can you get the deriving to work ona newtype instance MVector s Foo = ....?On Tue, May 13, 2014 at 9:39 PM, John Lato <jwlato@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 -> Intetc.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 representationalbut 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 Intam I correct that the current machinery isn't up to handling this?Thanks,John
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