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