Re: vector and GeneralizedNewtypeDeriving

Not by anything I've tried yet, no. On Tue, May 13, 2014 at 10:40 PM, Carter Schonwald < carter.schonwald@gmail.com> wrote:

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 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

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users