Replying by email because I'm on a train. Simon Huh. Put otherwise, your point is this. Suppose we have the following kind for `(->)`: (->) :: forall v1 v2 r1 r2. TYPE v1 r1 -> TYPE v2 r2 -> Type To coerce from (C a -> Int) to (a -> Int) we'd have to cough up a coercion `g`: g : (->) Vanilla Vanilla Ptr Ptr (C a) Int ~R (->) Constraint Vanilla Ptr Ptr a Int And now (Nth 1 g :: Vanilla ~R Constraint). Nothing about `KindCo` there; it's just that `(->)` takes some kind arguments. But that can only happen if `(->)` has suitable roles. What if it doesn't? What if we just had an axiom axArrow v :: (->) Vanilla v ~R (->) Constraint v or something like that. Then we get [W] g : (->) Vanilla Vanilla Ptr Ptr (C a) Int ~R (->) Constraint Vanilla Ptr Ptr a Int We decompose partly and solve thus g = (axArrow Vanilla) <Ptr> <Ptr> axC <Int> Simon From: noreply@phabricator.haskell.org [mailto:noreply@phabricator.haskell.org] Sent: 31 January 2017 12:51 To: Simon Peyton Jones <simonpj@microsoft.com> Subject: [Differential] [Commented On] D2038: [WIP] TysPrim: Generalize kind of (->) goldfire added a comment. View Revision<https://phabricator.haskell.org/D2038> In D2038#89360<https://phabricator.haskell.org/D2038#89360>, @simonpj<https://phabricator.haskell.org/p/simonpj/> wrote: To avoid being able to extract ContraintRep ~R LiftedPtrRep we decided to weaken one of the coercion constructors, the one that gets a kind coercion from a type coercion. We don't need it, and it's awkward here. The problem is that we need it with this patch. I was able to weaken this coercion constructor (KindCo) in my patch D3023<https://phabricator.haskell.org/D3023>, but this patch uses it in a fundamental way that we can't get around. To wit: class C a where meth :: a axC :: (C a :: Constraint) ~R (a :: Type) Now, we wish to cast C a -> a to a -> a.. This cast will look like (->) ?? <LiftedRep> axC <a>. What goes in the ??? It's got to be something involving KindCo axC, which is disallowed as per our earlier decision. Therein lies the problem. As for reify: Yes, I'm agreed with that email. But is that implemented yet? Is a design settled on? I don't see a ghc-proposal. Are we wiling to take a dependency on that work in order to get this done? To be clear, my chief worry isn't that these problems cannot be solved by any means -- I'm just worried about the timing of this all and our desire to get 8.2 out the door. REPOSITORY rGHC Glasgow Haskell Compiler REVISION DETAIL https://phabricator.haskell.org/D2038 EMAIL PREFERENCES https://phabricator.haskell.org/settings/panel/emailpreferences/ To: bgamari, goldfire, austin Cc: simonpj, RyanGlScott, thomie