I would be very much opposed, personally, to such an unsafe global pragma. Something like
deriving instance {-# UnsafeNewtypeDerivation #-} Foo Bar
would seem more reasonable. I'd also really like to see a less awkward story for these unboxed vector types, ideally one that would also enable unboxed non-vector containers more easily. I imagine I'm not the only one interested in unboxed priority queues and maps and sets and things.
The notes to #9220 explain this fairly well, I think.
But, Wren's desire is reasonable. The real reason that GND can't work here is that the safety condition lives outside the type system.
It seems to me like one way forward is to allow an unsafe GND, via some pragma. It would be dead-easy to implement (modulo surface syntax): just change `coerce` to `unsafeCoerce` in the produced code.
Richard
On Mar 24, 2015, at 10:25 AM, wren romano <winterkoninkje@gmail.com> wrote:
> On Tue, Mar 24, 2015 at 10:15 AM, Reid Barton <rwbarton@gmail.com> wrote:
>> On Tue, Mar 24, 2015 at 9:57 AM, wren romano <winterkoninkje@gmail.com>
>> wrote:
>>>
>>> So I was working on porting my libraries to work for GHC 7.10 and I
>>> ran into a snag. Unlike the concrete type Data.Array.Array, the
>>> concrete type Data.Array.Unboxed.UArray is defined as being nominal in
>>> the second type argument— is this a bug?
>>
>> No, see https://ghc.haskell.org/trac/ghc/ticket/9220.
>
> Fair enough.
>
>
> To be clearer, my actual goal is to automatically derive the (IArray
> UArray) instance for a newtype. The newtype has semantic content (like
> Sum or Product), but no representational content (i.e., New and Old
> have the same size and the same valid bit patterns). This worked fine
> in GHC 7.8 and prior, and should still be fine since it doesn't run
> into the sorts of issues raised by that bug report. Again, the goal is
> to automatically derive the instance rather than doing it manually,
> since I really want to just inherit the underlying instance at the new
> type.
>
> --
> Live well,
> ~wren
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