vector and GeneralizedNewtypeDeriving

[John Lato]

Not by anything I've tried yet, no.


On Tue, May 13, 2014 at 10:40 PM, Carter Schonwald <
carter.schonwald at gmail.com> wrote:

> 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 <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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