vector and GeneralizedNewtypeDeriving

[Carter Schonwald]

this is an issue i'll be running into shortly, otoh I don't think many
folks are writing new unboxed vector instances and related engineering :)


On Wed, May 14, 2014 at 10:02 PM, John Lato <jwlato at gmail.com> wrote:

> 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 <eir at 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 <jwlato at 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
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>>
>>
>
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