vector and GeneralizedNewtypeDeriving

[Carter Schonwald]

can you get the deriving to work on
a newtype instance MVector s Foo = ....
?


On Tue, May 13, 2014 at 9:39 PM, John Lato <jwlato at gmail.com> wrote:

> 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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