But that can only happen if `(->)` has suitable roles.
What if it doesn’t?
The “correct” roles for (->) of the kind you gave is `nominal nominal nominal nominal representational representational`. That is, the dependent arguments are nominal, and the others are representational. This is because all kind-level coercions are nominal. You seem to be suggesting giving (->) different roles. I honestly don’t know what that would mean -- normally, GHC prevents you from specifying a weaker role than it would infer. It smells pretty foul to me, but I can’t quite put my finger on what would go wrong at the moment.
What if we just had an axiom
axArrow v :: (->) Vanilla v
~R (->) Constraint v
I think we’d also need one for results... but maybe not.
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>
And this works only if we weaken (->)’s roles.
This whole road just feels like the wrong way, as soon as we started contemplating a heterogeneous axiom, which are ruled out in the literature, even when we have kind equalities.
I think the Right Answer is to get rid of newtype-classes & fix reify, and I’m worried that anything short of that will fail catastrophically at some point. Otherwise, it’s patches on top of patches.
I don’t think there is disagreement here, but there is the question about what to do for 8.2.... and unless we’re ready to roll out the new reify, I think the best course of action is to delay the new Typeable and all this Constraint v Type stuff until 8.4. (The new levity polymorphism stuff already committed is hunky-dory.)
Richard
Simon
goldfire added a comment. |
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 patchD3023, 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.
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rGHC Glasgow Haskell Compiler
To: bgamari, goldfire, austin
Cc: simonpj, RyanGlScott, thomie