Relax Note [Phantom type variables in kinds] to allow for phantom levity vars
While implementing the unlifted datatypes proposal (!2218 (closed)), I realised that an edge case violates the kinding rules (Note [Phantom type variables in kinds]
, in particular). Specifically
type T :: forall (l :: Levity). TYPE (BoxedRep l)
data T = MkT
Note that
-
T
has a nullary data conMkT
-
T
is levity-polymorphic
That means the type of MkT
, forall {l::Levity}. T @l
, is ill-kinded! We have T @l :: TYPE (BoxedRep l)
and the forall
is supposed to bind the l
. But the kind rule for forall a. TYPE rep
demands that a
is not free in rep
:
ty : TYPE rep
`a` is not free in rep
(FORALL1) -----------------------
forall a. ty : TYPE rep
I argue that there is no harm in allowing TYPE (BoxedRep l)
here, because l
does not affect the representation of T
. I'm unaware of any other implications this might have.
We have to be a bit careful with evaluation semantics, but we have better means (#15532 (comment 239845), #17521) to decide whether levity polymorphism is OK at the term level. In any instance, MkT
is a data constructor and thus is a value under both call-by-need and call-by-value semantics. I'm reasonably positive that allowing MkT
to be well-kinded is OK. You can't do anything with it at the term level unless you apply it to a Levity
anyway.