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