Re: Type roles for UArray?

On Tue, Mar 24, 2015 at 10:15 AM, Reid Barton wrote:

On Tue, Mar 24, 2015 at 9:57 AM, wren romano 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