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:
- Include a
[TyVarBndr]
field inDataInstD
andNewtypeInstD
where eachTyVarBndr
is aKindedTV
. - Walk over the
Type
s in aDataInstD
/NewtypeInstD
and ensure that every occurrence of aVarT
is surrounded withSigT
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 |