Type roles for UArray?

[David Feuer]

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.
On Mar 24, 2015 10:45 AM, "Richard Eisenberg" <eir at cis.upenn.edu> wrote:

> 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 at gmail.com>
> wrote:
>
> > On Tue, Mar 24, 2015 at 10:15 AM, Reid Barton <rwbarton at gmail.com>
> wrote:
> >> On Tue, Mar 24, 2015 at 9:57 AM, wren romano <winterkoninkje at 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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>
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