Broken GeneralizedNewtypeDeriving in ghc 7.8 about vector

[JohnLato]

I don't think this is currently fixable within vector. Role annotations
aren't allowed on type or data families currently, and I don't know any
other way to get ghc to make the coercion.

For now, I've used
http://hackage.haskell.org/package/vector-th-unbox instead of GND.
On Apr 15, 2014 4:44 AM, "Edgar Gomes" [email protected] wrote:

Some of my code is broken with the newest version of ghc (7.8.2).

I'm using GeneralizedNewtypeDeriving to deriving instances of
Data.Vector.Unbox using the following:

data VoxelPos = VoxelPos
{-# UNPACK #-} !Int
{-# UNPACK #-} !Int
{-# UNPACK #-} !Int
deriving (Show, Eq, Ord)

newtype FacePos = FacePos VoxelPos deriving ( Eq, Hashable, NFData, G.Vector U.Vector, M.MVector U.MVector, U.Unbox)

where VoxelPos have manual rolled instances using (Int, Int, Int):

newtype instance U.MVector s VoxelPos = MV_VoxelPos (U.MVector s (Int, Int, Int))
newtype instance U.Vector VoxelPos = V_VoxelPos (U.Vector (Int, Int, Int))
instance U.Unbox VoxelPos
instance M.MVector U.MVector VoxelPos where
basicLength (MV_VoxelPos v) ...
...

and this was working with the previous versions of ghc. But after
upgrading ghc, I get the following error:

Could not coerce from ‘U.MVector s (Int, Int, Int)’ to ‘U.MVector
s FacePos’
because the second type argument of ‘U.MVector’ has role Nominal,
but the arguments ‘(Int, Int, Int)’ and ‘FacePos’ differ
arising from the coercion of the method ‘M.basicLength’ from type
‘forall s. U.MVector s VoxelPos -> Int’ to type
‘forall s. U.MVector s FacePos -> Int’
Possible fix:
use a standalone 'deriving instance' declaration,
so you can specify the instance context yourself
When deriving the instance for (M.MVector U.MVector FacePos)
which, I think, is because of addition of roles. I know that roles improves safety when using GeneralizedNewtypeDeriving which is, of course, really good!

What are the possible solutions to solve this?
It seems necessary to enforce the role of MVector, something like this:

type role MVector _ representational

—
Reply to this email directly or view it on GitHubhttps://github.com//issues/16
.

[hvr]

See also recent thread on glasgow-haskell-users

[cartazio]

this is a ghc side problem that we can't fix in vector, and requires some future evolution of ghc's Roles tooling to enable again

[cartazio]

closing because ghc doesn't have higher kinded roles, so we cant help GND sadness here