vector and GeneralizedNewtypeDeriving

[John Lato]

Hello,

Prior to ghc-7.8, it was possible to do this:

> module M where
>
> import qualified Data.Vector.Generic.Base as G
> import qualified Data.Vector.Generic.Mutable as M
> import Data.Vector.Unboxed.Base -- provides MVector and Vector
>
> newtype Foo = Foo Int deriving (Eq, Show, Num,
>     M.MVector MVector, G.Vector Vector, Unbox)

M.MVector is defined as

> class MVector v a where
>     basicLength :: v s a -> Int
etc.

With ghc-7.8 this no longer compiles due to an unsafe coercion, as MVector
s Foo and MVector s Int have different types.  The error suggests trying
-XStandaloneDeriving to manually specify the context, however I don't see
any way that will help in this case.

For that matter, I don't see any way to fix this in the vector package
either.  We might think to define

> type role M.MVector nominal representational

but that doesn't work as both parameters to M.MVector require a nominal
role (and it's probably not what we really want anyway).  Furthermore
Data.Vector.Unboxed.Base.MVector (which fills in at `v` in the instance) is
a data family, so we're stuck at that point also.

So given this situation, is there any way to automatically derive Vector
instances from newtypes?

tl;dr: I would really like to be able to do:

> coerce (someVector :: Vector Foo) :: Vector Int

am I correct that the current machinery isn't up to handling this?

Thanks,
John
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