Hi Richard, Thanks for pointing me to the ticket; I agree that's the issue (although I'm glad to have you and Simon confirm it). I've summarized the issue and raised the priority, and Simon linked to this thread.
I would have expected this would have affected a lot users, but as I haven't heard many complaints (and nobody else said anything here!) maybe the impact is smaller than I thought. Thanks, John On Wed, May 14, 2014 at 6:02 AM, Richard Eisenberg <e...@cis.upenn.edu>wrote: > Is this an instance of https://ghc.haskell.org/trac/ghc/ticket/8177 ? I > think so. > > The problem boils down to the fact that Vector and MVector are data > families and are thus (currently) exempted from the roles mechanism. (Or, > more properly, may *only* have nominal roles.) There is no technical reason > for this restriction. It's just that the feature would take a few solid > days of work to implement and I wasn't aware of a concrete use case. > > Here is such a use case. > > If you agree that you've hit #8177, please post to that bug report and > raise the priority to High -- being able to coerce Vectors seems very > reasonable indeed, and we should support it. I doubt the feature will land > in 7.8.3 (depending on the timeline for that release), but I'll get to it > eventually. (Or, if you feel this is more critical in the larger picture, > shout more loudly on the ticket and perhaps I can squeeze it in before > 7.8.3.) > > Thanks, > Richard > > On 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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