change. Therefore, I'm inclined to implement option (2) instead.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture