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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